Basics of computed tomography

Maxine Murray

Computed tomography was introduced into clinical practice in Britain in the early 1970s, and since then there has been a meteoric rise in the number of scanners throughout the country. Scanners were developed to study the brain in cross section, but other applications soon became apparent and today computed tomography of the chest, abdomen, and pelvis is commonplace.

Advantages of computed tomography

Conventional radiographs depict a three dimensional object as a two dimensional image. Their main limitation is that overlying tissues are superimposed on the image. Computed tomography overcomes this problem by scanning thin slices of the body with a narrow x ray beam which rotates around the body, producing an image of each slice as a cross section of the body and showing each of the tissues in a 10 mm slice. Another limitation of the conventional radiograph is its inability to distinguish between two tissues with similar density, such as soft tissue and fluid. Computed tomography can differentiate between tissues of similar density because of the narrow x ray beam and the use of "windowing" (see below).

How it works

The information acquired by computed tomography is stored on computer as digital raw data and an image can be displayed on a video monitor or printed on to x ray film. The image is made up of a matrix of thousands of tiny squares or pixels (65000 pixels in a conventional image). Each pixel has a computed tomography number (measured in Hounsfield units) attributed to it. The computed tomography number is a measure of how much of the initial x ray beam is absorbed by the tissues at each point in the body. This varies according to the density of the tissues. The denser the tissue is the higher the computed tomography number, ranging from 1000 HU (air) to 1000 HU (bone) (fig 1).

Fig 1 - Hounsfield numbers of various tissues

In an ideal world the image would be displayed with a different shade of grey for every different computed tomography number. However, the human eye is unable to distinguish between 2000 shades of grey so a system of windowing is used. The radiographer selects the range of computed tomography numbers to be displayed and all the numbers within this range are spread over the available grey scale. By convention, high computed tomography numbers are displayed as white and low as black.

To image an area of the body in which many of the tissues have a similar density - for example, the mediastinum or abdomen - a narrow range of computed tomography numbers is selected. These can be spread out over the available grey scale so that two tissues with only a little difference in density will be ascribed separate shades and can therefore be differentiated. For example, a window width of 500 is often used for imaging the mediastinum, with a window level of 39. The level refers to the Hounsfield unit at the centre of the selected window. Thus, in this case, all pixels within the range - 211 to 289 will be displayed. Most of the lungs (largely air) will have computed tomography numbers below - 211 and will therefore appear completely black on the final image (fig 2a).

If you want to see all the lungs you need a much wider window (fig 2b). The value of windowing is that the raw data from a single scan can be displayed in different ways to give useful images of a wide range of tissues.

Fig 2 - Computed tomogram of the chest at the level of the hila.

(a) imaging on mediastinal windows (level 39, width 500) gives good mediastinal detail but the lungs are completely black.

(b) imaging on lung windows (level 2775, width 850) gives good lung detail but the mediastinum is completely white

Drawbacks

The benefits of computed tomography don't come without a price. Firstly, computed tomography is considerably more expensive than conventional radiography. As a rough guide, a chest x ray might cost £10-£25 whereas a computed tomogram of the chest could cost £50 to £150. Secondly, the x ray dose from computed tomography is much larger than for conventional radiography. During computed tomography of the chest the patient receives the equivalent dose to that from 400 chest radiographs. Most radiology departments therefore insist that all requests for computed tomography are vetted by a radiologist.

Another factor to bear in mind when requesting computed tomography is patient preparation. For scans of the abdomen patients often have to drink several large cupfuls of contrast agent about an hour before scanning. The contrast agent has a high density so in the scan the entire bowel is highlighted in white and readily distinguished from possible intra-abdominal masses. For scans of the pelvis patients are often given rectal contrast, and female patients may be asked to insert a tampon in the vagina, particularly in cases of pelvic malignancy. Intravenous contrast may also be given to differentiate vessels from enlarged lymph nodes.

Because of the large x ray dose from computed tomography it has been superseded by magnetic resonance imaging in many instances. You should always consider ultrasound or magnetic resonance imaging as an alternative to computed tomography in children as they are particularly sensitive to radiation. Nevertheless, computed tomography scanners are more widely available than magnetic resonance imaging scanners and will continue to provide an important means of investigation for the foreseeable future, particularly in the investigation of head injury and staging of tumours.

