RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

1. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY Part 18: Optimization of protection in CT scanner Practical exercise - Quality Control

2. Contents Quality control tests • CT accuracy, uniformity, linearity. and noise, • Low and high contrast resolution • Z-axis sensitivity • Alignment, table top travel accuracy • Gantry tilt measurement • Dosimetry 18: Optimization of protection in CT scanner

3. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 18: Optimization of protection in CT scanner Topic 1: Quality Control

4. Physical parameters of CT image Image quality • May be expressed in terms of physical parameters such as uniformity, linearity, noise, spatial resolution, low contrast resolution • Image quality depends on the technical characteristics of the CT scanner, the exposure factors used and image viewing conditions. • Quality may be assessed by quantitative measurement using test phantoms, and by the appearance of artifacts. • Measurements should be conducted regularly 18: Optimization of protection in CT scanner

5. Scanner performance: technical parameters (I) Test Phantoms: • Test phantoms of a standardized human shape or test objects of a particular shape, size and structure, are used for the purposes of calibration and evaluation of the performances of CT scanners • Phantoms should allow for evaluation of CT number; uniformity; noise; spatial resolution; slice thickness; dose; positioning of table top 18: Optimization of protection in CT scanner

6. Scanner performance: technical parameters (II) CT Number Accuracy • CT number depends on tube voltage, filtration, object thickness • CT number of water is by definition equal to 0 • Measured CT number should be < ± 4 HU in the central ROI CT Number Linearity • The linear relationship between the calculated CT number and the linear attenuation coefficient of each element of the object • Deviations from linearity should be < ± 5 HU CT Number Uniformity • The CT number of each pixel in the image of an homogeneous object should be the same over the image area • The difference in the CT number between a peripheral and a central region of an homogeneous test object should be < 8 HU • Differences are largely due to beam hardening 18: Optimization of protection in CT scanner

7. Scanner performance: technical parameters (III) Noise • The local statistical fluctuation (standard deviation) of CT numbers in a homogeneous Region Of Interest (ROI) • Noise strongly affects the low contrast resolution • Noise is dependent on the radiation dose • Image noise should be measured over an area of about 10% of the cross-sectional area of the test object. • Goal— obtain an image with an acceptable level of noise while keeping the patient dose as low as reasonably achievable = Noise 1 dose 18: Optimization of protection in CT scanner

8. Scanner performance: technical parameters (IV) Spatial Resolution • The high contrast resolution determines the minimum size of details visualized in the plane of the slice with a contrast >10%. It is affected by: • the reconstruction algorithm • the detector width • the effective slice thickness • the object to detector distance • the x-ray tube focal spot size • the matrix size. 18: Optimization of protection in CT scanner

9. Scanner performance: technical parameters (V) Spatial Resolution • The low contrast resolution determines the size of detail that can be visibly reproduced when there is only a small difference in density relative to the surrounding area • Low contrast resolution is limited by noise. • The perception threshold in relation to contrast and detail size can be determined by creating contrast-detail curve. 18: Optimization of protection in CT scanner

10. Scanner performance: technical parameters (VI) Slice Thickness • The slice thickness is determined in the center of the field of view as the distance between the two points on the sensitivity profile along the axis of rotation at which response has fallen to 50%. • The use of post-patient collimation to reduce the width of the image slice leads to very significant increases in the patient dose Positioning of couch • The accuracy of positioning of the patient couch is evaluated by moving the loaded couch a defined distance and moving it back to the start position 18: Optimization of protection in CT scanner

11. Minimum requirements: CT scanner (I) • Image noise The Standard Deviation of CT numbers in the central 500 mm2 ROI for a water or tissue equivalent phantom should not deviate more than 20% from the baseline. • CT number values The deviation in the CT number values for water or tissue equivalent material and materials of different densities should <± 20 HU or 5%. • CT number uniformity The SD of the CT number over a 500 mm2 region of interest for water or tissue equivalent material at the center and at the periphery of phantom should vary by less than 1.5% of the baseline 18: Optimization of protection in CT scanner

12. Minimum requirements: CT scanner (II) • Computed tomography dose index (CTDI) The CTDI for a single slice for each available beam shaping filter and for each available slice thickness should not deviate more than ± 20% from the baseline. • Irradiated slice thickness The FWHM of the dose profile should not differ more than ± 20% from baseline. • High contrast resolution (spatial resolution) The FWHM of the point spread function of a pin, or the edge response function of an edge should not differ more than ± 20% from baseline. • Low contrast resolution Polystyrene pins of 0.35 cm diameter inserted in a uniform body water phantom should be visible in the image. 18: Optimization of protection in CT scanner

13. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 18: Optimization of protection in CT scanner Topic 2: Noise

