Fluke Biomedical 76-414 - Manual (Page 8)

Nuclear Associates 76-414-4150
Operators Manual
2-2
Where:
μ
w
= Linear attenuation coefficient of water.
μ
x
= Linear attenuation coefficient of material of interest.
(CT)
w
= CT number of water.
(CT)
x
= CT number of material of interest.
3) CT Conditions of Operation Means
All selectable parameters governing the operation of a CT x-ray system including nominal
tomographic section thickness, filtration, and the technique factors as defined in §1020.30(b)(36).
4) CT Number Means
The number used to represent the x-ray attenuation associated with each elemental area of the
CT image.
5) [Reserved]
6) CT Dosimetry Phantom Means
The phantom used for determination of the dose delivered by a CT x-ray system. The phantom
shall be a right circular cylinder of polymethl-methacrylate of density 1.19±0.01 grams per cubic
centimeter. The phantom shall be at least 14 centimeters in length and shall have diameters of
32.0 centimeters for testing any CT system designed to image any section of the body (whole
body scanners) and 16.0 centimeters for any system designed to image the head (head
scanners) or for any whole body scanner operated in the head scanning mode. The phantom
shall provide means for the placement of a dosimeter(s) along its axis of rotation and along a line
parallel to the axis of rotation 1.0 centimeter from the outer surface and within the phantom.
Means for the placement of a dosimeter(s) or alignment device at other locations may be
provided for convenience. The means used for placement of a dosimeter(s) (i.e., hole size) and
the type of dosimeter(s) used is at the discretion of the manufacturer. Any effect on the doses
measured due to the removal of phantom material to accommodate dosimeters shall be
accounted for through appropriate corrections to the reported data or included in the statement of
maximum deviation for the values obtained using the phantom.
7) Dose Profile Means
The dose as a function of position along a line.
8) Modulation Transfer Function Means
The modulus of the Fourier transform of the impulse response of the system.
9) Multiple Tomogram System Means
A CT x-ray system which obtains x-ray transmission data simultaneously during a single scan to
produce more than one tomogram.
10) Noise Means
The standard deviation of the fluctuations in CT number expressed as a percent of the
attenuation coefficient of water.
Its estimate (S
n
) is calculated using the
following expression: