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Dose reduction technique in ct scan

-CT scan technique for dose reduction
-Iterative algorithm
- Ct scan technology

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Dose reduction technique in ct scan

  1. 1. Mohd Aiman bin Azmardi Pegawai Pengimejan, Jabatan Pengimejan Diagnostik Hospital Kulim 11th August 2015
  2. 2.  Introduction  Dose in CT Scan  Dose Reduction Technique  Equipment Design  What is dose in CT Scan..?  Acquisition Parameters  mAs  ATM  kVp  Image Reconstruction  FBP  Iterative Reconstruction  Scan Technique  Recommendation  Conclusion  References
  3. 3.  CT scan advancement in technology  Near to isotropic pixel – high resolution  Fast scan – cover ROI in short time  Advance post processing (e.g.: MIP, 3D reconstruction, VRT and many more)  Weakness?
  4. 4.  CT scan lead high dose compared to other imaging modalities  50 times riskier from plain abdomen radiograph  MSCT compared to single slice  Increase 10% and 34% of effective dose per patient  CT scan induce cancer arise rapidly  ALARA
  5. 5.  The most powerful dose reduction tool in radiation protection -justification of a study,  However, where an examination is undertaken, the emphasis must be on dose optimization,  achieving the required image quality at the lowest possible dose level.  This can be approached in two ways  design of dose efficient equipment,  through the optimization of scan protocols.
  6. 6.  Siemens intoduced in the SOMATOM Sensation 64 CT-system is equipped with the 0 MHU STRATON x-ray tube.  This newly developed X-ray tube offers  significantly reduced cooling times for shorter interscan delays  increased power reserves.
  7. 7.  Double Z sampling is two overlapping projections result  doubling the scan (more data)  without an increase in dose.  Advantages  Slice widht can be reduce e.g: it is possible to reconstruct 0.6mm slices at any pitch less than 1.5) with best image quality  Improve image quality  decreasing slice width  and removing windmill artifacts, without increasing the radiation dose Z axis direction
  8. 8.  Windmili artifact can be reduced which commonly found in MSCT images in the vicinity of sharp contrast in axial direction.
  9. 9.  Dual energy CT scans are a relatively new form of CT scanning that use separate X-ray energies to make images. Images can be generated:  by the simultaneous use of two X-ray tubes (‘dual source’);  by using an X-ray detector with separate layers to detect two different energy ranges (‘dual layer’); or  by using a single scanner to scan twice using two different energy levels (electronic kVp switching).
  10. 10.  Advantages  CT angiography. Dual energy scans can amplify the iodine signal of contrast agents, improving the delineation of arteries. They can also better distinguish iodine from calcium, therefore allowing better bone subtraction around vessels; for example, at the skull base.  CT of the kidney, ureter and bladder (CT KUB). Dual energy CT KUB scans can reliably distinguish urate from non-urate calculi.  CT imaging around metal implants. Dual energy CT can significantly reduce the streak artefact normally associated with metal implants and allow better visualisation; for example, around spinal rods or hip replacements.
  11. 11.  With fast kV-switching, Dual Energy data can be acquired by rapidly switching the tube voltage between CT projections.  Disadvantages  lower number of projections are available to create each image; reduced image quality  In addition, only the kVp can be modulated between individual projections. Resulting over- exposure in the highkV projections or under-exposure in the low-kV projections
  12. 12.  Idealized dual layer detector technology: In reality a certain amount of high and low-energy photons are registered in both layers which significantly reduces spectral separation  Disadvantages  Detector not able to distinguish between high and low energy photons. Both high and low energy photons are absorbed in both layers  The construction of this detector requires two photodiodes, which significantly increases electronic noise. Leads to inferior image quality for dual and single energy images.
  13. 13.  Slow kV-switching: Both kV and mA are switched between half rotations of the gantry, either in sequence or in spiral modes  Disadvantages  The time needed to switch from 80 kV to 140 kV and adjust the mA is typically in the order of 100 ms.  During this time, the patient is exposed to radiation that does not provide useful information.  Thus, this method does not follow the ALARA (“as low as reasonably achievable”) principle.
  14. 14.  Dual Energy imaging means that the system uses two X-ray sources simultaneously at different energy levels.  This makes it possible to differentiate  between fat, soft tissue, and bone,  and also between calcifications and contrast material (iodine) on the basis of their unique energy-dependent attenuation profiles.
