Chapter 4


GLOSSARY

This glossary contains descriptions of commonly-used technical terms in CT as an aid to understanding the Guidelines. The bold-faced typed words in the explanatory text indicates that they occur elsewhere in the glossary.

artefact (structured noise): The appearance in the CT image of details not present in the scanned object. The main components of structured noise are due to a form of partial volume effect and to beam hardening. Both effects usually result in streaking artefacts, which are observed in regions of high contrast when there is a sharp discontinuity in object density, such as at air-tissue, air-bone and metal-tissue boundaries. Streaking will also arise from mechanical misalignment within the scanner and, in clinical practice, from patient motion and the use of high-density contrast media.

attenuation: Reduction of the radiation intensity, upon passage through matter, resulting from all types of interaction.

back projection: Mathematical procedure for the reconstruction of the CT image, based on the smearing of the individual rays within a view (projection) back along the direction in which they were measured. Spatial filtration (convolution) of the raw data is necessary before back projection in order to reduce artefacts.

beam hardening: The process of filtration of a polychromatic beam by the preferential absorption of lower energy photons in tissue, with a subsequent increase in effective energy. The associated artefacts are of particular significance in quantitative computed tomography (QCT).

calibration of a CT-scanner: Correction procedures used to take account of variations in beam intensity or detector efficiency in order to achieve homogeneity within the field of view and accuracy of CT number. Calibration procedures include scanning air or an appropriate test phantom.

collimation: Geometrical limitation of the extent of the radiation beam in the z-direction.

computed tomography dose index (CTDI): Integral along a line parallel to the axis of rotation (z) of the dose profile (D(z)), measured free-in-air or in a CT dosimetry phantom for a single slice, divided by the nominal slice thickness (T):

 (mGy)

In practice, it is convenient to use a pencil ionisation chamber with an active length of 100 mm so as to provide a measurement of CTDI100 (mGy to air).

computed tomography number (CT number): Number used to represent the mean x-ray attenuation associated with each elemental area of the CT image. Numbers are normally expressed in terms of Hounsfield units (HU). Measured values of attenuation are transformed into CT numbers using the international Hounsfield scale:

 (HU)
where µ is the effective linear attenuation coefficient for the x-ray beam.
The CT number scale is defined so that water has a value of 0 HU and air a value of -1000 HU.

contrast: In relation to the radiation emerging from an irradiated object, if the photon fluence at some reference point is Ø0, and at an adjacent point is Ø1, the contrast can be defined as (Ø1 - Ø0) / Ø0. Contrast can also be expressed in terms of energy fluence or exposure.

contrast enhancement: Administration of intraveneous or intraarterial contrast increase the visibility of low contrast structures due to increased density of vessels and organs/tissue containing contrast media.

contrast resolution: See low contrast resolution.

convolution: The mathematical process by which raw data undergo spatial filtration prior to back projection.

couch increment: Distance by which position of patient couch (table) is changed between individual slices in serial scanning or the distance the couch position is changed during one 360o rotation of the tube during helical scanning.

CT dosimetry phantoms: Cylinders of polymethylmethacrylate (PMMA) used for standard measurements of dose in CT, having a diameter of 16 cm (head phantom) or 32 cm (body phantom) and a length of at least 14 cm. The phantoms are constructed with removable inserts parallel to the axis to allow the positioning of a dosemeter at the centre and 1 cm from the outer surface (periphery).

CT number: Abbreviation for computed tomography number.

CTDI: Abbreviation for computed tomography dose index.

CTDIair: Value of CTDI determined free-in-air.

CTDIw: See weighted CTDI.

detector: A single element of a detector array, which produces an electrical or light signal in response to stimulation by x-rays.

detector array: The entire assembly of detectors, including their interspace material, arranged along an arc or circumference (depending on scanner technology) of a circle centred on the axis of rotation.

detector efficiency: for each detector contained in a detector array, the ratio between the number of pulses recorded and the number of x-ray photons incident on the detector.

detector width: In a detector array, the distance between the two opposite faces of any single detector.

diagnostic reference level: Advisory dose levels set by professional bodies to prompt local reviews of practice if consistently exceeded.

display matrix: The array of rows and columns of pixels in the displayed image, typically between 512 x 512 and 1024 x 1024. It may be equal to or larger than the size of the reconstruction matrix due to interpolation procedures.

dose descriptor: measurable parameter, such as CTDIair, CTDIw or DLP, from which the effective dose or the organ dose delivered to a patient in a CT examination can be estimated, or the performances of different CT scanners can be compared.

dose-length product (DLP): Dose descriptor used as an indicator of overall exposure for a complete CT examination in order to allow comparison of performance against a reference dose value set for the purpose of promoting optimisation of patient protection.

