The CT scanners that were used for the data collection in this study include; one General Electric (GE), two Philips, one Toshiba and one Siemens machines brands. The scanners were manufactured and installed in the radiology centres between 2012 and 2016. The number of slices of the Scanners ranged from 16 to 640. The operating tube voltage of the CT scanners ranged between 100 kV and 140 kV, with AEC activated protocols and fixed tube current settings. Specifications of the various CTs used in the study are shown in Table 1.
Table 1
Specifications of the various CTs used in the study.
Manufacturers
|
Scanner Model/Scan Mode
|
Philips
|
Brilliance 64, Multislice, Axial and Helical Modes
|
Siemens
|
Emotions 16, Multislice, Axial and Helical Modes
|
General Electric
|
Lightspeed VCT 64, Multislice, Axial and Helical
Modes
|
Toshiba
|
Toshiba-Aquilion ONE, Multislice, Axial and Helical Modes
|
The data distribution in the survey in the slice technology of the detector-row/per the scanners were: Between 16 and 32 was 32%, 64–128 was 31%, and 320/640–37% of the survey sample. To ensure uniformity of data collection, the data was collected using the IAEA structured questionnaire for each patient between 2016 and 2020. Research assistance together with the help of the radiographers in the various CT facilities completed the CT scan parameters displayed on the CT console together with those retrieved from the clinically patients dosed report. The imaging parameters of interest include the input parameters (scan length, slice thickness, pitch factor, kVp, mA and exposure time) and output parameters (CTDIvol and DLP). Data was collected for routine adult head CT, chest CT and abdomen CT examinations for paediatric patients between the ages of 0–16 years old. As part of the inclusive criterion experienced radiologists reviewed the images of the patient data that were collected and those that were classified as passed were included in the study. The inclusion criteria for the sample selection among other things include children under 16 years
Basic Data Collection Protocol
Basic patient information was collected between 2016 and 2022 at all the imaging centres across the country. Approximately 200 MDCT exams of patient data for CT of the Head, Chest, abdomen-Pelvis were collected from five CT facilities. This was assessed from the PAC system and the registry with prior approval by the ethics committee of the various facilities. Outpatients who sought CT examination for a variety of diagnostic requests other than those linked with any pathological problem related to the region of interest made up the study's sample population of 200 patients out of a total of 300 patients.
The methodology used include four processes; the quality control measurement process, to ensure optimum performance of the machine and phantom data in other to compare with image data; the retrospective measurement method to collect patient and equipment performance data; estimation of CTDI and DLP.
a) Quality Control Measurement
Equipment specific daily QC were performed on each of the five CT equipment. This was to ensure that basic CT components work correctly. This enables the evaluation of image quality and CT performance as indicated by producers and by international standards. For proper CT Quality Control examination, the scanner readings were validated to standard measurement. The daily necessary scanning procedure and protocol of the various manufacturers were used to complete the calibration check, as illustrated in Fig. 1.
b) The retrospective measurement method
The methodology includes using retrospective method to collect patient data, including input parameters such as protocol and procedure used, exposure time, kVp, mA, pitch factor and scan length that are associated with the output parameters such as DLP and CTDIVOL. The derived dose parameters such as effective and organ dose were also obtained and analyzed. This information was obtained from two main sources. Firstly, from the programmable logic controller (control console) based on which each recorded data was matched with the serial number of the parameters on the dose report and recorded. Secondly from the MeVisLab image application software that was also used to view the image data recorded and compared with the recorded data from the console. These data were then analyzed.
c) The CT dosimetry: CTDI and DLP.
i) Principle for CTDI and DLP Measurement
The computed tomography dose index (CTDI) is the dosage in CT determined using a Polymethyl methacrylate (PMMA) phantom with a diameter of 16 cm or 32 cm. The dosimeter was serially inserted into the center and periphery holes, and the results estimated using Eq. 1, as shown in Fig. 2. From the weighted CTDIW, the volume CTDIVOL was also estimated using Eq. 3. Additionally, the CTDIVOL was also obtained from the retrospective image.
Measurement of CTDI and DLP
MVL DICOM application software standard supplement was issued in 2007 for the reporting of dose parameters in CT (DICOM, 2007). This became mandatory for all manufacturers of CT equipment. It requires a report summary should be given for the whole patient examination and the accumulated dose applied after every CT scan. The patient bio data, study information and the general equipment information are stored within the general part of the structured report. This development enabled the obvious difficulties in measuring the distribution of dose within the body during CT imaging to be overcome since 2007 using image data. More practical dosimetric quantities captured as part of image data were applied to readily estimate dose parameters from closely related measurements. The practical dosimetric quantities, CTDIW, CTDIVOL and DLP, were recorded directly.
On the image data, using MVL platform detail information of the CTDIvol and DLP were available for recording as shown in Fig. 2.
The weighted CTDI (CTDIW) was estimated by multiplying the volume CTDI (CTDIVOL) by the pitch factor expressed mathematically as:
CTDIW = 0.813 ∗ CTDIVOL, 5