06.CT6.pptx the best lecturer in CT technical

06.CT6.pptx the best lecturer in CT technical

1. بسم الله الرحمن الرحيم ALZAEM ALAZHARI UNIVERSITY Faculty of radiological sciences and medical imaging Quality management Lec ( 6 ) Computed tomography QC

2. introduction A current computed tomography (CT) system is composed of numerous electronic parts and computers that generate and process huge amounts of data. Because of the system's complexity, a quality control program is essential to ensure optimal system performance and image quality with the least amount of radiation dose to the patient.

3. Movement of the x-ray tube is not helical . It just appears that way because the patient moves through the plane of rotation during imaging.

4. A , During multislice helical computed tomography, image data are continuously sampled. B , Interpolation of data is performed to reconstruct the image in any transverse plane.

5. ACCEPTANCE TESTING Typically, the installation of most CT units is immediately followed by extensive acceptance testing by qualified physicists. Acceptance testing consists of measuring radiologic and electromechanical performance, analyzing image performance, and evaluating the system components.

6. ACCEPTANCE TESTING The results of the acceptance tests are used to identify system components that may need only slight adjustments and defective parts that should be replaced. At the conclusion of the acceptance testing, scans are taken of standard objects so that their images, CT numbers, and standard deviations can be recorded as a baseline for future measurements of the system's performance.

7. ROUTINE TESTING To establish more consistency in the performance measurements of CT scanners, federal performance standards state that the vendors of CT systems manufactured after September 1985 are required to supply the following: Instructions for performing quality control tests. Schedule for testing. Allowable variations for the indicated parameters. Method to store and record the QC data. Dose information in the form of a CT dose index.

8. ROUTINE TESTING In addition, each vendor is required to supply phantoms capable of testing the following parameters: contrast scale, noise, slice thickness. spatial resolution capabilities for both high- and low-contrast objects. The mean CT number of water or other reference material.

9. Couch incrementation Because the couch (table) moves through the gantry for CT examinations, the couch incrementation must be precise to ensure accurate patient position. All that is needed for this test is a measuring tape. Procedure: Before scanning the patient, note the starting position of the couch. Scan the patient as usual and note the end position of the couch. Using the measuring tape, measure the distance the couch moved and compare with the intended couch movement.

10. Contrast Scale and the Mean CT Number of Water Contrast scale is defined as the change in linear attenuation coefficient per CT number relative to water. The contrast scale is determined by the CT numbers for air (-1000 HU) and water (0 HU). This test is done to determine if the scanner is assigning CT numbers that correspond to the appropriate tissue. This is important because many radiologists use CT numbers to identify suspected pathology in the image. To measure contrast scale it is necessary to calculate the CT number of known materials.

11. Contrast Scale and the Mean CT Number of Water Water is the reference material used to determine CT numbers because it constitutes up the most of soft tissue mass, is easy to obtain, and is completely reproducible. Because water has a CT number value of zero, tissues with densities greater than water will have positive CT numbers, and those with densities less than water will have negative CT numbers. A plastic, water-filled phantom is commonly used to measure the contrast scale of a CT system.

12. Contrast Scale and the Mean CT Number of Water (Daily) Procedure: Using a specific technique, take a single scan through the phantom. On the reconstructed image, place ROI 200-300 pixels in center of field and take measurement. From the pixels located within the ROI calculate the two parameters, the mean CT number and the standard deviation of the CT numbers. On a monthly basis move the cursor outside of the phantom on the reconstructed image and perform the ROI function over air.

13. Contrast Scale and the Mean CT Number of Water (Daily) Expected results: CT number of water equal to zero, but range of +/- 3 at center of image is acceptable, and +/- 5 HU at peripheral locations. Cause of failure is usually miscalibration of the algorithm that generates CT numbers.

14. Contrast Scale and the Mean CT Number of Water In this example the ROI measured 0.07 HU with a standard deviation of 3.33 HU.

15. High-Contrast Spatial Resolution (monthly) The parameters that influence the high-contrast spatial resolution of a CT scanner include: Scanner design (focal spot size, detector size and spacing, magnification). Image reconstruction (pixel size, reconstruction algorithm, slice thickness). Sampling (number of rays per projection and number of projections). image display capabilities (display matrix).

16. High-Contrast Spatial Resolution Procedure: Take a single scan through the test object (Phantom with equally spaced holes drilled in the plastic, used to measure high-contrast spatial resolution). On the resultant image determine which row has the smallest set of holes in which all the holes can be clearly identified. This is known as the limiting resolution of the CT scanner.

17. High-Contrast Spatial Resolution The limiting high-contrast spatial resolution of a CT scanner is measured in line pairs per centimetre. The range of 0.45 to 1.5 lp /mm represents the typical high-contrast resolution of CT scanners used today. Even though many modern scanners have the ability to resolve holes as small as 0.3 mm, the spatial resolution of CT scanners is still much lower than that of conventional radiography.

18. High-Contrast Spatial Resolution To validate this test of the bar pattern is measured using a box ROI (yellow arrow). The box ROI should be sized until it fits into the pattern. Measure the standard deviation of the pixels in this ROI to get a quantitative assessment of changes in system resolution. The standard deviation measurement should be 40 +/- 4. This should be compared to the baseline measurement at equipment acceptance for accuracy

19. Low-Contrast Resolution Compared with conventional radiography, CT provides superior low contrast resolution. Typically, contrast resolution is expressed in one of two ways) the smallest diameter of an object with a specific contrast that can be detected or the smallest difference in x-ray attenuation that can be discriminated for an object of a specific diameter. A phantom consisting of test objects, such as holes drilled into plastic, is used for this test. The rows of holes should be of varying sizes and filled with a liquid that has a CT number that differs from the CT number of the plastic by approximately 0.5% ex(polystyrene membrane).