  15. 15.  Volume Computed Tomography Dose Index (CTDIvol) is a standardized parameter to measure Scanner Radiation Output  CTDIvol is NOT patient dose  CTDIvol is reported in units of mGy for either a 16-cm (for head exams) or 32-cm (for body exams) diameter acrylic phantom  For the same technique settings, the CTDIvol reported for the 16-cm phantom is about twice that of the 32-cm phantom  The reported CTDIvol is based on measurements made by the manufacturer in a factory setting  In these slides, the term "patient dose" is used to describe the absorbed dose to a patient, while the generic term "dose" refers to CTDIvol
  16. 16. CTDIvol is not patient dose  The relationship between the two depends on many factors, including patient size and composition  AAPM Report 204 introduces a parameter known as the Size Specific Dose Estimate (SSDE) to allow estimation of patient dose based on CTDIvol and patient size  For the same CTDIvol, a smaller patient will tend to have a higher patient dose than a larger patient
  17. 17. Both patients scanned with the same CTDIvol Patient dose will be higher for the smaller patient CTDIvol = 20 mGy CTDIvol = 20 mGy 120 kVp at 200 mAs 120 kVp at 200 mAs 32 cm Phantom 32 cm Phantom
  18. 18. Smaller patient scanned with a lower CTDIvol Patient doses will be approximately equal CTDIvol = 10 mGy CTDIvol = 20 mGy 120 kVp at 100 mAs 120 kVp at 200 mAs 32 cm Phantom 32 cm Phantom
  19. 19.  CTDIvol provides information about the amount of radiation used to perform the study  CTDIvol is a useful index to track across patients and protocols for quality assurance purposes  CTDIvol can be used as a metric to compare protocols across different practices and scanners when related variables, such as resultant image quality, are also taken in account  The ACR Dose Index Registry (DIR) allows comparison across institutions of CTDIvol for similar exam types (e.g., routine head exam)
  20. 20.  The Dose Length Product (DLP) is also calculated by the scanner  DLP is the product of the length of the irradiated scan volume and the average CTDIvol over that distance  DLP has units of mGy*cm
  21. 21.  The relationships between acquisition parameters and CTDIvol described in the following slides assume all other parameters are held constant  The relationship between a parameter and CTDIvol is often described as proportional in some way  The symbol  is used to indicate “proportional to”  Directly proportional means that a change in the parameter results in the same change in CTDIvol  Example: Doubling the rotation time from 0.5 to 1.0 seconds will double the CTDIvol  Inversely proportional means that a change in a parameter has the opposite effect on CTDIvol  Example: Doubling the pitch from 1 to 2 will reduce the CTDIvol by half
  22. 22.  Acquisition Parameters define the technique that will be used and how the scan will proceed  Acquisition Parameters are set in the user interface where scans are prescribed  Changing a single Acquisition Parameter while holding everything else constant will typically affect the CTDIvol for that scan  The following slides describe what that affect is for each parameter
  23. 23. CT Scanners offer a variety of Scan Modes which describe how the table moves during an exam  Scan Modes include  Axial  Helical or Spiral  Dynamic The Acquisition Parameters that affect CTDIvol may change amongst different Scan Modes
  24. 24.  In the Dynamic Scan Mode multiple acquisitions covering the same body region are acquired. Examples of these study types include:  Perfusion Studies  Bolus Tracking Studies  Test Bolus Studies  Dynamic Scans often have large CTDIvol values because the scanner reports the sum of the CTDIvol values from each rotation  The reported CTDIvol is NOT skin dose or organ dose
  25. 25. Is the movement of the table through the bore of the scanner over a full 360 degree rotation Units: millimeters/rotation or millimeters/second The parameter is known both as Table Feed (helical/spiral acquisition) & Table Increment (axial acquisition) Table Feed affects CTDIvol through its inclusion in Pitch (discussed later)
  26. 26.  Is the combination of the number of data channels and the width of the detector associated with each data channel  The Detector Configuration determines the Beam Width or Beam Collimation (nT), which is the number of channels (n) times the detector width associated with each data channel (T)  For a selected detector width per data channel, a smaller total Beam Collimation usually has a higher CTDIvol than a larger Beam Collimation  Example: On a 16 slice scanner with a detector width per channel of 1.25 mm, a collimation of 4x1.25mm is generally less dose efficient than a collimation of 16x1.25mm Users should monitor CTDIvol values when changing detector configuration
