Liz ParvinFrom simple X-ray photographs to computer images produced by magnetic resonance imaging there are a whole range of different techniques available to doctors for looking inside our bodies. How do they decide which one is most appropriate for a particular patient with particular symptoms?Well that is just one of the questions we will be addressing here at the busy Princess Margaret Hospital in Swindon. This video compilation is related to the medical imaging part of the course. After this introductory band you will find four other bands, each one associated with one of the four general subject areas of the course. Let’s start by looking at the physical mechanisms involved. I am joined by Alan Davies who is head of medical physics here at the Princess Margaret Hospital. Alan, would you like to just tell us something about the range of techniques you have available here in the hospital. Well the most common of all the medical imaging techniques is still the conventional X-ray. We have nine main X-ray rooms in this hospital. Additionally we have a number of ultrasound sets. We have a gamma camera for nuclear medicine imaging. We have a CT scanner that is a computer tomography scanner and an MRI unit – a magnetic resonance imager. Great. Well shall we go and look at some of the modern technology you have got here? Let’s go and look in the MR and CT unit. Right. What's the purpose of this? Security barrier is it? Yes. On the other side of this barrier both the CT and MRI units, both of which have their attendant risks. The risk associated with CT can be well confined within the scanner itself. On the other hand the very strong magnetic field of the MRI scanner can potentially cause effects outside of the room. So this is the first level of access control for this unit. Anybody who goes beyond this barrier will be closely supervised and will be required to fill in a fairly detailed questionnaire to ensure that they won't be affected by the strong magnetic field. But as we have already been through that with you we will go on. Thank you. So this is where patients come for CT and MRI scans. Shall we start with CT? Would you like to explain to me how these images here are formed? OK. Well we have got two video displays here that we can see. One is a conventional closed circuit television display, which allows us to monitor the patient during the examination. No patient there at the moment! No. That’s right. The larger display has the cross section image that the CT scanner produces. This is one of a head. The CT scanner uses an X-ray tube similar to those which are used in conventional X-ray sets but this X-ray tube rotates around the patient and detectors on the other side of the patient pick up that signal and from the whole data that is acquired during that single rotation a slice of the patient can be acquired and displayed on the screen. So CT scans are using X-rays, ionizing radiation. That's right. Now, gamma rays are also another form of ionizing radiation. How do you use those to do medical imagining? In nuclear medicine we image the distribution of radionuclide’s within patients. The radio nuclide will be injected generally into the patient and depending upon the compound to which it is attached it will spread throughout one of the organ systems or perhaps more than one organ system of the body and the gamma camera is capable of imaging that distribution. The important – or one of the important differences to note between something like CT and nuclear medicine is in CT we are imaging anatomy. In nuclear medicine we are looking at the function of the organs rather than the anatomy. Well let's move on to techniques that rely on non-ionizing radiation. Now this is the MRI machine. These images look very similar to the CT images we saw earlier on. Yes both CT and MR produce very high quality cross-sectional images through the body but that is really where the similarity between the two techniques ends. So how are these images produced? Well the patient here is placed inside a very large, static magnetic field and as they are placed in that field the protons which make up the nuclei of the hydrogen atoms will align along the magnetic field. As they align they will also precess around their own axis. The frequency of that precession is going to be determined by the magnitude of the static magnetic field. If we were to now switch on a radio frequency source tuned in precisely to the rate of that precession then a resonance effect will take place and the protons will absorb the radio frequency energy. Switching off the source of the radio frequency energy will then allow the protons to give up their energy, again as a radio frequency signal and that can be detected by a sensitive coil or aerial and that signal gives us a measure of the proton density and information about the local chemical environment of the protons. So the signal you get back depends upon the environment of the hydrogen atoms. Is that right? That's right. As well as the density of the hydrogen atoms the local chemical environment which they find themselves in also affects the size of the signal that we get from the system. So you get a different signal depending upon whether it's in fat or water or .. That's exactly right. It certainly gives you some wonderful images doesn't it? They really are very nice. Yes. So we have looked at CT and talked about gamma cameras and MRI and all of those involve electromagnetic radiation. Are there any other kind of waves that we can use to get images? Well ultrasound of course is used for imaging. Right. Can we have a look at that? So this is your ultrasound machine is it? This is one of the Doppler ultrasound machines that we have in the hospital. Now unlike the other techniques that we have looked at ultrasound employs no ionizing radiation and has a wide range of applications within medicine. The one that most people will be familiar with of course is ante-natal screening for pregnant mothers, used for sizing the foetus and monitoring the progression of the foetus. But it has a wide range of applications beyond that: in the cardiac area, for looking at livers. And in this case we have a Doppler ultrasound scan of a carotid artery. I could demonstrate how we do that using the transducer we have got here which acts both as a transmitter and a receiver of ultrasound energy. It will simply be a question of placing the transducer on your neck with a suitable coupling gel to ensure that we don’t loose any of the high frequency ultrasound energy and with a bit of careful positioning we could get an image similar to the one we have got on the screen. So what is happening here? The ultrasound is going in to my neck and being reflected back out again. Is that right? That's right. The ultrasound is reflected from any boundary within the body and the