Imaging in medicine
Imaging in medicine

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Imaging in medicine

2 X-ray imaging

2.1 Introduction

X-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.

Figure 2
Figure 2: Planar X-ray of ankle and extended foot

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]

Download this video clip.Video player: Planar x-ray images
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Transcript: Planar x-ray images

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.
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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.

Figure 3
Figure 3: X-ray unit used to image Alan's skull

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