  27. 27. Acquisition Parameter Settings
  28. 28.  Is the Table Feed per gantry rotation divided by the beam width/collimation  Pitch is the ratio of two distances and therefore has no units  Users should monitor other parameters when changing Pitch. The scanner may or may not automatically compensate for changes in Pitch (for example, by changing the tube current) to maintain the planned CTDIvol. CTDIvol 1/Pitch: Hitachi, Toshiba (no AEC) CTDIvol independent of Pitch: GE, Siemens, Philips, Neusoft, Toshiba (AEC)
  29. 29.  CTDIvol may not change in the expected manner if the scanner automatically adjust other parameters when the pitch is changed  The relationships between CTDIvol and pitch for the different vendors are described below  CTDIvol inversely proportional to change in pitch: Hitachi, NeuroLogica  CTDIvol constant when pitch is changed due to changes to other parameters: GE, Neusoft, Philips and Siemens  The relationship between CTDIvol and pitch depends on scan mode or Software version: Toshiba
  30. 30. Pitch < 1 Beam Width has some overlap at each view angle from rotation to rotation Pitch = 1 No overlap of Beam Width at each view angle and no view angles not covered at certain table positions Pitch > 1 Some view angles are not covered by the beam width at certain table positions Acquisition Parameter Settings
  31. 31. Determines the number of electrons accelerated across the x-ray tube per unit time Units: milliAmperes (mA) CTDIvol is directly proportional to Tube Current CTDIvol Tube Current
  32. 32.  Is the electrical potential applied across the x-ray tube to accelerate electrons toward the target material  Units: kiloVolts (kV or kVp)  CTDIvol is approximately proportional to the square of the percentage change in Tube Potential n old new kV kV       volCTDI
  33. 33.  Research done by reducing kVp improved enhancement of iodinated contrast media as a dye in the vascular and no significance increase in image noise.  Contrast to noise ratio(CNR) will increase at lower tube potentials, radiation dose may be reduced to achieve similar or improved iodine CNR compared to high kV. This will lead to reducing the dose to the patient.  With phantom 10cm the require dose at 80kV is only 35% of the amount required at 120kV and at 100kV the required dose is 62% of the amount required at 120kV.  With the 25cm phantom, the doses are 46% of the 120kV dose at 80kV and 63% of the 120-kV dose at 100 kV
  34. 34. Graph shows the relative radiation dose required at each tube potential to obtain the same iodine CNR For all three phantoms. For the 10-cm phantom, 35% of the 120-kV dose is required at 80 kV to achieve the same iodine CNR as at 120 kV, and 62% of the 120kV Dose is required At 100 kV. For the 25-cm Phantom, 46% of the 120kV dose Is requiredAt 80 kV, and 63% of the 120-kV dose is required at 100 kV
  35. 35. Axial contrast material–enhanced multidetector CT images obtained in a 58-year-old man during late hepatic arterial phase with (left) protocol A (140 kVp), (right) protocol B (80 kVp)((Marin et al., 2010)
  36. 36.  However selecting the appropriate kVp is not straightforward task  Scanning speed, motion artifacts, patient size and diagnostic task must be considered and carefully evaluated before the patients undergo CT scan examination
  37. 37. Is the product of Tube Current and the Exposure Time per Rotation Units: milliAmpere-seconds (mAs) CTDIvol is directly proportional to Tube Current Time Product CTDIvol Tube Current Time Product
  38. 38.  dose ~ tube current(constant tube potential, scanning time & slice thickness)  Weight or size based – 50% dose reduction  One research done reduction from 100mAs to 40mAs in sinus examination (findings – chronic and acute sinusitis)  Low dose (40mAs) can be clearly visualize; no significant difference dspite increase in noise (graininess)  Reduction should be made without degrade the image quality
  39. 39. Figure 1.Axial and Sagittal images of MDCT scans obtained at 100 mAs (A, B) and 40 mAs (C, D) at the level of the maxillary sinuses show complete opacification of the right maxillary sinus (star) and air-fluid levelin the left maxillary sinus (arrow). No significant difference in the diagnostic image quality of these tworadiological findings of these two scans.
  40. 40. Figure 2.Coronal reformatted images of MDCT scans obtained at 100 mAs (A) and 40 mAs (B) at the level of osteomeatal complex showing a normal right osteomeatal complex and a blocked left osteomeatal complex (arrow). This structure is clearly identified and correctly assessed in these two scans of this patient.