time it takes for the reflected ultrasound energy to get back to the transducer is used to calculate the depth from that boundary within the patient and therefore using a large number of transducers which make up the array an image of the structure can be formed. What about using the Doppler Effect in ultrasound? I believe that is possible. That's right. On the display here we have a demonstration of that. The colours represent the flow of the blood. If I start the tape going then we can see that the blue colour is showing blood flowing towards the transducers and the orange flowing away from the transducer. So we can see that the transducer is just fixed in the centre here and we are getting no Doppler signal directly underneath it.X-rays and CT use X-ray part of the electromagnetic spectrum.Medical ultrasound uses sound waves rather than electromagnetic radiation. The frequency of the sound waves is usually between 1 and 15MHz.Magnetic resonance images uses radio waves (usually around 50–150MHz).Radionuclide imaging (not actually shown in the video clip, but mentioned by Alan Davis) uses gamma radiation.2 X-ray imaging2.1 IntroductionX-ray imaging is probably the best known and most widely used of the imaging techniques we will cover. Not only are X-rays the longest established means of producing images of the internal structure of the body, but X-ray imaging is also the workhorse technique for all radiology departments.X-rays are high-energy electromagnetic radiation; those used for diagnostic imaging typically have photon energies between 20 and 120 keV.In X-ray imaging, a beam of X-rays is passed through the patient and detected on the opposite side. As the X-rays pass through the body some are either absorbed or scattered. This attenuation depends on the thickness of the material and on its attenuation coefficient. The equation that describes the reduction in intensity is
where I0 and I are the intensity before and after passing through material of thickness x and μ is the attenuation coefficient.Bone has a much higher attenuation coefficient than soft tissue, which in turn has a higher coefficient than air. This means that, if a patient is placed in a uniform beam of X-rays, radiation that has passed through soft tissue will have a higher intensity at the detector than radiation that has passed through bone. Hence the image will show good contrast between bone and soft tissue and, similarly, between soft tissue and lungs. X-ray images usually show the high attenuation material (e.g. bone) as white and the low attenuation material as black.Watch the video clip below which shows Alan having X-rays of his skull taken by a radiographer, following his arrival by ambulance to the Accident and Emergency Department with a possible head injury and skull fracture.Click to view the clip about planar x-ray images [2 minutes 45 seconds]
Planar x-ray images
NarratorX-ray Assuming the patient is not in a life-threatening condition, the doctor assessing the patient will often want to gain a rapid image of the patient’s injuries. Frequently an X-ray offers a quick and effective means of achieving this. This patient is being taken to have a skull X-ray. The radiographer inserts a cassette to record the X-ray image. In this case it is not a traditional screen film cassette but an array of phosphors. These store luminance as a function of X-ray exposure and act as a “glow memory” of the X-ray intensity at each point. The beam must be carefully aligned to the area of the patient under examination. The white light highlights the area that will be exposed to the Xray beam. In the case of a skull X-ray it is important to avoid the eyes if possible, as they are very sensitive to ionizing radiation. Having checked the alignment, the radiographer withdraws behind a lead glass screen, selects the exposure and proceeds with the X-ray. After exposure the cassette reader converts the intensity of each element of the phosphor array into a digital record, and can print out an image for the radiologist to examine. In this case the image shows a fracture to the skull, but the doctor is concerned that there could be damage to the top of the spine, something that won’t show up in a planar X-ray. As a result the doctor will send this patient for a CT scan.Figure 3 shows the X-ray unit used to image Alan's skull. It's main components – the x-ray source, collimator, patient couch, and film and cassette holder – are labelled.2.2 Components of an X-ray unit2.2.1 X-ray sourceX-rays are produced when energetic electrons strike a metal target. The X-ray source consists of an evacuated tube containing a cathode, from which the electrons are emitted, and an anode, which supports the target material where the X-rays are produced. Only about 1 per cent of the energy used is emitted as X-rays – the remainder is dissipated as heat in the anode. In most systems the anode is rotated so that the electrons strike only a small portion at any one time and the rest of the anode can cool. The X-rays are emitted from the tube via a radio-translucent exit window. Some of the X-rays are given off at an energy that depends on the nature of the target material. These are known as characteristic radiation (see Figure 5). There is also a broad spectrum of radiation known as bremsstrahlung (braking radiation); it is this radiation that is used in most diagnostic procedures (see Figure 5). However, both types contribute to the radiation dose given to the patient.The maximum photon energy is determined by the voltage applied to the tube (known as kVp). The peak of the X-ray spectrum is at about half of this energy. kVp values vary between about 20 kV for very thin body parts and 120 kV for a pelvic X-ray. Filters can be placed in front of the exit window to eliminate low-energy X-rays that do not contribute to the final image.2.2.2 CollimatorThe dimensions of the emerging X-ray beam can be altered by the collimator. This helps to ensure that only the region of interest is exposed to the X-rays.2.2.3 CouchThe couch or patient trolley must be radio-translucent (i.e. it allows through most of the X-rays). Nonetheless there is some interaction between the couch and the X-rays and this can be a cause of scattered radiation.2.2.4 Film cassette and gridAs the X-rays pass through the patient some of them will be scattered and will therefore not follow the expected line through the patient. If these reach the detector they will blur the image. Some of the scattered radiation can be removed by a grid, usually oscillating, placed between the patient and the detector.Analogue imaging systems use either film alone (rarely) or a combination of a film and fluorescent material (phosphor). The phosphor fluoresces and produces visible light which is recorded by the film. The film is then developed.Digital imaging systems