(LCR) The low contrast resolution describes the ability to discriminate between tissues with slightly differences in attenuation properties. The LCR depends on the stochastic noise.
The low contrast resolution is usually expressed as the minimum detectable size of an image structure, for a fixed percentage difference in contrast relative to the adjacent background.
A strength of computed tomography (CT) is its ability to visualize structures of low contrast in an object, a task that is limited by noise and is closely associated with the radiation dose. For example, a reduction of the dose at constant spatial resolution affects the visibility of structures with low contrast (e.g. vessels in the liver), due to increased noise. The visibility of these low contrast structures can partly be improved by decreasing the spatial resolution, while keeping the dose constant.

See also CT Number, Image Quality and Low Contrast Detectability.

(MPI) The myocardial perfusion scan is the most common nuclear medicine procedure in cardiac imaging and allows assessing the blood-flow patterns to the heart muscles. The comparison of the radiopharmaceutical distribution after stress and at rest provides information on myocardial viability and cardiac perfusion abnormalities. ECG-gated myocardial perfusion imaging allows the assessment of global and regional myocardial function such as wall motion abnormalities.
The diagnostic accuracy of myocardial perfusion scintigraphy (also abbreviated MPS) allows reliable risk stratification and guides the selection of patients for further interventions, such as revascularization. MPI also has particular advantages over alternative techniques in the management of a number of patient subgroups, including women, the elderly, and those with diabetes. The use of this type of cardiac scintigraphy is associated with greater cost effectiveness of treatment, in terms of life-years saved, particularly in these special patient groups.
Myocardial perfusion scintigrams are acquired with a gamma camera. Single photon emission computed tomography (SPECT) is preferred over planar imaging because of the three dimensional nature of the images and their superior contrast resolution.
Common MPI radiopharmaceuticals, approved by the U.S. Food and Drug Administration (FDA) include: Tl-201 and the Tc-99m-labeled radiopharmaceuticals, such as sestamibi, tetrofosmin, and teboroxime for single-photon imaging. Rb-82 is used for positron emission tomography (PET) imaging.

See also Gated Blood Pool Scintigraphy, Myocardial Late Enhancement, Cardiac MRI and Echocardiography.

(PSF) The point spread function describes the response of an imaging system to a point or impulse input. A hypothetical point object will generally have an extended (blurred) image resulting from the imaging process; this is the point spread function characterizing the imaging process. Considering any object as composed of an assembly of point objects, knowledge of the PSF permits the prediction of how the object will be imaged, assuming linearity of the imaging process.

See also Resolution, Phantom and Daily Quality Assurance.

A computed tomography (CT) of the abdomen images the region from the thoracic diaphragm to the pelvic groin. The computed tomography technique uses x-rays to differentiate tissues by their different radiation absorption rates.
Oral contrast material can be given to opacify the bowel before scanning. An i.v. injection of a contrast agent (x-ray dye) improves the visualization of organs like liver, spleen, pancreas and kidneys and provides additional information about the blood supply.
Spiral- or helical CT, including improvements in detector technology support faster image acquisition with higher quality. Advanced CT systems can usually obtain a CT scan of the whole abdomen during a single breath hold. This speed increases the detection of small lesions (caused by differences in breathing on consecutive scans) and is beneficial especially in pediatric, elderly or critically-ill patients.
Changes in patient weight require variations in x-ray tube potential to maintain constant detector energy fluence. An increased x-ray tube potential improves the contrast to noise resolution (CNR).

An abdominal CT is typically used to help diagnose the cause of abdominal pain and diseases such as:
Other indications for CT scanning of the abdomen/pelvis include planning radiation treatments, guide biopsies and other minimally invasive procedures. Advanced techniques include for example 3D CT angiography, multiphasic contrast-enhanced imaging, virtual cystoscopy, virtual colonoscopy, CT urography and CT densitometry.

See also Contrast Enhanced Computed Tomography.

A pixel is a picture element (pix, abbreviation of pictures + element). Tomographic images are composed of several pixels; the pixel size is determined by the used field of view and the number of elements in the display image matrix. The corresponding size of the pixel may be smaller than the actual spatial resolution.
Pixels do not have a fixed size; their diameters are generally measured in micrometers (microns). Although the pixel is not a unit of measurement itself, pixels are often used to measure the resolution (or sharpness) of images. As a hypothetical example, a 600 x 1000 pixel image has 4 times the pixel density and is thus 4 times sharper than a 300 x 500 pixel image, assuming the two images have the same physical size.