  41. 41.  Many CT scanners automatically adjust the technique parameters (and as a result the CTDIvol) to achieve a desired level of image quality and/or to reduce dose  Dose Modulation and Reduction techniques vary by scanner manufacturer, model and software version
  42. 42.  Automatically adapts the Tube Current or Tube Potential according to patient attenuation to achieve a specified image quality  Automatic adjustment of Tube Current may not occur when Tube Potential is changed  Centering the patient in the gantry is VITAL for most AEC systems  AEC aims to deliver a specified image quality across a range of patient sizes. It tends to increase CTDIvol for large patients and decrease it for small patients relative to a reference patient size The use of Automatic Exposure Control may decrease or increase CTDIvol depending on the patient size and body area imaged and image quality requested
  43. 43. Is the AEC parameter that is set by the user to define the desired level of image quality Changing the Image Quality Reference Parameter will affect the CTDIvol The effect on CTDIvol when changing the Image Quality Reference Parameter is vendor dependent Dose Modulation and
  44. 44.  A change in the Image Quality Reference Parameter will affect the CTDIvol  Setting the parameter for “increased” image quality (e.g., lower noise) will result in more dose  Setting the parameter for “decreased” image quality (e.g., more noise) will result in less dose Dose Modulation and
  45. 45.  Is an AEC feature that adjusts the Tube Current as the x-ray tube rotates around the patient to compensate for attenuation changes with view angle  Angular Tube Current Modulation is used to adjust the Tube Current to attempt to deliver similar dose to the detector at all view anglesThe use of Angular Tube Current Modulation may decrease or increase CTDIvol depending on the patient size and body area imaged and image quality requested Dose Modulation and
  46. 46.  Angular Tube Current Modulation uses information from one or two view localizers
  47. 47. Is an AEC feature that adjusts the Tube Current as patient attenuation changes in the longitudinal direction The CT Localizer Radiograph is used to estimate patient attenuation The use of Longitudinal Tube Current Modulation may decrease or increase CTDIvol depending on the patient size and body area imaged and image quality requested Dose Modulation and
  48. 48.  Longitudinal Tube Current Modulation uses information from one or two view localizers Dose Modulation and
  49. 49.  Is an AEC feature that incorporates the properties of both Angular and Longitudinal Tube Current Modulation to  Adjust the Tube Current based on the patient’s overall attenuation  Modulate the Tube Current in the angular (X-Y) and longitudinal (Z) dimensions to adapt to the patient’s shape The use of Angular and Longitudinal Tube Current Modulation may decrease or increase CTDIvol depending on the patient size and body area imaged and image quality requested Dose Modulation and
  50. 50. Dose Modulation and
  51. 51.  AEC is available in modern CT scan machine  Reduction dose 40 – 50% dose  The penetration adjust according to patient specific attenuation an all three planes  Technique; depend on patient size, attenuation profile and scanning parameter
  52. 52. Figure. Graph of tube current (mA) superimposed on a CT projection radiograph shows the variation in tube current as a function of time (and, hence, table position along the z-axis) at spiral CT in a 6-year-old child. An adult scanning protocol and an AEC system.
  53. 53. Is an AEC feature that allows for the tube current to be decreased or turned off over radiosensitive organs on the patient periphery, such as the breasts or eye lenses To maintain image quality, tube current may need to be increased at other view angles The use of Organ-Based Tube Current Modulation may reduce the absorbed dose to organs at the surface of the body but may increase the absorbed dose to other organs Dose Modulation and
  54. 54. Gantry Gantry Conventional Organ-Based Modulation Dose Modulation and
  55. 55. Is an AEC feature that selects the tube potential according to the diagnostic task and patient size in order to achieve the desired image quality at a lower CTDIvol The use of Automatic Tube Potential Selection is intended to decrease CTDIvol while achieving the image quality required for a specific diagnostic task and patient attenuation Dose Modulation and
  56. 56.  Tube Potential is not modulated in the same fashion as Tube Current  It does not change with different tube positions (view angles) around the patient  The Tube Potential for a specific patient, anatomic region and diagnostic tasks is selected and held constant for that acquisition, though it may be changed to a different tube potential for a different diagnostic task Dose Modulation and
  57. 57. 62 Reconstruct an image from tomographic scan data Schematic of Projection and Reconstruction
  58. 58. 63  How are we going to solve this problem ?  Filtered Back Projection
  59. 59. 64  How does BP work ?