use a variety of methods to record the intensity for each pixel in the image. This can then be displayed on a VDU or printed out on film.Activity 2See if you can answer the following questions. Please think about your answer before clicking to reveal the answer.What happens to most of the electrons which strike the anode in the X-ray source, and approximately what percentage of electrons produce X-rays?Most of the electrons produce heat, only 1 per cent of the electrons produce X-rays.Activity 3Why is a grid place in front of the film?The grid reduces the amount of scattered X-rays which reach the film, thus producing a clearer or sharper image.Activity 4Can you think of some advantages and disadvantages of X-ray imaging?The advantages of X-ray imaging are:X-ray images are relatively cheap to produce;images can be acquired quickly and are particularly useful for a rapid initial assessment;X-ray images are readily accessible.The disadvantages are:ionizing radiation dose;2-D image of a 3-D object;contrast can be poor.The disadvantage of planar X-ray imaging, producing a two dimensional representation of a three dimensional object, is overcome by computed tomography (CT) imaging.3 Computed tomographyThe aim of computed tomography (CT) is to produce an image of a slice of the body. (The Greek word ‘tomos’ means slice.) This is achieved by rotating a thin, fan-shaped beam of X-rays around the patient and measuring the intensity on the opposite side of the patient with a very large number of detectors.The following video clip shows Alan having a CT scan of his head to see if he has a base of skull fracture. Listen and watch the video clip carefully, with the following questions in mind:Activity 5What are the main differences between the skull X-rays and CT images produced of Alan?What colour do the skull bone and brain tissue appear on the CT images and why?How does the video say a two dimensional ‘slice’ is produced through the body?Click to view the clip about computed tomography [2 minutes 47 seconds]
Computed Tomography (CT)
An initial X-ray of this patient revealed a fracture; however the doctor responsible is also worried that there might be damage to the top of the spine - so a CT scan is being performed. Alan is now being placed in the CT scanner and the radiographer uses a laser beam to place his head in precisely the right location. CT scanning is essentially an X-ray procedure, but the X-ray source is rotated around the patient and the intensity recorded on the opposite side of the patient. Using data from a large number of angles a computer reconstruction can produce a two-dimensional map of the tissues in a slice of the body. Planning is done by taking a pilot scan. The patient is moved through the gantry, but the source remains stationary on one side. This produces an image similar to a planar X-ray. This pilot scan is taken in what is called the saggital plane. The final images will be in the axial plane. The concern over the potential for injury to the top of the spine has been dismissed, but there is a concern that there may be some damage to the brain, so it’s decided to do an MRI scan.The skull X-rays showed ‘projections’ of Alan's skull (a 3D object represented in 2D) and the brain tissue could not be seen. The CT slices show the skull and internal brain tissue as a series of ‘slices’, and therefore, in much more detail.The skull bone appears white and the brain tissue grey. This is because the skull attenuates X-rays to a greater extent than the brain tissue, and just like a planar X-ray the more a tissue attenuates X-rays, the whiter it will appear in the final image.The X-ray source is rotated around the patient and the intensity recorded on the opposite side of the patient. Using data from a large number of angles a computer reconstruction can produce a two-dimensional map of the tissues in a slice of the body.Let's look at the answer to the final question in more detail. First we will consider exactly how the X-ray source rotates around the patient.In modern scanners the source and the detectors rotate around the patient at more than one revolution per second. In the older scanners the couch was moved after one rotation and the subsequent rotation had to be in the opposite direction to avoid twisting the cables (called ‘stop and go’ by Dr Klaus Klingenbeck in the following video clip). However, with the introduction of slip-rings it became possible to keep the source and detectors rotating continuously and to move the couch at the same time. This means that the X-ray source describes a helix around the patient and a set of data covering the complete volume of the patient can be collected. This is known as spiral (or helical) scanning and has the advantage that the data for the entire thorax or other section of the body can be collected in one breath hold. More recently, multislice scanners, in which there is more than one arc of detectors, allow even faster data collection.Click to view part 1 of the clip about X-rays and CT [2 minutes 1 second]
X-rays and CT - part 1
Elizabeth Parvin:This collimator can be adjusted electronically to define a slice as thin as 1 mm or as thick as 10 mm, depending on clinical requirements. Now, what about rotation of the gantry? Klaus: For a standard CT scan we need to acquire projection data over a full rotation of 360 degrees, and at the time when the high voltage was still supplied at the tube via cables we had to reverse the direction of rotation from scan to scan in order not to twist the cables. And of course continuous data acquisition wasn’t possible at this time, only stop and go from scan to scan. But things are different today – look at this.Elizabeth Parvin: Continuous rotation of the gantry was impossible before the development of electrical slip-rings for getting power into the X-ray tube and data out of the detectors. Klaus: The weight of the rotating parts adds up to almost a ton, and we can accelerate the machine to an angular speed of one rotation in three quarters of a second. The radial acceleration at this speed is 4 to 5 times the gravitational acceleration.Elizabeth Parvin: In the original version of the third generation scanner, the problem of the twisting cables meant that the scanner had to reverse direction after each slice. This limited the speed of the scan as well as causing wear and tear on the components through repeated accelerations and decelerations. Continuous rotation overcomes all the disadvantages of “stop and go” and, when combined with continuous patient feed into the scanner, creates the so-called spiral scan.Now let's consider how a ‘slice’ through the patient can be reconstructed by the transmission data obtained at a large number of different angles, using a technique called ‘filtered back projection’. The following video clip will look at the transmission data from simple objects and how back projection, and finally filtered back projection, are used to reconstruct the original object from the transmission data. This clip introduces some complex topics – you only need a general overview of how CT image reconstructions work.Click to view part 2 of the clip about X-rays and CT [8 minutes 36 seconds]
