spiral order matrix ibookmark suggest edit

Spiral order matrix ibookmark suggest edit skin#
Spiral order matrix ibookmark suggest edit series#

A different approach was needed: one based on actual measurements inside patient-representative phantoms.Ī first step is to understand the nature of the radiation inside the phantom and to investigate the shape of the radiation beam in the z-direction. Though commonly used for high-energy beams in radiation therapy, such dose–depth data are too sensitive to differences in x-ray spectra and tissue attenuation to be of value in CT. By comparison, point B inside a symmetric cylindric patient receives the same amount of radiation from all tube locations during the rotation.Įarly attempts were made to estimate CT doses by using measurements of dose versus depth summed over all x-ray tube angles and positions. For each tube location, different amounts of radiation reach point A, depending on the depth of A (i.e., how much tissue must be penetrated) and the amount of internal scatter.

However, point A also receives some (albeit less) radiation when the tube is at every point during its rotation, even when the tube is on the opposite side of the patient.

During a 360° rotation, point A receives much radiation when the x-ray tube is above the patient (anterior entry surface). For example, consider 2 points, A and B, which are, respectively, near the anterior surface and at the center of a cylindric patient. It is no longer clear where (on the surface of or inside the patient) the maximum dose occurs, nor is calculation of the dose at any point in or on the patient straightforward. Such measurements are usually expressed as exposure per milliampere second (mR/mAs, or more properly today, mGy air kerma/mAs).ĭuring scanning of a CT slice, on the other hand, the x-ray beam enters from all directions at some point during the scan.

Spiral order matrix ibookmark suggest edit skin#

The calculation of entrance skin exposure is straightforward, using measurements of in-air ionization chamber exposure at several x-ray tube kilovoltages covering the clinical range. Therefore, it has been reasonable to use radiation exposure to the entry surface (referred to as entrance skin exposure) as an indicator of radiation risk when different techniques, receptors, and x-ray machines are compared. In those types of examinations, the radiation dose to the patient maximizes where the x-ray beam enters the surface of the skin.

Spiral order matrix ibookmark suggest edit series#

This article, the second in a series of continuing education articles on the principles of CT, focuses first on CT dosimetry and radiation dose and then on CT image quality.īefore CT came into use, planar radiography and fluoroscopy comprised all non–nuclear medicine uses of ionizing radiation in imaging. As a result, diagnostically acceptable image quality at acceptable doses of radiation requires appropriately designed clinical protocols, including appropriate kilovolt peaks, amperages, slice thicknesses, and reconstruction filters. Spatial resolution is ultimately limited by sampling, but both image noise and resolution are strongly affected by the reconstruction filter. A strength of CT is its ability to visualize structures of low contrast in a subject, a task that is limited primarily by noise and is therefore closely associated with radiation dose: The higher the dose contributing to the image, the less apparent is image noise and the easier it is to perceive low-contrast structures. CT image quality, as in most imaging, is described in terms of contrast, spatial resolution, image noise, and artifacts. A weighted average of the CT dose index measured at the center and periphery of dose phantoms provides a convenient single-number estimate of patient dose for a procedure, and this value (or a related indicator that includes the scanned length) is often displayed on the operator's console. The most commonly used dose descriptor is CT dose index, which represents the dose to a location (e.g., depth) in a scanned volume from a complete series of slices. This article discusses CT radiation dose, the measurement of CT dose, and CT image quality.