Ch.6 key words

For medical imaging

ALARA

Acronym for “as low as reasonably achievable” which refers to minimizing exposure to patient, self, and others

Anode

The positive side of the x-ray tube that becomes a target for the source of electrons (the cathode).

Aperture Size

The diameter of the hole on a scanner in which the patient is being positioned,

also called the bore

Artifact

Unwanted image abnormalities, such as blurring, star, and beam-hardening artifacts that can be caused by patient motion, anatomy, design of the scanner, or system failure.

Attenuation

Removal of photons and electrons from a radiation beam by scatter or absorption as it travels through a medium.

Backscatter

The reflection of radiation back towards its source after interacting with a material

Bremsstrahlung

German term for “braking” radiation, which is a common reaction between an

electron and target material inside the x-ray tube.

Cathode

One of the electrodes found in the x-ray tube that represents the negative side of the tube.

Characteristic Radiation

Within the x-ray tube, radiation that is created by the direct

interaction of cathode electrons with inner-shell electrons of the target material.

Compton Scattering

Produced when an x-ray photon interacts with an outer-shell orbital

electron with sufficient energy to eject it from orbit and alter its own path.

Cone-beam Computed Tomography

A form of CT using wider x-ray beam angles for scanning, thus yielding the ability to scan a much larger volume within one rotation. It differs from fan beam CT in that the CT detector is an area detector. At certain degree intervals during the rotation of the gantry, single projection images are acquired. Net result is a three-dimensional reconstruction data set, which can project images in three orthogonal planes (axial, sagittal, and coronal)

Contrast

The tonal range of densities from black to white or the number of shades of gray (in Radiation Therapy focus on the shades of gray).

Computed Radiography

Digital radiographic imaging

Density

Degree of darkening on the image (In Radiation Therapy focus on the blacks and whites).

Detectors

Solid-state detectors in a computed tomography (CT) scanner are designed to

convert radiation to light.

Detector Array

A key component of the CT scanner which senses the x-ray radiation which is partially attenuated by tissues in the patient’s body and converts them into digital signals.

Filament

Small coil of wire made of thoriated tungsten, which has an extremely high melting point (3380°C)

Differential Absorption

The human body consists of varying quantities of air, fat, water, muscle, and bone, each with its own absorption properties (differential absorption) or absorption coefficient. Denser structures and those of higher-than-average atomic numbers appear as lighter areas on the image because of their higher rates of attenuation. Air, as a result of its extremely low density and fat and of its relatively low atomic number, appears as dark areas, whereas bone, which is dense and has a high atomic number (Z), appears as a light shadow. These differential absorption characteristics of anatomy is the basis of radiographic imaging.

Film/Image Density

The degree of darkening (black and whites) apparent on a film or other form of imaging device.

Flat-Panel Detectors

Imaging device that uses amorphous silicon and solid-state integrated circuit technology to produce images with quality far superior to conventional film-screen combinations.

Gadolinium

Non-iodine-based intravenous contrast agent used for computed tomography and magnetic resonance imaging scans. Gadolinium helps differentiate between edema and a tumor.

Image Contrast

The element of imaging that provides visual evidence of differential absorption rates of various body tissues. Image contrast has been described as the tonal ranges of densities from black to white or the number of shades of gray in the image

Image Fusion

Process of combining images from different modalities with a computed tomography (CT) image. Properly fused images combine the enhanced imaging capabilities of magnetic resonance imaging (MRI) and/or positron emission tomography (PET) with the spatial accuracy of CT. Anatomy can be defined on any of the image data sets and can then be displayed on the CT image.

Interfraction Uncertainties

Variations in setup that exist between each setup (e.g., inaccuracies in positioning, variations in machine characteristics, patient differences)

Inverse Square Law

Mathematical relationship that describes the change in beam intensity as the distance from the source changes, where intensity is inversely proportional to the distance squared.

Ionization

This occurs when electrons collide with tungsten orbital electrons that have

sufficient energy to overcome their binding energy and eject them from orbit.

Onboard Imaging

The software and hardware requirements of a linear accelerator that

provide the user daily image verification capabilities

Oncogene

Gene that regulates the development and growth of cancerous tissues.

Magnetic Resonance Imaging

Diagnostic, nonionizing means of visualizing internal anatomy through noninvasive means. Imaging is based on the magnetic properties of the hydrogen nuclei.

Magnetic Resonance Imaging Safety Zones

The zones indicated by the American college of Radiology to limit access to the MRI for safety reasons

Zone 1:

Area accessible to the general public.

Zone 2:

More strictly supervised area. This is usually the area where patients are

screened.

Zone 3:

Area restricted to MR personnel; located outside the treatment room.

Zone 4:

MR scanner room.

Noise

This is the grainy appearance of an image

Nuclear Medicine Imaging

Branch of medicine that uses radioisotopes in the diagnosis and

treatment of disease

Pixels

Small, discrete elements that make up an image.

Positron Emission Tomography/ Positron Emission Tomography-Computed Tomography

Nuclear medicine procedure that has become useful in oncology to examine the biochemical or physiological (functional) aspects of a tumor. This diagnostic tool is useful in determining the physiology of the organ in question. Beneficial diagnostic tool that may be useful in determining differences between necrosis and malignancy, which are associated with areas of high metabolism.

Resolution

The relationship between the number of pixels or voxels. The greater the number of pixels in an image, the greater the resolution.

Scatter

Produced when an x-ray photon interacts with an outer-shell orbital electron with sufficient energy to eject it from orbit and alter its own path

Spatial Resolution

AKA high contrast, is specified in terms of line pairs that are equal-sized

black and white bars.

Ultrasound

Imaging that involves high frequency sound waves sent into the body. Echo waves as the sound bounces back from various tissue interfaces are recorded.

Voxels

Short for volume elements, is a discrete unit of volume that makes up a three-

dimensional object. It is the 3D equivalent of a pixel in 2D imaging.

Window Level

Represents the central Hounsfield unit of all the computed tomography (CT)

numbers within the window width.