Indications for computed tomography

I can cover only the most common indications for computed tomography in this article, but if you want more information the Royal College of Radiologists' guidelines detail the relative merits of different radiological investigations.(1)

HEAD

Computed tomography is the preferred investigation for patients with head injuries. This is because computed tomography is more widely available than magnetic resonance imaging (particularly out of hours) and because it is more difficult to monitor semiconscious patients during magnetic resonance imaging. Much of the monitoring and resuscitation equipment is ferromagnetic and cannot be allowed into the room containing the magnet. The computed tomogram may show a skull fracture but, more importantly, it will show underlying brain damage, intracerebral haemorrhage, or evidence of raised intracranial pressure. Computed tomography is also used to exclude subarachnoid haemorrhage in patients with acute onset of headache and is a simple method of excluding haemorrhage after a stroke in patients in whom anticoagulation is contraindicated.

Magnetic resonance imaging is generally better for investigating space occupying lesions, but computed tomography will show calcification in lesions such as meningiomas or gliomas, which may help in the differential diagnosis. Computed tomography is also useful for showing destruction or expansion of bone in diseases of the sinuses and orbits since the conical bone gives no signal on magnetic resonance imaging.

CHEST

Computed tomography of the chest has been revolutionised by two new developments in scanning. These are the ability to scan quickly with the new spiral scanners and the ability to acquire very thin slices (high resolution computed tomography).

Until recently, the chest was scanned in slices 1 cm thick at 1 cm intervals. For each slice patients had to hold their breath for about five seconds, and the entire scan took about 10 minutes. With spiral computed tomography scanners the entire chest can be scanned during a single breath. This also means that only a single bolus of intravenous contrast is needed and small intrapulmonary abnormalities such as a pulmonary metastasis will not be missed because of variations in inspiratory effort.

Contrast enhanced computed tomography of the chest is usually used for staging lung cancer and for assessing masses in the mediastinum. Another useful application is in the assessment of aortic dissection as the dissection flap can be clearly seen (fig 3).




Fig 3 - Spiral computed tomography of the chest after intravenous contrast. The ascending aorta is dilated and a dissection flap is seen within the lumen of the ascending (long arrow), and descending aorta (short arrow)






High resolution computed tomography is used to image the lung parenchyma. The thin slices (1.5 mm) improve the resolution, making it possible to diagnose and assess the extent of diseases such as lung fibrosis, sarcoid, and bronchiectasis. In certain conditions the appearances are so characteristic that open lung biopsy can be avoided. It is now possible to predict which patients with lung fibrosis should improve with steroids on the basis of the computed tomography findings.

ABDOMEN

Computed tomography is used in the staging of tumours, both to outline the primary tumour and to assess spread (for example, to the liver, adrenal glands, peritoneum, or lymph nodes). It is also used to determine the response to chemotherapy. In the future, magnetic resonance imaging will probably be used for some of these roles.

Computed tomography can be valuable in the preoperative assessment of complex masses and in evaluating postoperative complications, such as focal fluid collections or anastomotic leaks. It also has a role in evaluating the complications of pancreatitis, such as pseudo tumours and pancreatic abscess or necrosis, and will often provide a more complete evaluation in patients in whom gaseous filling of the bowel (for example, due to ileus) precludes ultrasound assessment. Like ultrasound, computed tomography can be used to guide biopsies and drainage procedures.

TRAUMA

As well as its role in head trauma, computed tomography is particularly useful in showing complex fractures of the pelvis. Pelvic fractures are notoriously difficult to assess in plain radiographs because of superimposition of bone. This problem is removed with computed tomography, and the extent of fractures and site of any free bony fragments, particularly within the joint, can be identified before surgery. Computed tomography will also show the extent of any pelvic haematoma. This is particularly difficult to detect in plain radiographs and is a common cause of morbidity and even death in patients with pelvic trauma.

Computed tomography may also be helpful in excluding cervical spine fractures in trauma patients in whom the plain radiographs are inadequate because positioning was difficult. However, remember that if you request computed tomography of the neck you must be specific. Thin sections are required to exclude a fracture, and 4050 scans would be required for the entire neck. Contrast enhanced computed tomography is used to investigate possible splenic, renal, or hepatic rupture and will often show free air or free fluid.