Radiation Dose Reduction Strategy for CT Protocols: Successful Implementation in Neuroradiology Section

Appendix E1

Comparison of CT Scanners

The clinical images that were compared in this study were acquired on two different CT scanners (16-section and 64-section) using different imaging protocols and notably different reconstruction section thicknesses (Tables 1–4). To compare the doses delivered on the two types of scanners, a parameter called "dose efficiency" (E_2/1), defined as the dose ratio between acquisition setting 2 and acquisition setting 1, was measured for each scanner. To obtain this value, a homogeneous cylindric phantom filled with water and having a diameter of 16 cm was scanned by using various acquisition parameter settings on each of the systems. According to the physics that governs CT, the standard deviation σ of pixel values (image noise) is inversely proportional to the square root of the dose and the reconstructed section thickness:

(E1)

where IN = image noise, D_vol = dose, and H = section thickness.

To compare two different acquisition settings, the E_2/1 factor corresponding to the gain in dose made when going from setting 1 to setting 2 for a given image noise level was calculated according to Equation (E2), based on cylindric phantom measurements. The E_2/1 factors for unenhanced CT imaging protocols of the brain are provided in Table E1.

(E2)

Table E1. Gains in Dose Efficiency (E_2/1) between 16-Section and 64-Section Scanners for Unenhanced CT Imaging Protocols of the Brain

Table E1

Note.—NA = not applicable.

Table E1 shows that for the 16-section scanner, comparing a 4 × 2.5-mm acquisition to a 4 × 3.75-mm acquisition results in a dose gain (E_2/1) that is less than 1.0 but smaller than the expected ratio of the reconstructed section thicknesses (ie, 0.87 vs 0.66 [2.5/3.75]). The explanation for this is that in this system, the geometric dose efficiency is substantially lower in the 4 &tomes; 2.5-mm configuration (geometric dose efficiency = 82.43%) than in the 4 × 3.75-mm configuration (geometric dose efficiency = 92.04%). As expected when scanning a homogeneous cylindric phantom, no major differences between modulated and fixed-current acquisitions were observed. Another interesting result is that the use of a smaller reconstruction section thickness (2.5 mm) on the 64-section scanner compared the greater thickness (3.75 mm) on the 16-section scanner results in a dose gain of 17%&150;40% and not to the expected dose penalty of 66%. This behavior is certainly due to the better geometric dose efficiency of the 64-section system together with the use of higher-efficiency interpolation algorithms. Similar observations were made for the unenhanced CT protocols for the spine and for CT angiography.

Criteria Used in the Review for Judging the Diagnostic Acceptability of CT Studies (Tables E2–E4)

Table E2. Imaging Quality Criteria and Grading for Unenhanced CT of the Brain in Adult and Pediatric Patients

Table E2

Table E3. Imaging Quality Criteria and Grading for Unenhanced CT of the Cervical Spine in Adult Patients

Table E3

Table E4. Imaging Quality Criteria and Grading for CT Angiography in Adult Patients

Table E4

This Article

	Abstract
	Full Text
	Submit a response

Services

	Email this article to a friend
	Alert me to new issues of the journal