Performance evaluation of computed tomography is not significant in the Republic of Cameroon in general and more particularly in the city of Yaoundé. Therefore, this work aimed to analyze image performances of 05 CT scanners in 05 medical facilities in the city of Yaoundé, by using the Catphan 700 phantom and test procedures of the Atomic Energy Agency. Five quality control tests were performed on each CT-Scanner. All the CT scanners evaluated showed good results and were not disapproved in three of the five tests performed. The difference between the measured spatial resolution for each of the five CT scanners and the spatial resolution specified by the manufacturer was very high. The low contrast resolution of four of the five CT scanners was found low when compared to the low contrast resolution specified by the manufacturers.
Quality assurance (QA) methods in diagnostic radiology aims to promote the effective use of radiation through achieving and maintaining appropriate image quality while delivering a minimal, dose to the patient.
Routine quality control (QC) procedures in computed tomography (CT) centers in Cameroon are not given much attention due to no/limited certified staffs trained to carry out this task.
The IAEA recommends acceptance tests and periodically QA-tests of CT-scanners with respect to radiation dose and image quality [
Publications by the International Electrotechnical Commission (IEC), the American Association of Physicists in Medicine (AAPM) and the Institute of Physics and Engineering in Medicine (IPEM) form a basis for the monitoring of CT systems. IEC publish recommendations and acceptance values for international use, and provide assistance in performing QA-tests on CT-scanners. IEC recommends the mean CT-number of a central region of interest (ROI) in a uniformity device not to deviate by more than 4 HU from the nominal values specified by the manufacturer of the CT-scanner for the specific material of the test device [
The uniformity (the deviation in mean CT-number between central and peripheral regions) must not be greater than 4 HU at acceptance [
Different types of Image phantoms are used worldwide to evaluate image performance of CT-scanners. The Catphan Phantoms manufactured by the Phantom Laboratory are certainly the most popular. One of those phantoms, the Catphan 700 phantom is a diagnostic imaging tool specially designed for comprehensive evaluation of axial, spiral, multi-slice, cone beam, and volume CT scanners from the point of view of maximum performance [
Garayoa and Castro [
The assessment of CT scanners image quality performance with Catphan phantoms is very important since those phantoms are also used by the manufacturers to test the image performances of theirs CT scanners after their manufacturing. This enables a comparison with the results obtained by the manufacturers.
To perform the experimental study, 05 CT scanners were analyzed in Yaoundé a city of about three millions of inhabitants. The total number of CT scanners in the city of Yaoundé is 10, however due to regular failures all the CT scanners are not always available. The sample consisted of different manufacturers and models of CT scanners, as shown in
A phantom, manufactured by Phantom Laboratory, Catphan model 700, serial number 700158 [
The CT scanners were evaluated in five quality control tests in order to verify theirs adequacy.
The geometry sensitometry module CTP682 has a pair of 23˚ wire ramps oriented parallel to the x-axis at 0˚ rotation. These wire ramps were used to measure slice widths. The radiation profile width aimed to verify that it meets IAEA specifications. Following the related IAEA publication, the tolerance limit
CT scanner n˚ | Manufacturer/Model |
---|---|
1 | Toshiba/Aquilion 16 |
2 | Toshiba/Alexion |
3 | Neusoft/Neuviz 16 |
4 | Toshiba/Aquilion 16 |
5 | Toshiba/Aquilion 16 |
is 1 mm for slice widths upper than 2 mm.
The consistency of the CT-numbers in the image of a homogenous material was measured in the uniformity module (CTP712) in the phantoms. Uniformity of the CT-numbers was measured manually by placing ROIs, one in the middle and four in the periphery of the module (clock positions 12, 3, 6 and 9). The observer used ROIs with a diameter of 10% of the diameter of the image of the uniformity module, in agreement with IEC 61223-3-5. As stated in the IAEA publication, tolerance limit for uniformity is ±10HU.
The geometry sensitometry module CTP682 contains inserts with different known densities for measurements of CT-numbers and linearity of CT-numbers for different mass densities. Catphan 700 has inserts made from teflon, acrylic, low density polyethylene (LDPE), air, polymethylpentene (PMP), delrin, bone 20%, bone 50% and polystyrene. It also has a water container that was filled with distilled water.
The procedure to evaluate the accuracy of the CT number is similar to the test for evaluation of field uniformity. After scanning the uniform section of the phantom, the ROI tool is used to measure the mean CT number for water and other materials. The IAEA publication [
Module 714 contains the 30 line pair/cm gauge for visual evaluation of high resolution ranging from 1 through 30 line pair/cm. Spatial resolution was measured by counting the numbers of resolved group of Lp/cm, assessed by three independent investigators.
The scan parameters in this test were as recommended by each manufacturer of the CT scanner. The value of the spatial resolution measured was then compared with the value specified by the manufacturer. as recommended by the IAEA publication [
Module CTP515 contains low contrast subslice and supraslice targets with diameters 2 - 15 mm, and contrast levels of 0.3%, 0.5% and 1.0%, used to evaluate the ability to differentiate objects with slightly different densities. Low contrast was measured by counting numbers of visible targets, assessed by three independent investigators.