X-rays and CT - part 2
Elizabeth Parvin: Let’s consider how these images of body slices are generated from the data recorded by the scanner. The basic mathematics behind this problem was actually solved in general terms as long ago as 1917. In many ways, it’s rather like trying to calculate the positions of all the trees in a forest from photographs taken through the forest in various different directions. Here’s Dr. James Cubillo of Coventry University. James Cubillo: The tomographic imaging process begins of course with data acquisition. This is referred to as the forward projection. The data are in the form of attenuation profiles, obtained at equal intervals as the instrument rotates about the body. I’ll demonstrate that with this simple object. Here we have a square object with uniform high attenuation. This is the imaging plane. If we irradiate the object with a parallel beam of X-rays from the left here you can see that we obtain an attenuation profile here. Outside of the object area we have zero attenuation. Where we have the object we have uniform high attenuation level. As we rotate the instrument through some angle you can see that we still have zero attenuation outside of the object here, uniform attenuation here between these two points where we have equal path lengths. Between these two points the path length is gradually increasing and we see that we have an attenuation profile which also increases uniformly. At 45 degrees you can see that we have maximum attenuation here corresponding to the diagonal of the square which of course is maximum for this object, between these two points we have uniformly increasing attenuation as the path length increases. And the process continues as we rotate the instrument relative to the object. Elizabeth Parvin: In these forward projections, James has actually rotated the object and fixed the direction of the beam, to make it easier to compare one attenuation profile with the next. However, in an actual scan and in the reconstructions to come, it’s the instrument that rotates and not the object. James: Now let’s look at a slightly more complicated scene in which we have two squares, each of high uniform attenuation. We irradiate the scene as before and you can see that we have rectangular attenuation profiles here for each of the squares. Rotating the beam through 45 degrees, we have two triangular profiles corresponding to each of these two squares. Then at 90 degrees we’re back to the rectangular profiles. However at 135 degrees the two squares are in line and therefore we have maximum attenuation corresponding to the maximum path length of these two squares in line. Elizabeth Parvin: These attenuation profiles and knowledge of the direction of each forward projection are all that’s required to reconstruct the original object. This process is known as simple back projection. James: Here we have the original object. This scale here is known as the hot body scale, with white and yellow representing high attenuation and red and black representing low attenuation. Applying a back projection at zero degrees we have an equal likelihood of the objects occurring anywhere along these paths. Now if we apply a back projection at 90 degrees we can see that surprisingly enough we have four candidate positions for these squares leaving us with an ambiguous situation. It is only when we add the 45 degree projection that we confirm the positions of the two squares. Going on further to the 135 degree projection, again the position is reinforced once more. Elizabeth Parvin: This ambiguity would not have existed with the single square. And this illustrates a general point. The more complex the images are, the more projections are required to avoid ambiguity in the reconstructed image. James: Now, if we go on further to reconstructing the whole image, starting with zero degree projection and building up at 15 degree intervals, we can see the two squares forming with each projection. And now with the completed image, with 24 back projections we can see clearly the two squares that we started with. However they do appear slightly blurred. Elizabeth Parvin: Convolution of the object with the point-spread function leads to a blurring effect, as you can see more clearly in this three-dimensional mesh plot of the same data. This problem is inherent to the back projection process and cannot be avoided, even by taking extra projections. But as you can see, a simple object, consisting of two squares, can still be reconstructed reasonably well. However, when simple back projection is applied to real CT data, the blurring effect seems to be much more serious. There are two reasons for this. Firstly, the objects imaged in a real CT scan have a whole range of density values whereas the squares in James’ model have just one, and secondly, a much higher spatial resolution is required. Nevertheless, the technique can be improved by filtering the data before carrying out the reconstruction. And this is known as filtered back projection. James: Here is an infinite ramp filter that can be applied to the reconstruction. Along the horizontal axis you can see the spatial frequency, and on the vertical axis you can see the relative amplitude of the signal passed by the filter. You can see that the filter itself discriminates against low frequencies in favour of the high frequencies. However, with such a filter we will run into aliasing problems, and so we need to apply the Nyquist criteria to cut off the filter at half the maximum spatial sampling frequency as we have here. Now let’s see what we get when we apply this filter to the back projections we saw earlier. Here we have the two original objects, and here is the filtered back projection. As you can see we get a much clearer, sharper image. Although it is still not perfect, much of the blurring has been removed. And here is the mesh plot. There is obviously a vast improvement over the simple back projection. And here we have a comparison of the simple back projection compared with the filtered back projection with their respective mesh plots. And the improvement is clear to see. Elizabeth Parvin: OK. So, filtering the data greatly improves the reconstruction of this simple model. But how well does it perform on the pelvic slice we saw earlier? Well, as you can see, we now have a clear, sharp image in which we can identify the top of the femur, the bladder, and the spinal cord – a vast improvement over the blurred image we obtained