20. Low-Contrast Resolution This low contrast detectability phantom image displays various sized holes used to determine low contrast. This test measures the scanners ability to detect an objects density when it is close to background density.

21. Low-Contrast Resolution Low-contrast resolution is determined as the difference in HU of objects and background. To get an accurate measure of low contrast we need to know the CT number for the polystyrene membrane. This is accomplished by taking the CT number for water over an area that does not include the membrane. A second measurement is taken over an area that includes the membrane superimposed on water (called water plus the membrane). The CT number taken for water is subtracted from the water plus membrane to get the CT number for the membrane.

22. Low-Contrast Resolution To perform this test, measure using a box ROI above and below the membrane in the water section (labelled A and D in the phantom image below). Take a box ROI in the polystyrene membrane above the holes (labelled B) and below the holes (labelled C). Subtract "A" from "B" and subtract "D" from "C". When the measurements are completed and recorded adjust the Window Width to 20 and the Window Level to the CT number recorded for water. Contrast measurements should be within equipment manufacturer specifications.

23. Noise and Uniformity (Weekly) Noise represents the portion of the CT image that contains no useful information. It is defined as the random variation of CT numbers about a mean value when an image of a uniform object is obtained. The contrast resolution of a CT system is primarily determined by the amount of noise in the images. Noise produces a "salt and-pepper" appearance or grainy quality in the image. The sources of noise in a CT image include quantum (statistical) noise, electronic noise, object size, reconstruction algorithms, and artefacts.

24. Noise and Uniformity Factors under the influence of the technologist that affect the amount of noise in an image are pixel size, slice thickness, and technique factors. Methods to minimize noise and help provide uniform images include tube warmups and daily system calibrations. Because the amount of noise contained in an image is inversely proportional to the total amount of radiation absorbed, noise can be measured by obtaining the mean and standard deviation of the CT numbers within an ROI.

25. Noise and Uniformity Uniformity refers to the ability of the CT scanner to yield the same CT number regardless of the location of the ROI within a homogenous object. A simple 20-cm water phantom can be used to measure noise and uniformity in CT Procedure: Take a scan through the water phantom, and position a cursor over the resultant image in 3-6 different locations. The cursor should be positioned in the center, at the top, and at the sides of the image. At each cursor location, take an ROI measurement and record the standard deviation and mean CT number.

26. Noise and Uniformity For uniformity measurements, the uniformity (of the CT number of water) should not vary more than ±2 HU from the center of the phantom to the periphery.

27. Slice Thickness The slice thickness in CT is determined primarily by the collimators. The position of the-collimators determines the width of the slice that falls within the view of each detector. Another factor affecting slice thickness is focal spot size. The focal spot size can influence the penumbra, or sharpness of the edge of the x-ray beam, which can cause the edge of the slice to spread. Focal spot size in CT is determined by the technique factors and/or algorithm selected for the scan parameters.

28. Slice Thickness Measurements of slice thickness are determined using a phantom that includes a ramp, spiral, or step-wedge in the test objects. The test objects have known measurements and provide a standard to compare with the scanner. Typically, the test objects are aligned obliquely to the scan plane

29. Slice Thickness Procedure: Perform three separate scans through the test object, each with a specified thickness (10 mm, 5 mm, 1 mm). Display the images using the manufacturer's recommendations. If a ramp is used, the length of the resultant image gives the slice width. For a spiral test object, the resultant image will contain an arc, with the arc length giving the slice width. When a step-wedge is used, the slice width can be determined by the number of steps that are imaged

30. Slice Thickness On the figure The quality assurance phantom is used to perform several tests including the slice thickness test. (ramp pattern). For a slice thickness of 5 mm or greater, the slice thickness should not vary more than ±1 mm from the intended slice thickness. For a slice thickness of 5 mm or less, the slice thickness should not vary more than ±0.5mm.

31. Linearity Linearity refers to the relationship between CT numbers and the linear attenuation values of the scanned object with a particular kVp value. When linearity is present within an image, it is an indication that subject contrast is constant across the range of CT numbers within the image. A standard phantom containing materials with known physical and x-ray absorption properties is used for this test.

32. Linearity Procedure: Take a single scan through the appropriate phantom. Plot the average CT numbers as a function of the attenuation values corresponding to the materials within the phantom.

33. Linearity Over time, these values can vary because of changes in system components. The plotted values should demonstrate a straight line between the average CT numbers and the linear attenuation coefficients. Any deviation from the straight line could indicate that inaccurate CT numbers are being generated or the scanner is malfunctioning.

34. Patient Dose It is important that personnel monitor the amount of radiation to which patients and staff are exposed. It is equally important for a CT technologist to realize that the patient dose can increase with changes in slice thickness, kVp , and mAs . Image quality is not always performed at maximum resolution because it requires an increase in patient exposure and patient dose.

35. Patient Dose Specially designed ionization chambers or thermoluminescent dosimeters (TLD) are used to measure the radiation dose. These specially designed radiation detectors are capable of providing measurements from which the dose can be calculated for the exposure factors used (slice thickness, mAs , and kVp ).

36. Patient Dose This is a picture of a pencil ionization chamber that is inserted into a head or body phantom for patient dose calibration testing. A head phantom of 16 cm diameter is used for head dose calculation. 32 cm diameter body phantom is used for calibrating dose.

37. Patient Dose The pencil ionization chamber is inserted into the center hole (A) and peripheral holes that are 1 cm from the surface (A through E) in the 12 o’clock, 3 o’clock, 6 o’clock, and 9 o’clock positions to determine various simulated patient doses.