 (mGy cm)

where i represents each scan sequence forming part of an examination, and CTDIw is the weighted CTDI for each of the N slices of thickness T (cm) in the sequence.

dose profile: Representation of the dose as a function of position along a line perpendicular to the tomographic plane.

dosimetry phantom: See CT dosimetry phantom.

dynamic scanning: A method of obtaining CT scans in rapid sequence so as, for example, to follow the passage of contrast material through vessels or tissue, or to decrease examination time.

effective dose: Risk-related quantity used as indicator of overall patient dose. It is defined by the International Commission on Radiological Protection (ICRP) in Publication 60 (1991) as the sum of the weighted absorbed doses in all tissues and organs of the body:

 (mSv)

where DT is the absorbed dose (mGy) in tissue T due to radiation R, wR is the weighting factor for radiation R and wT is the weighting factor for tissue T. For x-rays, wR is equal to unity.

exposure factors: The settings of x-ray tube voltage (kV), tube current (mA) and exposure time (s).

exposure time: Duration of emission of radiation by the x-ray tube (seconds) for an individual slice in axial scanning or total acquisition time for helical scanning.

field of view (FOV): The maximum diameter of the reconstructed image.

filter: Mathematical procedure used for the convolution of the attenuation profiles and the consequent reconstruction of the CT-image.

focal spot: The effective area on the x-ray tube anode from which x-rays are emitted. The size of the focalspot has influence on spatial resolution.

full width at half maximum (FWHM): Interval parallel to the abscissa between the points on a curve with the value of one-half of the maximum of the symmetrical curve.

gantry: Scanner structure containing the x-ray tube, collimators and the detector array.

gantry aperture: Diameter of the physical opening of the gantry through which the patient is moved for the examination.

gantry tilt: The angle between the vertical plane, and the plane containing the x-ray fan beam and the detector array.

helical CT: A particular technique of scanning in which there is continuous rotation of the x-ray tube coupled with continuous linear translation of the patient through the gantry aperture in order to achieve volumetric data acquisition. Also known as spiral or volume CT.

high contrast resolution: See spatial resolution.

HU (hounsfield units): See CT number.

imaging volume: See volume of investigation.

intensity: The quantity of radiation energy flowing through unit area in unit time.

interpolation: A mathematical method of averaging or smoothing images that are being displayed on a larger number of pixels than that for which they were originally reconstructed.

inter-slice distance: The distance between the adjacent nominal margins of consecutive slices in serial CT scanning. It is dependent upon the couch increment between slices.

linearity: In CT, the extent to which the CT number of a given material is exactly proportional to its density (in HU unit).

linear attenuation coefficient: The fractional reduction in intensity per unit thickness of material as an x-ray beam passes through an absorber. For a polychromatic beam, the effective linear attenuation coefficient depends on the effective energy of the beam, and the density and atomic number (composition) of the material.

kernel: See filter.

low contrast resolution: A measure of the ability to discriminate between structures with slightly differing attenuation properties (CT number). It depends on the stochastic noise and is usually expressed as the minimum detectable size of detail discernable in the image, for a fixed percentage difference in contrast relative to the adjacent background.

Monte Carlo Technique: A technique for obtaining an approximate solution to certain mathematical and physical problems, characteristically involving the replacement of a probability distribution by sample values, usually performed using a computer.

multiple scan average dose (MSAD): The MSAD is the average dose across the central slice from a series of N slices (each of thickness T) when there is a constant increment I between successive slices:

 (mGy)

where DN,I(z) is the multiple scan dose profile along a line parallel to the axis of rotation (z).
For a sufficient number of slices such that the first and the last in the series do not contribute any significant dose over the width of the central slice:

 (mGy)

noise: Noise is the point-to-point variation in image density that does not contain useful information. The magnitude of noise is indicated by the percentage standard deviation of the CT numbers within a region of interest in the image of a uniform substance (generally water), relative to the difference in CT numbers between water and air.

nominal (tomographic) slice thickness: The slice thickness selected and indicated at the control panel of the CT scanner.

number of measurements: The total number of attenuation values measured during the acquisition of the raw data for a single slice.

packing factor: In relation to dosimetry for serial CT, the packing factor (p) is used to spread the radiation density evenly over the volume of investigation when the slices are not contiguous. For a series of N slices, each of thickness T, and with a couch increment I such that the total scan length is L:

  

p = 1 for contiguous slices
p > 1 for overlapping slices
p < 1 for gaps between slices.

partial volume effect: The inaccuracy in CT number caused by the presence of a structure within only part of a slice. Such effects become less important as the slice thickness is reduced.