14. Imaging performance (Noise) • Noise is assessed using cylindrical phantoms, which are either filled with water or made of a tissue equivalent material • Once an axial image of the phantom has been acquired, noise is measures as the standard deviation in CT number in a region of interest (ROI) placed in the center of the image 18: Optimization of protection in CT scanner

15. Imaging performance (Noise) Region of interest (ROI) 18: Optimization of protection in CT scanner

16. Imaging performance (Noise) • Noise values are given in manufacturers’ specifications are quoted for a specific phantom (e.g., manufacturer’s QA phantom) and for specified scan techniques • These conditions must be matched exactly for the purposes of the acceptance test • Manufacturers quote noise at a particular surface dose • If this is the case, dose for axial scans can be measured by taping an ion chamber to the surface of the phantom 18: Optimization of protection in CT scanner

17. Imaging performance (Noise) • Baseline noise values should be obtained for several scan protocols that will be used clinically, using the routine QC noise phantom • To ensure that noise figures are both accurate and representative, it is essential to find the mean value from several scans (10 scans.) 18: Optimization of protection in CT scanner

18. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 18: Optimization of protection in CT scanner Topic 3: CT uniformity and linearity

19. CT number uniformity • CT number uniformity can be assessed at the same time as measuring noise, by placing four additional ROI (N, E, S and W) at positions near the edge of the image of a uniform phantom • The mean CT number is then noted for these four regions, as well as the center • The deviation from the central value should be calculated • The CT number uniformity should be measured for large fields of view 18: Optimization of protection in CT scanner

20. CT number uniformity Axial image of a homogenous phantom 18: Optimization of protection in CT scanner

21. CT number uniformity • CT number linearity is assessed using a phantom containing inserts of a number of different materials (materials should cover a wide range of CT numbers) • One example of a suitable phantom to use at acceptance is the Catphan (The Phantom Laboratory, Salem, NY), which contains four inserts with CT numbers ranging from approximately -1000HU to +1000HU 18: Optimization of protection in CT scanner

22. CT number uniformity 18: Optimization of protection in CT scanner

23. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 18: Optimization of protection in CT scanner Topic 4: Low and high contrast resolution

24. Low contrast resolution • Low contrast resolution (or low contrast detectability) is often quoted in specification documentation as the smallest visible object at a given contrast for a given dose • Since this measurement relates directly to imaging performance, it is an important value to verify at acceptance testing • At least 20 images of the low contrast insert (LCR) should be acquired and then viewed by at least 3 observers under optimal viewing conditions to obtain an average 18: Optimization of protection in CT scanner

25. Low contrast resolution Typical image of the Catphan LCR insert 18: Optimization of protection in CT scanner

26. Spatial resolution (high contrast) • There exist two broad categories of measuring techniques : • analysis of the point spread function, usually by calculation of the modulation transfer function (MTF) • either objective analysis or visual assessment of images of a resolution bar phantom. • The resolution is quoted as the spatial frequency (in line pairs/cm) at which the modulation falls to 50%, 10% or 2% MTF. • These figures are often given for more than one reconstruction algorithm, e.g., for standard and high-resolution scans. 18: Optimization of protection in CT scanner

27. Spatial resolution (high contrast) • The number of line pairs per cm just visible in the image is approximately equivalent to the 2% value of the MTF • This result can then be compared with the 2% MTF, as quoted in the manufacturer’s specifications 18: Optimization of protection in CT scanner

28. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 18: Optimization of protection in CT scanner Topic 5: Z-axis sensitivity

29. Z-Sensitivity (Imaged slice width) Axial measurements • Phantoms used for axial measurements may contain thin metal plates, wires or arrays of air holes, inclined at an angle to the image plane • Manufacturers should be able to supply an appropriate phantom or, alternatively, the Catphan contains an insert suitable for this test Note: to obtain meaningful measurements, the thickness of the plates, wires or holes cannot be greater than the nominal slice width concerned. This may create problems for the sub-millimetre slice widths offered on multi-slice scanners. 18: Optimization of protection in CT scanner

30. Z-Sensitivity (Imaged slice width) Axial measurements • Phantoms manufacturers may quote the tolerance for each nominal slice width setting in their specification documentation • Z-sensitivity measurements in axial mode can be used to check that imaged slice widths are within the tolerances given • This can also be used in conjunction with irradiated slice width measurements to assess the accuracy of post patient collimation and to calculate the geometric efficiency for the scanner 18: Optimization of protection in CT scanner

31. Z-Sensitivity (Imaged slice width) Plan view of a test object used to measure imaged slice widths for axial scans 18: Optimization of protection in CT scanner

32. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 18: Optimization of protection in CT scanner Topic 6: Alignment

33. Alignment of indicating lights with scan, coronal and sagittal planes • Several methods can be used to perform these tests • The techniques described here are straightforward to implement and require minimal test equipment. 18: Optimization of protection in CT scanner

