Chapter 11 Computed Tomography

Understanding the Basics and Recognizing Normal Anatomy

Introduction to CT

A computed tomographic (CT) image is composed of a matrix of thousands of tiny squares called pixels, each of which is computer-assigned a CT number from −1000 to +1000 measured in something called Hounsfield units (HU), after Sir Godfrey Hounsfield, the man credited with developing the first CT scanner (for which he won the Nobel prize in Medicine in 1979 along with Allan Cormack).

• A tomogram is a slice through the body measured in millimeters that allows for the accurate localization of objects in that section, unlike conventional radiographs that superimpose all of the structures within a given field of view.

• A CT scanner uses x-rays transmitted to multiple detectors connected to a computer, which processes the data though various algorithms to produce images of diagnostic quality.

The CT number will vary according to the density of the tissue scanned and is a measure of how much of the x-ray beam is absorbed by the tissues at each point in the scan. By convention, air is assigned a Hounsfield number of −1000 HU and bone is about 400 to 600 HU (fat is −40 to −100, water is 0, and soft tissue 20 to 100).

CT images are displayed or viewed using a range of Hounsfield numbers preselected to best demonstrate the tissues being studied (for example, from −100 to +300) and anything within that range of CT numbers is displayed over the levels of density in the available gray scale.

This range of densities is called the window or window-width setting, and the number within that range that is arbitrarily chosen to be the center of the gray scale is called the center or the window level.

Denser substances that absorb more x-rays have high CT numbers, are said to demonstrate increased attenuation, and are displayed as whiter densities on CT scans.

• On conventional radiographs, these substances (like metal and calcium) would also appear whiter and be said to have increased density or be more opaque.

Less dense substances that absorb fewer x-rays have low CT numbers, are said to demonstrate decreased attenuation, and are displayed as blacker densities on CT scans.

• On conventional radiographs, these substances (like air and fat) would also appear as blacker densities and be said to have decreased density (or increased lucency).

High-resolution computed tomography (HRCT), or thin-slice CT, is now a potential part of every chest CT, making these studies particularly useful for the imaging and characterization of diffuse parenchymal lung diseases.

Spiral CT (helical CT) uses a continuously rotating x-ray source and detector array combined with constant table movement to permit data acquisition through the chest or abdomen so quickly it can be done while the patient holds a single breath.

• This results in an extremely rapid acquisition of data that has no “gaps” between slices, which, in turn, allows for seamless reconstruction (also called reformatting) of those images in almost any plane. As importantly, the resolution of the reconstructions is equivalent to the clarity of the original scan with most modern scanners.

• Traditionally, CT images were viewed mostly in the axial plane. With volumetric data acquisition and seamless reconstruction, CT scans now provide diagnostically useful images in the coronal and sagittal planes as well as three-dimensional reconstructions that can be viewed at any angle.

Multislice CT (also called multidetector CT) makes use of multiple rows of detectors that allow for images to be reconstructed in a range of slice thicknesses after the scan is completed without reimaging the patient.

• Because of increasingly sophisticated arrays of detectors and acquisition of as many as 256 slices simultaneously, multislice CT scanners permit very fast imaging (head to toe in less than 30 seconds) that has allowed for development of new applications for CT, such as virtual colonoscopy (see Chapter 18) and virtual bronchoscopy, cardiac calcium scoring, and CT coronary angiography (see Chapter 9).

• Such examinations can contain a thousand or more images so that the older convention of filming each image for study on a view box is impractical and such examinations are almost always viewed on computer workstations.

So, you might ask, why not scan everyone at the thinnest possible slice thickness from head to toe?

• The reason is that doing so might expose a patient to an unnecessarily high dose of radiation. The radiation dose delivered by CT studies is dependent on many factors, including the type of equipment, the energy of the x-rays used to produce the images, and the size of the patient. The economic and personal costs of evaluating unexpected and often inconsequential findings may also outweigh any potential benefit from whole-body screening.

• Dose-reducing measures are being employed, including the use of optimized CT settings, reduction in the x-ray energy used, limiting the number of repeat scans, and assuring—through appropriate consultation—that the benefits derived from obtaining the study outweigh any potential risks of the radiation exposure.

Intravenous Contrast in CT Scanning

CT scans can be performed with or without the intravenous administration of iodinated contrast material but, in general, they yield more diagnostic information that is more easily recognizable when intravenous contrast can be used.