pitch factor: In relation to helical CT, ratio of the patient couch travel in horizontal direction per rotation of the x-ray tube divided by the product of the number of tomographic sections produced by a single rotation of the x-ray tube N times the nominal tomographic slice thickness T:

  
where:
delta dis the patient couch travel in horizontal direction
Nis the number of tomographic sections produced by a single rotation of the x-ray tube
Tis the nominal tomographic slice thickness.

pixel: Individual square picture element of a digital image display, being the two-dimensional representation in HU of a voxel within the scanned slice. Pixel size is determined by the diameter of the field of view and the number of elements in the display matrix.

polymethylmethacrylate (PMMA): Polymethylmethacrylate, a polymer plastic commercially available for example as Perspex or Lucite.

profile of CT numbers: Representation of the CT numbers of the pixels along a specified direction in a CT image.

quantitative computed tomography (QCT): The use of CT images and the corresponding CT numbers for quantitative characterization of organs or tissues. QCT is most-widely used in relation to the determination of bone mineral content and treatment planning in radiotherapy.

radiographic exposure: Product of tube current and exposure time.

raw data: The values of x-ray detector response from all views and rays within a scan. These data are convolved with the convolution filter and undergo back projection to produce a CT image.

ray: The narrow beam of x-rays from the tube focal spot to a single detector within a detector array, giving rise to a detector reading. Each view or projection is composed of numerous rays.

reconstruction algorithm: Mathematical procedure used to convert raw data into an image. Different algorithms are used to emphasize, enhance, or improve certain aspects of the data.

reconstruction matrix: The array of rows and columns of pixels in the reconstructed image.

region of interest (ROI): Localised part of an image defined by the operator which is of particular interest at a given time.

ring artefacts: Circular artefacts, usually found in third-generation scanners, caused by faulty calibration or a defect in detector function.

scanning: The process of recording x-ray attenuation data through a slice of an object, from which images are reconstructed.

scan projection radiograph (SPR): Generic name for the digital image obtained by linearly translating the patient through the gantry aperture during an x-ray exposure while the x-ray tube remains stationary. The SPR has a similar appearance to a plain radiograph and is used primarily for localizing the required region of scanning. Synonymous terms include radiographic mode and localizer image, together with the proprietary names Pilot scan, Scanogram, Scanoscope, Scoutview, Surview and Topogram.

scan time: The time interval between the beginning and the end of the acquisition of attenuation data for a single exposure. For some CT scanners, this may be longer than the exposure time due to the pulsing of x-ray emission.

scattered radiation: Secondary radiation belonging to the same radiation type as the original radiation, produced in the interaction of the original radiation with a material medium. The interaction can be characterized by a reduction in radiation energy and/or by a change in the direction of the radiation.

sensitivity profile: Relative response of a system for CT as a function of position along a line perpendicular to the tomographic plane.

signal to noise ratio: The ratio of the strength of the signal for information content in the image to the noise level (the standard deviation of the signal).

slice: Tomographic section (defined by position and thickness) of a test phantom or patient under investigation during a single CT exposure in serial scanning.

slice thickness: Effective thickness of the tomographic section, as measured by the full width at half maximum of the sensitivity profile in the centre of the scan field.

spatial resolution (or high contrast resolution): The ability to resolve different objects in the displayed CT image, when the difference in attenuation between the objects and the background is large compared to noise; normally a difference corresponding to at least one hundred HU is considered adequate.

spiral CT: See helical CT.

stability: The maintainance over time of constancy of CT numbers and uniformity.

standard examination: Outline of scanning procedure for a particular clinical indication that is generally accepted as being able to provide adequate clinical information in most of the patients examined.

test phantom: Object of particular shape, size and structure (including standardised representations of human form), used for the purposes of calibration and evaluation of performance of CT scanners.

uniformity: Consistency of the CT numbers in the image of a homogeneous material across the scan field.

volume CT: See helical CT.

volume of investigation (imaging volume): Entire volume of the region under investigation by scanning.

voxel: Elementary volume element (expressed in units of mm3 ) within the scanned slice of the object, with which CT numbers are associated.

weighted CTDI (CTDIw): An estimate of the average dose over a single slice in a CT dosimetry phantom that is used to allow comparison of performance against a reference dose value set for the purpose of promoting optimisation of patient protection.

 (mGy)

where CTDI100,c or p refer to measurements of CTDI100 at the centre (c) or periphery (p) of the head or body phantom for the settings used in clinical practice.

window level: The central value of the window (in HU) used for the display of the reconstructed image on the image monitor of the CT scanner.

window setting: The setting of the window level and the window width, selected for optimization of the grey scale levels in the displayed CT-image.

window width: The range of CT numbers within which the entire grey scale is displayed on the image monitor of the CT scanner.