34. Agreement between internal and external scan plane lights • Use an envelope-wrapped film (therapy verificaiton film) for the measurement. However, a piece of paper or card can also be used • The film is placed flat on the table and illuminated by the external scan plane light • The position of the light is marked on the film envelope and the table is moved automatically to the scan plane • If the distance between the internal and external lights is correct, the internal light should now coincide with the mark on the film envelope. 18: Optimization of protection in CT scanner

35. Co-incidence of internal scan plane lights and scan plane • Pin pricks are made in a piece of therapy verification film (or similar) along the line of the internal scan plane light • The film is wrapped around a phantom and scanned with a narrow axial and developed • Coincidence between the pin pricks and the x-ray beam exposure indicates alignment between the alignment lights and the scan plane 18: Optimization of protection in CT scanner

36. Co-incidence of internal scan plane lights and scan plane • For multi-slice scanners, pin pricks (and, thus, the internal alignment light) are usually found to coincide with the center of the four slices • To plan a scan so that the x-ray beam is centered over the internal scan plane lights at zero, it will be necessary to center the first and last slices symmetrically around zero (e.g. four slices from –7.5 mm to +7.5 mm on a 4 x 5 mm scan) 18: Optimization of protection in CT scanner

37. Co-incidence of internal scan plane lights and scan plane Z Pin pricks made in film at position of scan plane light X-ray beam exposure X 18: Optimization of protection in CT scanner

38. Coronal and Sagital plane lights • A long, thin object, with a high CT number relative to air, such as the ‘lead’ in a pencil or a straightened paper clip, can be used as a marker to perform this test • The marker is supported above the patient table and aligned, using the indicating lights, so that it is positioned at the isocentre, parallel to the z-axis and perpendicular to the scan plane • If indicating lights are accurately aligned to the coronal and sagital planes, the marker should appear as a dot at exactly x = 0, y = 0 on the axial image. 18: Optimization of protection in CT scanner

39. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 18: Optimization of protection in CT scanner Topic 7: Table travel accuracy

40. Table travel accuracy • A ruler or tape measure placed alongside the table, can be used to check that the amount of table movement indicated on the gantry agrees with the actual distance moved. • A load of approximately 70- 80 kg should be placed on the table in order to simulate the weight of a patient. • The test should be performed twice: by driving the table top both away from and towards the gantry. 18: Optimization of protection in CT scanner

41. Table travel accuracy Assessment of distance indicator accuracy 18: Optimization of protection in CT scanner

42. Axial scan incrementation accuracy • Verification of incrementation accuracy between successive axial slices can be achieved by placing therapy verification film on the couch (in the isocentre plane) and exposing it to an axial scan sequence • Narrow slices separated by a couch increment greater than 1 slice width can be used, and the distance between the lines on the film measured 18: Optimization of protection in CT scanner

43. Table travel accuracy for helical scans • In helical scanning, it is not sufficient to use a simple mechanical test because the distance imaged depends on couch speed and scanner software • One method of assessing distance accuracy is to use an acrylic test object containing two small radio-opaque markers, separated by a fixed distance (e.g., 20 cm) • The test object is scanned in Scan Projection Radiography (SPR) mode and a helical scan is planned to start at the first marker and to end at a distance x from the first marker • If couch travel is accurate during the helical scan, the markers should be clearly seen on the first and final images of the series. 18: Optimization of protection in CT scanner

44. Couch travel accuracy for helical scans 18: Optimization of protection in CT scanner

45. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 18: Optimization of protection in CT scanner Topic 8: Gantry tilt measurement

46. Gantry Tilt • The accuracy of displayed gantry tilt can be assessed by supporting therapy verification film at the gantry end of the patient table • The film must be held vertically (e.g., by taping to an acrylic block), so that it is parallel to the sagital plane and intersects scan and coronal planes at right angles 18: Optimization of protection in CT scanner

47. Gantry Tilt • Three axial exposures are made using the same film: • one for the maximum superior gantry tilt, • one for the maximum inferior gantry tilt • one at 0º gantry tilt • The three scan planes should then be visible on the developed film • The angles + and - between scan planes at maximum tilt relative to that at 0º tilt should equal tilt angles displayed on the gantry. 18: Optimization of protection in CT scanner

48. Assessment of accuracy of gantry tilt 18: Optimization of protection in CT scanner

49. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 18: Optimization of protection in CT scanner Topic 9: Dosimetry

50. Dosimetry - CTDI in air • The Computed Tomography Dose Index (CTDI) in air can be measured using a 10 cm pencil ionization chamber, located in the scan plane at the isocentre • The ion chamber can be supported using a laboratory test stand and clamp 18: Optimization of protection in CT scanner