with simple back projection. Filtered back projection is the method of reconstruction used in modern CT imaging. The only difference from the process you’ve just seen is that the filters are rather more sophisticated than the cut-off ramp filter James used.Activity 6Now that you understand more about the CT imaging equipment and the images it produces, take a little time to consider what you think the advantages and disadvantages of this technique would be.The advantages of CT imaging are:excellent resolution and contrast;choice of tomographic or 3D-images available;relatively fast (compared with MRI);contrast medium can be used.The disadvantages are:larger dose of ionizing radiation than most planar X-ray procedures;equipment is costly and therefore not available at all hospitals;slower and more complex to undertake than most planar X-ray procedures.4 Magnetic resonance imagingMagnetic resonance imaging (MRI) is one of the newest techniques available for imaging in hospitals. It is a tomographic technique (i.e. it takes ‘slices’ through the body) based on the phenomenon of nuclear magnetic resonance (NMR). Technically it is a complex technique, but some understanding can be gained using qualitative arguments.X-rays are not used, but instead the patient is placed in a ‘strong’ magnetic field (around 30 000 times as strong as the Earth's magnetic field). The nuclei of hydrogen atoms (protons) like many other nuclei have a special property called nuclear spin. This means that in some respects they behave like tiny bar magnets, and in a static magnetic field they have just two possible orientations; either aligned with, or against, the magnetic field (see Figures 7 and 8 below). More protons align with the magnetic field, as this requires less energy. So, within the patient the net magnetisation within the tissues is aligned parallel to the applied magnetic field.Transitions away from this parallel state can be brought about by the application of a radiofrequency (rf) field, typically in the region of 20 to 100 MHz for many MRI scanners. The frequency required, which is referred to as the resonant or Larmor frequency, depends linearly on the strength of the static magnetic field. The rf field is applied in the form of a pulse of short (microseconds) duration. The signal detected by the scanner is the component of the net magnetisation vector perpendicular to the applied magnetic field.The signals are detected using specially designed and shaped radiofrequency coils (or antena), for example for heads, knees, etc. The fact that these coils can be placed immediately adjacent to an area of interest makes an important contribution to the quality of the final image.In the following video clip a head coil is used. As you are watching the video, pay particular attention to the main differences in image appearance between MRI and CT.Click to view the clip about magnetic resonance imaging [3 minutes 24 seconds]
Magnetic Resonance Imaging (MRI)
Thankfully, for this patient, a CT scan has already shown that the top of the spine hasn’t been fractured, but damage to the brain has yet to be ruled out. Magnetic resonance imaging, or MRI, with its excellent soft tissue discrimination, is the best way to check for this. To obtain images the patient is placed in a large static magnetic field, produced by a super-conducting magnet. This powerful field, typically 1.5 T in strength, causes the nuclei of the hydrogen atoms in the body to line up. But they can be flipped into another direction by a radio frequency pulse. The way these nuclei then relax back to their original position depends on the environment of the hydrogen atoms, in other words on tissue type. When imaging small parts of the body, specialized coils are used to detect the radio frequency signal given off by the relaxing nuclei. Here a head coil is being used. Laser alignment is used so that the patient can be moved to the correct position in the scanner. Because the powerful magnetic fields can disrupt pacemakers and cause heating of metal implants, every patient must be carefully checked before entering the scanner room. With MRI it is possible to choose any direction for the image slices. In this case the radiographer is taking an axial pilot scan to give a series of coronal images. There are many different imaging sequences that can be used, but most of them produce an image where the intensity depends mainly on one of three characteristics – T1, T2 and proton density. The intensity in these images depends largely on T2, the spin-spin relaxation. Substances with a long T2, such as water, appear bright. This is obvious if you look at the eyes in this image. Thankfully for this patient the images show no abnormality.Activity 7Take a moment to consider what you think are the main difference between MRI and CT images.MRI can be used to ‘slice’ the body in any direction. As well as producing ‘axial’ images, it can also produce sagittal or coronal images (or any plane in between.) The contrast can be changed in MRI images producing what were called T1, T2, or proton density weighted images. The contrast in CT images is fixed, depending on the attenuation coefficient.We will now look at T1, T2 and proton density weighting in a bit more detail. Once the net magnetisation has been excited away from its position parallel with the magnetic field by the rf pulse, it moves or ‘relaxes’ back to its original state. The times taken for this relaxation are governed by T1 and T2 time constants. These T1 and T2 values vary for different types of tissue.MR images are formed using a series of rf pulses in a carefully timed sequence. By varying the timings of these pulse sequences the final images can highlight the differences in T1 or T2. (It is also possible to set the timing within a pulse sequence so that the contrast is independent of both T1 and T2, and so depends on just proton density.)T1-weighted images show tissues with a large value of T1 (e.g. water) as dark. In T2-weighted images tissues with a large value of T2 are bright. Water has a high T2 so shows up bright in a T2 image. This feature can be used to highlight disease. Figures 9 and 10, below, show T2- and T1-weighted MR images.Activity 8You may think that since MRI uses strong magnetic fields and rf waves, rather than X-rays, it is completely safe. Actually, two safety hazards were mentioned in the video clip. Can you remember what they were?MRI scanners, magnetic fields and rf pulses can upset the operations of pacemakers. In addition, heating can occur in some metallic implants.Another major hazard of MRI scanners not mentioned in the video is the ‘projectile effect’. Ferromagnetic materials experience strong forces in the static magnetic field and objects such as a bunch of keys or an oxygen cylinder can become lethal weapons.The strength of the magnetic fields and the power levels for the radiofrequency irradiation are thought to be well below potentially damaging levels. However, as a precaution, MRI is not normally carried out during pregnancy.Activity 9Now that you understand more about MRI, take a little time to consider what you think the advantages and disadvantages of this technique would be.The advantages of MRI are:excellent spatial resolution;excellent soft tissue discrimination (for example, white and grey matter in the brain can be distinguished);no ionizing radiation;slices can be taken in any direction.The disadvantages are:high cost of equipment and maintenance;relatively slow;not suitable for all patients (e.g. those with a heart pacemaker).5 UltrasoundUltrasound imaging uses acoustic waves, rather than ionizing radiation, to form an image. The principle is rather like radar; a pulse of ultrasound (1–15 MHz) is sent out from the transducer and reflected from tissue boundaries. Measurement of the time taken for the pulse to return allows the distance to the reflecting boundary to be calculated.The important parameter determining the amount of reflection is known as the acoustic impedance (Z) of the tissue and is the product of acoustic velocity and density.The appearance of the returning echoes can be displayed in two principal ways. Firstly the amplitude of the echo can be shown as a vertical displacement against a horizontal time axis, which takes the appearance of a profile such as that typical of a mountain range. This is known as A (amplitude) mode.The alternative is to show the echo intensity as dots of varying brightness and this is known as B (brightness) mode. 2D imaging uses a large number of adjacent B-mode lines to form the final image, a B-scan.In a third mode, called M (movement) mode, a single line of a B-scan is chosen and the position of the reflecting boundaries is plotted as a function of time. A moving boundary will show up as a wavy trace. The shape of this trace can be indicative of critical features such as the operation of cardiac valves.Activity 10Look at these ultrasound images of a fetus, and the liver and kidneys. Do you think it was obtained using A-mode, B-mode or M-mode ultrasound imaging? They are both B-mode images. This is the type of ultrasound image we are all most familiar with.Activity 11Watch the following video clip.Click to view clip about ultrasound [5 minutes 24 seconds]
Ultrasound
This patient has been sent by his GP for an ultrasound investigation of the heart. Ultrasound has a number of key advantages; it is quick, cost effective, and has virtually no associated hazards. It is also far more acceptable to many patients than some other modalities. ECG electrodes are attached to the patient to record the electrical signals from the heart. This allows the ultrasound images to be linked to the heart cycle. Because even a thin layer of air will reflect the ultrasound signal, a coupling gel is used between the ultrasound transducer and the patient. Ultrasound is useful for rapid and reliable cardiac investigations, particularly as it is the only technique that can provide information about blood flow. However, as ultrasound doesn’t penetrate bone, the transducer has to be shaped so that the ultrasound beam will pass between the ribs. The transducer being used in this examination is a wide band transducer that transmits ultrasound at 1.75 MHz, a low enough frequency for good penetration of the heart. The images are formed by detecting the reflected second harmonic at 3.5 MHz. This gives a good compromise between spatial resolution and sensitivity. Paul has produced a parasternal view of the heart. First a standard B mode image. Now he has chosen a direction from the B mode image to look at in M mode. This allows the position of different parts of the heart to be plotted against time. By moving into colour Doppler mode, the blood flow through the heart can be visualized. Ultrasound reflected by blood flowing away from the transducer returns with a slightly lower frequency and shows up blue on the images. Conversely blood flowing towards the transducer is shown in red. Moving to the apex of the heart Paul uses a different Doppler technique. The frequency shift depends on the blood flow velocity, so a plot of frequency shift against time indicates flow velocity as a function of time, and this is plotted in yellow. Because the frequency shift is in the audible range it can be sent to a loudspeaker and a trained operator can use the sound for diagnostic purposes.What advantages of ultrasound imaging are mentioned?In addition to B-mode imaging, which other forms of ultrasound imaging are shown?Ultrasound imaging is quick, cost effective, has virtually no known hazards and is acceptable to most patients.M-mode, colour flow Doppler and flow velocity/time image.We will now look in a little more detail at the ultrasound transducer. Design of transducers is complex but they rely on the piezo-electric effect. When a voltage is applied across piezo-electric materials they change shape. If an a.c. voltage is used then the crystal vibrates at the same frequency and sound is produced.The process also works in reverse – sound incident on the crystal gives rise to an a.c. voltage. Thus the same crystal can be used to transmit and receive.Each transducer will be designed for use at a particular frequency. In general a higher frequency gives better resolution but lower penetration. So for large patients, or deep structures, the frequency will have to be low – perhaps 3 MHz; for small structures, such as the eye, frequencies can be much higher (e.g. 12 MHz). Some transducers are designed to transmit at one frequency and receive at another, allowing detection of the second harmonic reflections.Transducers can be designed for use externally or via body orifices such as the vagina (for uterine imaging), or the oesophagus (for heart imaging).6 Radionuclide imaging6.1 IntroductionRadionuclide imaging is a very valuable way of examining the function of an organ, as opposed to the more structural images obtained by other methods such as X-ray and CT.The basic principles of radionuclide imaging are as follows:a radioactive substance, usually combined with a biologically active compound, is injected into the patient;this targets a particular organ or tissue type;the radiation emitted is detected and used to form an image of, or the function of, that organ.The most common radioactive substances used emit gamma rays (usually in the energy range 100–300 keV). More than 95 per cent of all gamma camera imaging techniques use technetium-99m (Tc-99m). This radionuclide is particularly useful because:it produces only gamma rays (by isomeric transition);the gamma rays have an appropriate energy of 140 keV;the physical half-life of Tc-99m is about 6 hours, so injection and imaging