  60. 60. 65  Another diagram that illustrates this concept
  61. 61. 66  Without filter: • Point object becomes widely spread out
  62. 62. 67  With Filter: • Cancellations in the vicinity of the object • The result is a sharper reconstructed image
  63. 63. 68  Process is linear - if it works for a point object (at any location), then it will work for any image
  64. 64. Is a feature that uses the information acquired during the scan and repeated reconstruction steps to produce an image with less “noise” or better image quality (e.g., higher spatial resolution or decreased artifacts) than is achievable using standard reconstruction techniques The use of Iterative Reconstruction by itself may not decrease CTDIvol; with use of Iterative Reconstruction, image quality will change and this may allow a reduction in the CTDIvol by adjusting the acquisition parameters used for the exam Dose Modulation and
  65. 65.  Iterative Reconstruction may be completed using data in Image Space, Sinogram Space or a Model Based Approach  Changing/Turning On the %/Level of the iterative reconstruction used may or may not affect the CTDIvol of the scan and will affect the image quality of the final set of images  In consultation, the Radiologists and Medical Physicists at an institution may adjust the acquisition parameters for studies reconstructed using iterative reconstruction based on the imaging task, the patient population, the desired image quality, dose concerns and the needs of the interpreting Radiologist Dose Modulation and
  66. 66.  According to the Dictionary.com the iteration is define as problem solving or computational method in which a the most close to the approximation  First an initial estimate of the x-ray photon from the tube is made. Second an estimate is made of the x-ray detector counts that would be acquired in each projection with the use of forward projection (Nelson et al., 2011). Then the estimated forward projection before will be compared with the actual measured projection acquired by the CT system detector array (Nelson et al., 2011). The comparison between the estimate and the actual measured will be use to update the original estimate (Nelson et al., 2011). Then the whole process will be continuously repeated that would result from the revised x-ray photon distribution. The process done will results in the estimation of x-ray photon closer to the actual photon distribution
  67. 67.  Siemens  IRIS: Iterative Reconstruction in Image Space  SAFIRE: Sinogram-Affirmed Iterative Reconstruction  Thosiba  AIDR 3D: Adaptive Iterative Dose Reduction 3D  GE  ASIR: Adaptive Statistical Iterative Reconstruction  MBIR: Model-Based Iterative Reconstruction  Philips  iDose4
  68. 68. (A) FBP image of the right shoulder in the coronal plane reconstructed with a soft tissue algorithm. Note the horizontal streak artifacts because of aliasing from low x-ray photon counts through the bones. (B) FBP image of the right shoulder in the coronal plane reconstructed with a bone algorithm. Note the horizontal streak artifacts because of aliasing are even more noticeable with this reconstruction. (C) MBIR image of the right shoulder in the coronal plane depicted with a soft tissue window and level. Note excellent depiction of the soft tissue about the shoulder and significantly fewer streak artifacts. (D) MBIR image of the right shoulder in the coronal plane depicted with a bone window and level. Note excellent depiction of the comminuted fracture of the coronoid process (arrow)
  69. 69. Filtered back projection (FBP) (a) and SAFIRE (b) reconstructions at the same level of the ascending aorta. Image noise expressed as the standard deviation of the attenuation (HU) in the region of interest was significantly lower in images reconstructed using iterative reconstruction (circle in B) compared with those reconstructed using FBP (circle in A).
  70. 70.  Information about the CTDIvol planned for each scan is typically displayed before the exam on the user console  Information about the CTDIvol delivered by each scan is typically reported in a data page or DICOM structured dose report  Dose information provided after the exam typically also includes the DLP and the CTDI phantom size. These may also be included in information displayed before the scan.
  71. 71.  CTDIvol is displayed before a study is performed based on the selected technique parameters  It is important to check CTDIvol before a study is performed to ensure that the output of the scanner is appropriate for the specific patient and diagnostic task CTDIvol is displayed for each planned acquisition Dose Display
  72. 72.  Development of technology in CT scan make essential for radiographer cope with new skills and techniques.  Training should be provided  Increase knowledge  Know your THINGS. Avoid becoming PUSH BUTTON RADIOGRAPHER..!!
  73. 73.  Radiographers are integral part of a multidisciplinary team whose responsibility is to treat and manage patient came to the imaging department.  The role of the radiographer is to take care of the patients’ imaging before proceed to other treatment or investigation.  Current advances in technology have made it essential for nowadays radiographers to constantly learn and cope with new skills.  The increasing usage of CT scan facilities increases radiation doses to the staff and the population and this create a full awareness for continuous efforts in reducing the dose level.
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  77. 77. Mohd Aiman bin Azmardi Peg. Pengimejan Hospital Kulim

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