1 Current imaging techniques
A grainy Polaroid of the child in a mother's womb – an X-ray of a tibia fractured in a traffic accident – a report on a brain scan anxiously awaited. Very few of us have not had some connection with the techniques and practices of medical imaging. Often, these contacts are in periods of personal drama in which the medical images chart our physical status, the management of a condition and, in some cases, our future.
Imaging is a central feature of contemporary medicine. Along with chemically based techniques of analysis, it provides the clinician with ways of extending investigations that go beyond external observation and interview.
The video clip below will introduce the major imaging modalities in current use in hospitals. These are:
X-rays, including computed tomography (CT)
ultrasound
magnetic resonance imaging (MRI)
radionuclide imaging
The view offered in the clip is that of the underlying science, from the physical scientist's perspective, rather than that of the medical clinician.
Activity 1
As you watch the video clip, keep in mind the electro-magnetic spectrum shown in Figure 1 below and try to place the electromagnetic energy of each modality on the spectrum.
Click to view the introduction clip presented by Liz Parvin [7 minutes 50 seconds]
Download this video clip.Video player: Introduction clip presented by Liz Parvin
Transcript: Introduction clip presented by Liz Parvin
Liz Parvin
From 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.
Introduction clip presented by Liz Parvin
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Answer
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.