Section 360.APPENDIX D   Computed Tomography Dose Measurement Protocol

Radiation measurements shall be performed by a diagnostic imaging specialist with a calibrated radiation measuring device that is designed for computed tomography (CT) dose measurements.  The radiation measuring instrument shall have been calibrated within the previous 12 months with devices which have no more than a three-step (tertiary) calibration, traceable to the National Institute of Standards and Technology.  Measurements shall be specified in terms of the multiple scan average dose (MSAD) and shall be performed with a head phantom specifically designed for making CT dose measurements.

AGENCY NOTE:  There are two terms used to describe CT dosimetry measurements, the computed tomography dose index (CTDI) and the multiple scan average dose (MSAD).  Manufacturers of CT systems measure and report CTDI pursuant to the requirements of the Code of Federal Regulations, 21 CFR 1020.33(b)(1).  While the CTDI is carefully defined, it is difficult to measure accurately.  The MSAD is easily measured and was the CT dose descriptor used by the Center for Devices and Radiological Health (FDA) in the Nationwide Evaluation of X-Ray Trends (NEXT).  The CTDI is equivalent to the MSAD for a series of 14 contiguous scans spaced by the nominal tomographic thickness.  The MSAD was chosen as the dose descriptor for this Part due to the ease of measurement and the applicability of the data generated for comparisons with the results of the NEXT study.

a)         CT dose measurements shall be performed using a head phantom that meets the following requirements:

1)         The phantom shall be a right circular cylinder of polymethyl-methacrylate of density 1.19 plus or minus 0.01 grams per cubic centimeter.

2)         The phantom shall be at least 14 centimeters in length and shall have a diameter of 16 centimeters.

3)         The phantom shall provide means for the placement of a radiation measuring device in the center of the phantom along its axis of rotation.

b)         Set up procedure

1)         Place the phantom on the patient support device and in the patient head rest, if available.  Center the phantom in the CT gantry aperture and position the gantry so that it is perpendicular to the patient support device.  Align the phantom so that the tomographic plane is centered along the axis of the phantom.

2)         Make a single scan of the phantom and determine if the center of the phantom is aligned with the axis of rotation of the scanner.  If necessary, realign the phantom and repeat this procedure until the center of the phantom is aligned to within plus or minus 0.5 centimeters of the axis of rotation of the CT scanner.

3)         Place the radiation measuring device in the center of the phantom.

c)         Exposure measurement

1)         Select and record the technique factors and the tomographic section thickness most frequently used for a CT examination of the head.

            AGENCY NOTE:  If routine CT examinations of the head are performed at the facility using a different tomographic section thickness for the top or bottom part of the head, the larger tomographic section thickness should be used for measurement of the MSAD.

2)         Perform a single CT scan and record the exposure reading from the radiation measuring device.  Repeat this procedure, without advancing the table or phantom, three times for a total of four scans and determine the average exposure reading for a single scan.

d)         Calculation of MSAD

1)         The MSAD shall be calculated using the mathematical expression below:

            where:

AGENCY NOTE:  This calculation assumes tomographic sections are contiguous, without overlap of sections or gaps between sections.

            EXAMPLE:  The measurement is made with an ion chamber with an effective length of 100 millimeters and a calibration factor of 1.99.  The thickness of the tomographic section from subsection (c)(1) of this Section is 10 millimeters.  The average exposure reading from subsection (c)(2) of this Section is determined to be 306 mR.  The MSAD is calculated as follows:

            MSAD = (306 X 0.78 X 10(-3) X 1.99 X 100) / 10

            MSAD = 4.7 rad

2)         If the tomographic sections overlap, the MSAD must be multiplied by a fraction which is the thickness of the tomographic section divided by the scan increment.

            EXAMPLE:  Calculate the corrected MSAD for scan overlap technique, in a continuation of the above example, assume a scan increment of 5 millimeters.

            Corrected MSAD = MSAD X (T / scan increment)

            Corrected MSAD = 4.7 X (10 / 5)

            Corrected MSAD = 9.4 rad

(Source:  Amended at 22 Ill. Reg. 5904, effective March 13, 1998)