• All radiographic contrast agents, in general, are administered to increase the differences in density between two tissues. CT scans done with intravenous contrast are called contrast-enhanced or simply enhanced. Most of the time, the radiologist will choose the scanning parameters to optimize the CT study for the patient’s clinical issues. For example, different rates of contrast administration and timing of the scan will allow diagnostic enhancement of hepatic vessels versus the liver parenchyma.

• While it might sound like a wonderful idea to give everyone contrast for all CT studies, keep in mind that iodinated contrast can have adverse effects and produce serious reactions in susceptible individuals (Box 11-1).

Box 11-1 Contrast Reactions and Renal Failure

Intravenous contrast materials available today are nonionic, low-osmolar solutions containing a high concentration of iodine that circulate through the bloodstream, opacify those tissues and organs with high blood flow, are absorbed by x-ray (and therefore appear “whiter” on images), and are finally excreted in the urine by the kidneys.

In some patients (e.g., those with diabetes, dehydration, multiple myeloma) who have compromised renal function evidenced by creatinine > 1.5, iodinated contrast can produce a nephrotoxic effect resulting in acute tubular necrosis. Though usually reversible, in a small number of patients with underlying renal insufficiency, renal dysfunction may permanently worsen. This effect is dose related.

Iodinated contrast agents can sometimes produce mild side effects, including a feeling of warmth, nausea, vomiting, local irritation at the site of injection, itching, and hives; these side effects usually require no treatment. Occasional idiosyncratic, allergic-like reactions include itching, hives, and laryngeal irritation.

Asthmatics and those with a history of severe allergies or prior reactions to IV contrast have a higher likelihood of contrast reactions (but still very low overall) and may benefit from steroids, benadryl, and cimetidine administered prior and/or after injection. Prior shellfish allergy bears absolutely NO relationship to iodinated contrast reactions.

In about 0.01 to 0.04% of all patients, severe and idiosyncratic reactions to contrast can occur that can produce intense bronchospasm, laryngeal edema, circulatory collapse, and very rarely death (1 in 200,000 to 300,000).

Oral Contrast in CT Scanning

For abdominal and pelvic CT imaging, oral contrast may also be administered to define the bowel, although its use has diminished as the quality of CT images has improved. Oral contrast is usually not employed in chest CT scanning unless there is a question concerning the esophagus.

Orally administered contrast, frequently given in temporally divided doses to allow earlier contrast to reach the colon while later contrast opacifies the stomach, is utilized for most abdominal CT scans except those performed for trauma, the stone search study, and studies specifically directed towards evaluating vascular structures like the aorta.

One of two different types of oral contrast may be used. The most widely used is a dilute solution of barium sulfate, the same contrast agent employed in upper gastrointestinal studies and barium enemas. If there is concern for bowel perforation and the possibility that contrast may exit from the lumen of the bowel, an iodine-based, water-soluble contrast is sometimes used (Gastrografin). Contrast may also be introduced rectally to opacify the colon more quickly than it would take for orally administered contrast to reach the large bowel or through a Foley catheter to quickly opacify the urinary bladder.

You will probably not be required to make the decision of when or if to use contrast because the radiologist will usually tailor the examination to best answer the clinical question being asked, so it is always important to provide as much clinical information as possible when requesting a study.

Table 11-1 summarizes when intravenous and oral contrasts are utilized for particular problems.

By convention, CT scans, like most other radiologic studies, are viewed with the patient’s right on your left and the patient’s left on your right.

• If the patient is scanned in the supine position, as most usually are, the top of each image is anterior and the bottom of each image is posterior.

TABLE 11-1 CT SCANS: WHEN CONTRAST IS USED

IV Contrast Used	IV Contrast Usually Not Used
Chest
CT-pulmonary angiogram (CT-PA) for pulmonary embolism Evaluation of the mediastinum or hila for mass or adenopathy Detect aortic aneurysm or dissection Evaluate blunt or penetrating trauma Characterize pleural disease (metastases, empyema) CT densitometry of pulmonary masses Evaluate the coronary arteries	Evaluation of diffuse infiltrative lung diseases using HRCT Confirmation of the presence of a nodule suspected from conventional radiographs Detect pneumothorax/pneumomediastinum Calcium scoring for the coronary arteries Known allergies to contrast or renal failure
*Abdomen and Pelvis*
Evaluate for the presence of and/or to characterize a mass and to stage or follow up malignancies Trauma Abdominal pain (e.g., appendicitis) Detect aortic aneurysm or dissection	Virtual colonoscopy Search for a ureteral calculus
*When Oral Contrast Is Used*
Most cases of nontraumatic abdominal pain Inflammatory bowel disease Abdominal or pelvic abscess Locate the site of bowel perforation, including fistulae