CT

Anode

positively charged electrode that attracts electrons; in CT this is where the accelerated electrons from the cathode will strike to produce x-rays; large amounts of heat are generated from this process.

Beam Current

stream of electrons traveling from the cathode to the anode; measure in mA.

Beam Hardening Artifacts

Artifacts such as streaks, blurs and shadows which result from the lower energy photons that are preferentially filtered by the intervening tissues being absorbed more than high energy photons as the beam passes through the patient; effect is dependent on the composition of the tissue and the path of the beam thus these artifacts are commonly observed in larger patients.

Bowtie Filter

Pre-patient filter that reduces the dynamic range of the data acquisition system and improves noise.

Bremsstrahlung

x-ray emitted when a high energy electron is slowed down and/or deflected by a positively charged nucleus; major source of x-rays in CT.

Cathode

negatively charged electrode that acts as a source of electrons; in CT this heated filament boils off electrons and accelerates them towards the anode.

Characteristic X-Ray

Monoenergetic x-rays that are produced by electrons dropping to a lower energy shell in the atom. These x-rays may result from ionization of the atom by a process that removes an inner shell electron such as a photoelectric interaction.

Cone Beam

Used in 3rd generation CT machines. Increases the speed of the exam by increasing the area imaged by being able to reach multiple detectors in the fan beam array and multiple rings while constantly moving.

CTDI

Computed tomography dose index. The average dose imparted by a single axial acquisition to a standard 100mm pencil chamber dosimeter inside a PMMA phantom over the width of 14 CT slices.

Dose Length Product (DLP)

Represents the integrated dose in terms of the total scan length. This is an indicator of biologic risk.

Flat Filter

Pre-patient filter that removes low energy x-rays that would otherwise be absorbed by the patient.

Fluence

The total number of photons integrated over time. It is defined by the current and exposure time.

Focal Spot

The area on the anode of an x-ray tube or the target that is struck by electrons and from which the resulting x-rays are emitted.

Geometry

center-to-center spacing of detector elements decreases as the distance from the ring center increases; geometric corrections make the spacing uniform across the field of view.

Helical CT

Continuously rotating x-ray tube and detector.

High Contrast (Spatial) Resolution

The ability of the system to resolve high contrast objects of increasingly smaller size. Can be influenced by pixel size, reconstruction filters, and geometric resolution limits.

Hounsfield Unit

A unit of measured value placed into the pixels of a CT image. The value represents a relative density to water, which has a value of zero on the Hounsfield scale, with densities lower than water having a negative number and densities higher than water having a positive number.

Kernel

The image filter used in the CT reconstruction algorithm to calculate the sharpness of a reconstructed image.

Kilovolt Potential (kVp)

The peak voltage that can be applied between the cathode and anode of an x-ray tube; the potential difference between the cathode and anode.

Low Contrast Resolution

The ability of a system to resolve objects having small difference from background.

Low Dose CT

x-ray beam energy and beam current are set low to minimize radiation exposure to the patient.

Milliampere (mA). A unit of electric current that describes the flow of charge per second. Used to measure the number of photons generated by the x-ray tube in CT imaging.

Partial Volume Effect

When the size of an object begins to approach the resolution limits of an imaging system, resulting in a voxel containing more than one type of tissue.

Pitch

The ratio of table movement per revolution over the collimated slice thickness of one row of a multi-slice CT detector.

Postpatient Collimator

It removes scattered x-rays that would lead to image degradation.

Prepatient Collimator

It reduces the x-ray flux in the z-direction, and in single slice CT scanners it defines the slice thicknes

Scout Scan

An x-ray examination performed with the CT gantry in a fixed position to generate a planar image. The scout scan is used to define the body region to be imaged during the CT scan. This is sometimes referred to as a topogram.

Segmentation

replace tissue values in transmission or CT scans with known tissue values in order to reduce noise in the transmission attenuation correction maps

Sinogram

A collection of projections for one slice that are arranged by radial distance and angle.

Slip Ring Technology

Allows for continuous rotation of x-ray tube and eliminates the need for cables

Thermoionic Emission

A boiling off of electrons at the filament"; the process by which charge carriers, such as electrons or ions, move over a surface or some sort of energy barrier by the induction of heat.

Window/Level

represents the central Hounsfield unit of all the numbers within the window width.

Window Width

Range of Hounsfield Units displayed on an image