MI-42-0001 Rev. 19 195
QUALITY TESTING: Exposure Testing
8.5 Exposure Testing
8.5.1 Responsible Organization
Safety Procedures AIRO meets standards for CT medical equipment cited in 9.12 at point of delivery. After delivery, the
responsible organization must comply with the manufacturer’s safety and maintenance
requirements and follow all standards established by the applicable laws for radiological testing of
medical equipment.
The method to measure and verify CTDI on AIRO is provided below.
8.5.2 Measure and Verify CTDI
Equipment
Required
1. Polymethyl methacrylate (PMMA) CT dosimetry phantoms - Body phantom 32 cm diameter,
head phantom 16 cm diameter, at least 14 cm in length. Examples: Fluke Biomedical 76-415
(body) and 76-414 (head) phantom; Ludlum L-451 (body) and L-441 (head) phantom.
2. Electrometer / pencil ion chamber, 10 cm active detector length, with matched control unit.
Examples: Radcal 2026C control unit with 20X6-3CT ion chamber; Radcal Accu-Dose control
unit with 10X6-3CT in ion chamber.
Method 1. Place the body phantom on the patient table and
position it at the isocenter using the Ring
alignment lasers. Ensure that the phantom is centered along the axial, sagittal and coronal
planes.
2. Position the pencil ion-chamber in the center hole of the phantom; plug all other holes with
acrylic rods (included with the phantom). Set the control unit to read integrated dose, typically in
milli-Roentgens.
3. Perform an axial (sequential) scan with no Ring translation, at a typical abdomen technique
(example, 120 kV, 200 mA, NxT = 32x1.06 = 33.92 mm). Record the exposure displayed on the
base unit (mR). Designate the exposure readings as 'Center mR'.
4. Position the pencil ion-chamber at the 12 o'clock position, plugging the center hole with an
acrylic rod. Repeat step 3. Designate the exposure readings as 'Entrance mR'.
5. Repeat the same procedure (steps 1-4) with the head phantom, positioned at isocenter.
6. Calculate CTDI 100 as follows, for both body and head phantoms:
- CTDI 100, center (mGy)= [Center mR x 100 mm x CF x 0.0088] / [N x T]
- CTDI 100, entrance (mGy) = [Entrance mR x 100 mm x CF x 0.0088] / [N x T] where the 100
mm in the numerator is the active length of the pencil ion chamber,
- CF is the ion chamber correction factor, if specified by the manufacturer (typically 1.0),
0.0088 is the factor used to convert mR to mGy,
- and N x T is the beam thickness in mm (33.92 mm for the Mobius AIRO)
7. Measured CTDIw (CTDI weighted) as follows, for both body and head phantoms:
- CTDIw (mGy) = [CTDI 100, center x 0.333] + [CTDI 100, entrance x 0.667]
8. Compare the measured value against the calculated value. The calculated value is found by the
following formula;
- Body: (mA)*(0.0096)*(29.8229)
- Head: (mA)*(0.0096)*(39.7604)
9. Compare the measured results against the calculated results and calculate the percentage
difference. The percentage difference should fall within 20% and must fall within 30%.
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