can take place in a reasonable period of time but the patient does not remain radioactive for a long period;it can readily be combined with biologically active substances to form a variety of radiopharmaceuticals.Tc-99m is produced by the beta decay of molybdenum-99 by the reaction:6.2 Producing the radioactive substance (elution)In the radiopharmacy Tc-99m is produced in a generator.Mo-99, a product of the fission of uranium, is isolated from a nuclear reactor and absorbed on to an alumina column in the generator. When a saline solution is passed over the column, ion exchange results in the production of sodium pertechnetate. This can then be chemically manipulated to form a variety of compounds. The removal of the technetium by the passage of saline is known as elution.Conveniently, the optimum interval between elutions is 24 hours. As the half-life of Mo-99 is 66 hours the generator itself can be used for approximately one week. It is then returned to the supplier and replaced with a new one.Activity 12Click on the video clip to watch the elution process. Why do you think protective clothing needs to be worn?Click to view part 1 of the clip about radionuclide imaging [2 minutes 26 seconds]
Radionuclide Introduction
Clean Room The first step in radionuclide imaging is the preparation of the radiopharmaceutical. This must be carried out under stringent conditions of sterility. There are also strict guidelines to protect staff members from ionizing radiation. The radionuclide used for most imaging techniques is Technetium-99m. This is extracted from a generator on a daily basis by passing saline solution through the generator – a process known as elution. Inside the generator an ion exchange process results in the production of a solution of sodium pertechnetate. Having drawn off the sodium pertechnetate it is important to measure and record the activity of the eluate. In order to image a particular organ, the technetium must be combined with a substance that targets that organ. To do this the sodium pertechnetate must be chemically reacted with an appropriate compound. Here the technetium needs to be combined with macro-aggregated albumen in order to target the lungs. To do this the appropriate quantity of pertechnetate is diluted with saline and then added to the Pulmocis powder and shaken. This radiopharmaceutical will be ready for injection into the patient after fifteen minutes. Injection Room The patient’s identity must be checked by two members of staff before the injection is given. They must also check the radiopharmaceutical is the correct one for the procedure being carried out. A lead covered syringe offers protection to the personnel performing the injection. The time between injection of the radiopharmaceutical and imaging depends on the specific procedure. In some cases imaging is carried out immediately, in others there can be a delay of several hours.The pharmaceutical will be injected directly into the patient, so it must be produced in completely sterile conditions. The suit and mask are worn to prevent germs from the person reaching the eluate.6.2.1 Injecting the radioactive substanceThe injection may be given immediately before the imaging process, or there may, for certain procedures, be a delay of several hours.The patient's details and the dose being administered are carefully checked by two people before the injection is given. A lead-screened syringe is used to protect the staff from unnecessary radiation dose (see Figure 14).6.3 How a gamma camera worksActivity 13Before we look at a patient being imaged and some of the images which can be obtained using this technique, we will look in a bit more detail at how a gamma camera works. Watch the following video clip and note down the main components of the camera as you watch the clip.Click to view part 2 of the clip about radionuclide imaging [1 minute 16 seconds]
Positron emission tomography (PET)
NarratorPositron emission tomography (PET) is different from other gamma camera techniques. In PET the radionuclides used produce positrons. When an emitted positron collide with an electron in, say tissue, two gamma rays are produced which travel in exactly opposite directions. These gamma rays have a much higher energy than those normally used in gamma imaging, but if they can be detected, tomographic reconstruction methods can used to produce an image. Dedicated PET scanners have been around for several years, but most hospitals don’t have access to one. Recently dual headed gamma cameras with PET capability have been produced, and one of these is in use here. In order to be able to detect the 511keV gamma rays produced in PET, the camera heads on this scanner contain thicker sodium iodide crystals than are normally used for gamma imaging. Since the two gamma rays produced are emitted in opposite directions, the camera heads must be moved so that they are always opposite one another. Another difference is that since we are looking for co-incident opposite events, the collimators are not used; this has the added advantage that sensitivity is increased. This patient is having a brain scan. The radiophamaceutical used for this is a glucose analogue containing fluorine-18 which has a half life of about two hours. This substance is particularly useful in demonstrating metabolism. Scanners like this promise to make PET scanning, which has a wide range of potential uses, far more widely available.The main components of a gamma camera are:the sodium iodide crystal;the collimator; andthe photomultiplier tubes. Figure 15 shows the components of a gamma camera. Further detail on these components can be found below.6.3.1 CollimatorWithout a collimator, gamma rays from all directions would be collected by the crystal and no useful image could be obtained. Gamma rays cannot be focused by a lens but a collimator consisting of a series of holes in a lead plate can be used to select the direction of the rays falling on the crystal. Most collimators in use today are parallel hole collimators. A parallel hole collimator is shown schematically in Figure 16.The resolution and sensitivity of a collimator depend on a number of factors including:hole size (h);the thickness of the septa (s), the lead between the holes;the length of the holes (l);the energy of the gamma rays.Different collimators are selected for different procedures – e.g. Low Energy High Resolution.6.3.2 CrystalAlmost all modern gamma cameras use (thallium-doped) sodium iodide (NaI) as the scintillation crystal. A gamma photon interacts with the crystal to produce many photons of visible light.Sodium iodide is hygroscopic so cannot be left exposed to the air. The front surface is coated with a low atomic number metal that allows the gamma photons to pass through. The rear surface is covered with a transparent coating so that the