(CTA) A computed tomographic angiography or computerized tomography angiogram is a diagnostic imaging test that combines conventional CT technique with that of traditional angiography to create images of the blood vessels in the body - from brain vessels to arteries of the lungs, kidneys, arms and legs.
High resolution CT scans with thin slices and intravenous injection of iodinated contrast material provide detailed images of vascular anatomy and the adjacent bony structures. CTA requires rapid scanning as the imaging data are typically acquired during the first pass of a bolus of contrast medium. The selection of acquisition timing is important to optimize the contrast enhancement, which is dependent on contrast injection methods, imaging techniques and patient variations in weight, age and health. CT angiography is less invasive compared to conventional angiography and the data can be rendered in three dimensions.

CTA techniques are commonly used to:
See also Cardiovascular Imaging, Magnetic Resonance Angiography MRA, Coronary Angiogram, Computed Tomography Dose Index and Computed or Computerized Axial Tomography.

(CT or CAT scan) Computed tomography is a diagnostic imaging technique, previously also known as computerized axial tomography (CAT), computer-assisted tomography (CAT), computerized tomographic imaging, and reconstructive tomography (RT).
A CT scan is based on the measurement of the amount of energy that a tissue absorbs as a beam of radiation passes through it from a source to a detector. As the patient table moves through the CT scanner, the CT tube rotates within the circular opening and the set of x-ray detectors rotate in synchrony. The narrow, fan-shaped x-ray beam has widths ranging from 1 to 20 mm. The large number of accurate measurements with precisely controlled geometry is transformed by mathematical procedures to image data. Corresponding to CT slices of a certain thickness, a series of two-dimensional cross-sectional images is created.
A CT is acquired in the axial plane, while coronal and sagittal images can be rendered by computer reconstruction. Although a conventional radiography provides higher resolution for bone x-rays, CT can generate much more detailed images of the soft tissues. Contrast agents are often used for enhanced delineation of anatomy and allow additional 3D reconstructions of arteries and veins.
CT scans use a relatively high amount of ionizing radiation compared to conventional x-ray imaging procedures. Due to widespread use of CT imaging in medicine, the exposure to radiation from CT scans is an important issue. To put this into perspective, the FDA considers the risk of absorbed x-rays from CT scans to be very small. Even so, the FDA recommends avoiding unnecessary exposure to radiation during diagnostic imaging procedures, especially for children.
CT is also used in other than medical fields, such as nondestructive testing of materials including rock, bone, ceramic, metal and soft tissue.

See also Contrast Enhanced Computed Tomography.

(CTE) Computed tomography enterography is an imaging procedure to evaluate diseases affecting the mucosa and bowel wall of the small intestine. CTE uses oral contrast agents to improve bowel wall visualization. Several studies established that small bowel distention using negative oral contrast agent increases diagnostic performance of some abdomen CT studies.
The multi-detector row CT (MDCT) improves temporal and spatial resolution and 3D imaging processes offer a full examination of the small bowel with surrounding structures, depicting the small bowel inflammation associated with Crohn's disease by displaying mural hyperenhancement, stratification, and thickening.
CT enterography versus capsule endoscopy provides a non invasive study with comparable sensitivity, high specificity and overall accuracy.

See also Colonoscopy and Virtual Colonoscopy.

Contrast is the relative difference of intensities in two adjacent regions of an image. When referring to computed tomography (CT), contrast is defined as a difference in Hounsfield units between structures. The measurement of contrast resolution in CT imaging involves determining how easy it is to differentiate tissues whose CT density is similar to that of their surroundings. An image lacks contrast when there are no sharp differences between black and white. Brightness refers to the overall lightness or darkness of an image.
The contrast between air, soft tissue, and bones in x-ray and CT images is based on their different absorption of x-rays. Differences in tissue density, thickness and changes of the x-ray spectrum have consequences for image contrast, image noise as well as patient dose.
Optimized tube current, collimation, pitch and image reconstruction improves the contrast. Higher image contrast is produced by increased slice thickness, smaller matrix, and large field of view which results in large voxel size; high mAs to reduce noise; low pass filter.

See also Contrast Enhanced Computed Tomography.

X-rays contain a range of energies (polychromatic photons), the higher energies pass through the patient, the lower energies are absorbed or scattered by the body. Ideally, the x-ray beam should be monochromatic or composed of photons having the same energy. Strong filtration of the beam results in more uniformity. The more uniform the beam, the more accurate the attenuation values or CT numbers are for the scanned anatomical region.
There are two types of filtration utilized in CT:
Inherent tube filtration and filters made of aluminum or Teflon are utilized to shape the beam intensity by filtering out the undesirable x-rays with low energy. Filtration of the x-ray beam is usually done by the manufacturer prior to installation. The half value layer provides information about the energy characteristics of the x-ray beam. Too much filtration produces a loss of contrast in the x-ray image.
A mathematical filter such as a bone or soft tissue algorithm is included into the CT reconstruction process to enhance resolution of a particular anatomical region of interest.