The scan parameters in this test were as recommended by each manufacturer of the CT scanner. The value of the low contrast resolution measured was then compared with the value specified by the manufacturer. as recommended by the IAEA publication [
The nominal CT slice width in this test was 2 mm, the actual slice width was measured for each CT scanner, and the results are shown on
For all the CT scanners, the difference between the measured slice width and the nominal slice width is less than 1 mm. Therefore all the CT-scanners pass this test.
The results of this test are shown on
It appears that the measured spatial resolution for each of the five CT scanner is far below the manufacturer’s spatial resolution. However no CT scanner showed a spatial resolution of less than 8 Lp/cm. This shows that the equipment can still detail objects of small size and high contrast, such as small kidney stones [
The results of this test are shown on
The low contrast resolution for CT scanner 3 was as expected, but the low contrast resolution for the others CT scanners was lower than the manufacturer’s low contrast.
CT-scanner n˚ | Measured slice width |
---|---|
1 | 2.10 mm |
2 | 2.40 mm |
3 | 1.93 mm |
4 | 2.50 mm |
5 | 1.97 mm |
CT-scanner n˚ | Measured spatial resolution | Manufacturer’s spatial resolution |
---|---|---|
1 | 10 Lp/cm | 18 Lp/cm |
2 | 9 Lp/cm | 18 Lp/cm |
3 | 9 Lp/cm | 15 Lp/cm |
4 | 13 Lp/cm | 18 Lp/cm |
5 | 12 Lp/cm | 18 Lp/cm |
The results of this test are shown on
The measured CT numbers are shown on
CT-scanner n˚ | Measured low contrast resolution | Manufacturer’s low contrast resolution |
---|---|---|
1 | 7 mm @ 0.3% | 3 mm @ 0.3% |
2 | 5 mm @ 0.3% | 3 mm @ 0.3% |
3 | 4 mm @ 0.3% | 4 mm @ 0.3% |
4 | 5 mm @ 1% | 3 mm @ 0.3% |
5 | 4 mm @ 0.3% | 3 mm @ 0.3% |
CT-scanner n˚ | Measured field uniformity |
---|---|
1 | 2.4 HU |
2 | 4.2 HU |
3 | 1.64 HU |
4 | 2.73 HU |
5 | 1.1 HU |
Material | μ(cm−1) | CT-numbers (HU) | HU Range | ||||
---|---|---|---|---|---|---|---|
CT-scanner n˚ | |||||||
1 | 2 | 3 | 4 | 5 | |||
Air | 1.89E−04 | −991.4 | −996.6 | −971.48 | −1026.62 | −1010.4 | −1046: −986 |
Lung | 2.87E−02 | −823.2 | −827.3 | −805.61 | −833.01 | −838.3 | −925:−810 |
PMP | 1.36E−01 | −188.8 | −189.9 | −181.24 | −183.06 | −189.8 | −220:−172 |
LDPE | 1.51E−01 | −100.8 | −107.2 | −92.18 | −93.69 | −100.5 | −121: −87 |
Polystyrène | 1.59E−01 | −41.6 | −55.5 | −35.85 | −26.39 | −40.2 | − 65:−29 |
Water | 1.61E−01 | −1.4 | −1.9 | −2.43 | 7.19 | 1.7 | −7:7 |
Acrylic | 1.84E−01 | 119.5 | 122.5 | 111.16 | 126.98 | 118.6 | 92:137 |
Bone 20% | 1.78E−01 | 239.1 | 242.7 | 222.63 | 260.24 | 248.2 | 211: 263 |
Delrin® | 2.19E−01 | 321.3 | 325.3 | 339.18 | 363.13 | 356.1 | 344: 387 |
Bone 50% | 2.25E−01 | 669.7 | 669.6 | 640.62 | 749.11 | 720.4 | 667:783 |
Teflon | 3.05E−01 | 922.6 | 981.37 | 960.22 | 981.37 | 967.5 | 941:1060 |
The CT numbers for CT-scanner 5 was within the expected range as predicted in Catphan 700 manual. The CT numbers on others CT-scanners were found to be accurate for the majority of materials except for Delrin@, Polystyrene, Teflon, Bone 20% and Bone 50%. The CT number of Delrin@ was out of range for CT-scanners 1 and 2. The CT number of Polystyrene was out of range for CT-scanner 4. The CT number of Teflon was out of range for CT-scanner 1. The CT number of Bone 20% was out of range for CT-scanner 2. The CT number of Bone 50% was out of range for CT-scanner 3.
To establish a constancy of contrast scale over the range of CT numbers which is of clinical interest, the CT linearity was verified. This was performed by checking whether the CT numbers measured vary in a linear fashion with their linear attenuation coefficient values. [
From the analysis of tests, it was observed that the five CT scanners showed a high rate of approval for three of the five tests performed. The measured spatial resolution for each of the five CT scanners was far below the spatial resolution specified by the manufacturer. The low contrast resolution of four of the five CT scanners was found low when compared to the low contrast resolution specified by the manufacturer. However the measured spatial resolution was above the minimal value required for the performance of thoracic computed tomography [
Njiki, C.D., Ndjaka Manyol, J.E.M., Ebele Yigbedeck, Y., Abou’ou, D.W., Yimele, B.C. and Sabouang, J.F. (2018) Assessment of Image Quality Parameters for Computed Tomo- graphy in the City of Yaoundé. Open Journal of Radiology, 8, 37-44. https://doi.org/10.4236/ojrad.2018.81005