visible photons can pass through to the photomultiplier tubes.6.3.3 Photomultiplier tubes and detection circuitryThe visible photons are collected by an array of photomultiplier tubes behind the crystal. These convert each visible photon to an electron and then multiply the number of electrons sufficiently to give a voltage pulse. Because the number of visible photons is proportional to the energy of the incoming gamma ray, the height of the pulse depends on this energy. This gives a method of counting the numbers of gamma photons at different energies that reach the crystal.A resistive network connected to the photomultiplier tubes gives information on the spatial position (x,y) of each incoming gamma ray. The so-called ‘z coordinate’ corresponds to the pulse height and therefore the energy of the gamma ray.6.4 Taking the imageActivity 14Now watch this video clip of a patients lungs being imaged, called a VQ (ventilation quotient) scan. What are the two different types of acquisitions used called? What radioactive substance is used for each acquisition, and why?Click to view clip about planar scans [2 minutes 21 seconds]
Planar scans
NarratorThis patient has been admitted to hospital with a suspected pulmonary embolism. To investigate this, a VQ scan is going to be carried out. This examines both the blood supply, or perfusion, to lungs and the ventilation of the lungs. Two radionuclide’s, which emit gamma radiation at different energies, are used in this procedure. The patient has already received an injection of technesium combined with MAA. The particles of MAA tend to lodge in the very small capillaries of the lungs. This generator contains Rubidium-81, which decays to produce Krypton-81m. Air is passed through the generator and carries the Krypton to the patient’s lungs. The half life of Krypton-81 is 13 seconds, so the gas expired poses little hazard to the staff. (during capture) As the radionuclides decay, the detected gamma rays gradually build to form a useful image. The image on the left shows the technesium decay, and therefore represents the perfusion in the lungs. The image on the right is formed by the gamma rays emitted by the Krypton and shows ventilation. A full VQ examination is completed by acquiring images from several different angles. It is possible to carry out gamma camera imaging by recording data from a large number of angles. These can be processed to produce tomographic images. This is known as SPECT.The two different scans mentioned were a perfusion and ventilation scan, to look at the blood supply and air supply to the lungs, respectively. In order to do this simultaneously two different radionuclides that have different gamma ray energies are used. This is a valuable diagnostic test for a pulmonary embolism (a blood clot in the lungs).To image the perfusion the patient is given an injection of technetium-99m (gamma energy 140 keV, half-life 6 hours) combined with macro-aggregated albumin (MAA). The ‘large’ particles of MAA tend to lodge in the very small capillaries of the lungs and therefore give a good indication of where there is, or is not, a good blood supply.The ventilation can be imaged by asking the patient to breathe a mixture of air and krypton-81. This is produced from a generator containing the parent radionuclide, rubidium-81. As the patient is breathing the krypton-81, it shows which areas of the lungs the gas reaches, and therefore, which areas are ventilated. Another widely used procedure is a dynamic renogram. In this case the radionuclide imaged is one that is taken up and excreted by the kidneys. Successive images are collected over a period of time (e.g. every 15 seconds for 20 minutes) and can then be analysed to compare the function of the kidneys.Activity 15Now watch this final video clip on radionuclide images, on the single photon emission computed tomography (SPECT) imaging technique. The sequence shows cardiac (heart) images being obtained. How does SPECT produce slices through the body?Click to view clip about SPECT [3 minutes 23 seconds]
SPECT
This patient is being prepared to undergo a SPECT procedure to look at myocardial perfusion that is blood supply to the heart muscle. It is usual to scan the patient to look at the perfusion when the heart has been stressed, and also when the heart is rested. There can be several days between these two image acquisitions. Stressing of the heart is achieved in two ways. First by a pharmaceutical which has a similar effect on the heart to taking exercise, and also by squeezing a dumbbell. The heart needs to be monitored carefully during this period, so an ECG is recorded. Once the heart has been stressed, the radiopharmaceutical is injected. In this case, the pharmaceutical being used is technetium labelled tetrafosmim. In order to obtain tomographic images, two gamma cameras must be rotated around the patient, as close as possible to the heart. The patient’s arms must be held out of the way above his head. The cameras are now trained to move around the patient as closely as possible without touching him. The images are reconstructed from the data, using the same back projection techniques that are used in CT scanning. They are shown as slices through the heart in different directions, with the white part of the image corresponding to high take up of the radiopharmaceutical and blue representing low take up.Data is collected from a large number of directions by rotating one or more camera heads around the body. Then, tomographic images can be reconstructed, by filtered back projection, using a similar process to CT. The speed of rotation is much slower than that used in CT – the whole process taking at least 10 minutes.Images of a normally perfused heart muscle (myocardium), and a heart with an inferior and lateral wall defect are shown below.Activity 16Take a moment to consider what you think the advantages and disadvantages of radionuclide imaging would be compared with ultrasound, MRI, CT and X-rays.The advantages of radionuclide scans are:demonstration of functional information that often cannot be obtained in other ways;wide variety of organs can be imaged;tomographic and 3-D images available (SPECT).The disadvantages are:poor resolution;radiation dose to the patient;slow and labour intensive;specialised radiopharmacy and scanners are not readily available at all hospitals.7 ConclusionActivity 17Perhaps you are asking yourself why there are so many different imaging modalities. Is there not one that will do everything that is required? The answer, at the moment, is ‘No’. With most of the imaging techniques, we considered their advantages and disadvantages. Watch the following video clip, and note down what items the clinician should be considering when deciding which imaging technique to use.Click to view concluding clip [2 minutes 14 seconds]