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CTA Physics and Dosimetry

CTA Physics and Dosimetry. Nausheen Akhter, MD Division of Cardiology University of Illinois at Chicago December 21, 2007. Contents. Review of X-ray Production CT Components CT Scanner Generations CT Image Formation and Reconstruction Retrospective vs. Prospective Digitalization

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CTA Physics and Dosimetry

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  1. CTA Physics and Dosimetry Nausheen Akhter, MD Division of Cardiology University of Illinois at Chicago December 21, 2007

  2. Contents • Review of X-ray Production • CT Components • CT Scanner Generations • CT Image Formation and Reconstruction • Retrospective vs. Prospective • Digitalization • Hounsfield Unit • Radiation Dosimetery

  3. The CT Scan Defined • Computed tomography is an imaging modality that uses x-rays to image cross-sectional slices through the body. • X-rays are directed into a localized section of the patient. • Attenuated x-rays completely penetrate the patient and are detected. • Detected signals are then constructed into an image.

  4. Three paths of an electron (e-): (1) e- may directly collide with nucleus (unlikely) (2) e- may enter vicinity of nucleus, but not collide (3) e- collides with inner shell e- and outer shell e- shifts position X-ray Production

  5. X-ray Production • Paths (1) and (2) cause e- to lose much energy which is converted into x-ray photons. • Bremsstrahlung phenomena (aka. braking radiation): x-ray photons are created by paths (1) and (2) • Characteristic radiation: x-ray photons created by path (3) • 10-12% of x-ray beam emitted from CT x-ray tube

  6. X-ray Production

  7. CT Components • The gantry: • Aperture • Contains a rotating frame assembly (x-ray tube, detector array, data acquisition system) • Contains slip-ring and high voltage generator

  8. The patient table: X: Left to Right (saggital) Y: Anterior to Posterior (coronal) Z: Head to Feet (axial) CT Components

  9. The x-ray tube Generates and focuses the x-ray beam to strike a target material (Tungsten) and produce photons. Two electrodes: rotating anode and cathode High voltage generator CT Components

  10. Tube Current (mA) • When tube voltage (kV) is applied across the cathode and anode at a specific current (mA), the e- beam flows from the cathode and collides with the target material on the anode. • The cathode contains a heating filament wire. The temperature of this filament controls the current of e-. • Temperature of the filament is controlled by the tube current (mA). • The tube current (mA) determines the number of x-ray photons produced. • Higher mA  hotter filament  greater e- escape and create photons  greater x-ray dose to the patient.

  11. Tube Voltage (kV) • Tube voltage (kV) controls the x-ray photon energy level. • X-ray photon energy level determines how easily the x-ray penetrates the patient’s body and image quality. • 120 kV are higher energy and more penetrating than 100 kV.

  12. Collimation Prevents ‘scatter radiation’ and minimizes radiation dose Restrict the path of x-rays and define slice thickness Two types: pre-patient and post-patient CT Components

  13. CT Components • Detectors • In order for an x-ray photon to generate a signal, four steps must occur: (1) X-ray must enter a detector (ie. ‘capture’) (2) X-ray must collide with a detector atom (3) The collision must result in an electromagnetic conversion suitable for measurement (eg. Light) (4) This event must be amplified and conducted to a data acquisition system.

  14. Detectors Two types: gas (xenon) and solid-state Solid-state (aka. scintillation detector) is made of a solid crystalline substance which is more sensitive to x-rays at various angles. CT Components

  15. Linear Attenuation Coefficient • X-ray photons are attenuated via two processes: absorption and scatter • Amount of attenuation is dependent on: • Atomic number of the tissue, density of electrons in the tissue, thickness of the tissue, and energy of the x-ray photons • Lower energy photons are more easily attenuated than higher energy photons.

  16. Linear Attenuation Coefficient • Exponential decay • The number of photons detected is reduced in an exponential fashion as the thickness of the tissue traversed is increased.

  17. CT Components • Array processor • Primary function is the reconstruction of the projected attenuation raw data into a CT image.

  18. First Generation X-ray tube generated a thin ‘pencil beam’. Single detector Tube and detector translated across gantry while patient stationary Rotation angle 1 degree Scan time: 5 minutes CT Scanner Generations

  19. Second-generation X-ray ‘fan beam geometry’ Detector array (30) Rotation angle >5 degrees Scan time: 20 sec CT Scanner Generations

  20. Third generation Fan-beam x-rays Curved arc detector array (improved reproducibility of data, >750 detectors) Tube/detector array rotates around patient with a ‘continuous slip ring’ Scan time: < 1 sec Fourth generation CT Scanner Generations

  21. Scanning methods: The scout film Conventional/Axial/ Sequential CT Helical/Spiral CT CT Image Formation

  22. CT Image Formation • Conventional/Axial/Sequential CT • The x-ray tube rotates around a stationary patient. The patient table must be incremented after each image is acquired. • For a third generation scanners, a ‘projection’ is made up of all the x-rays emanating from the tube in one position. • Data measured at each projection is called the raw data.

  23. Helical/Spiral CT Continuous data collection. Tube rotates around the patient as the patient table moves at a fixed speed through the gantry. Need slip-ring scanner and high heat capacity tube The slice thickness of a helical scan is defined as the table increment/pitch CT Image Formation

  24. Advantages vs. Disadvantages • Advantages of Helical Scanning • More coverage in a breath-hold • No misregistration of slices • Less contrast injection • Gapless slices • Images may be constructed at any arbitrary position • Multiplanar reconstruction and 3D • Disadvantage of Helical Scanning • Continuous radiation

  25. Image Reconstruction • Most CT systems use a ‘filtered back projection algorithm’ for reconstruction. • Back projection is a summation technique of all of the projections. • A correction filter (aka. reconstruction filter or kernel) is applied to accentuate the edges and allow for exact representation of the original object. • ‘Convolution’ is the process of applying filtration.

  26. Filtered Back Projection

  27. Retrospective Reconstruction • Simultaneous recording of ECG with continuous x-ray data acquisition. • The ECG is retrospectively used to assign images to the respective phases of the cardiac cycle. • Retrospective, phase-specific segments of the R-R intervals are combined to reconstruct an axial slice. • Strength: oversampling allows for gap-less and motion-reduced imaging • Drawback: higher radiation

  28. Retrospective Reconstruction

  29. Prospective Reconstruction • “Step and shoot modality” • Images obtained at a pre-determined offset of the ECG-detected R wave • Only 1 image per detector per cardiac cycle • Strength: lower radiation • Drawbacks: inability to perform overlapping images, longer acquisition time

  30. Multi-planar Reconstruction and 3D • Multi-planar reconstruction is a post-processing technique that requires only the image data to reconstruct images in planes other than the plane of the original scan. • 3D surface rendering is another post-processing technique that reconstructs images of the surfaces of anatomical structures.

  31. Contrast Media Injection • Scan delay is determined by the contrast transit time • Bolus tracking technique • The coronary CT imaging sequence is initiated when the contrast enhancement in the ascending aorta reaches 100 Hu. • Timing bolus technique • Images acquired at level of carina every 2s starting 10s after injection of constrast (10-20 mL), followed by saline flush (30-50 mL).

  32. Timing Bolus Technique, Scan delay 20s

  33. Digitalization • The first step in generating a digital image is to divide the object into a grid of small regions. • Each region of the grid is called a ‘pixel’. • A grid of pixels is the image matrix. • Assign shades of gray to the pixels. • Gray-scale values for pixels within the reconstructed tomogram are defined as CT numbers or “Hounsfield units” (HU).

  34. Hounsfield Unit (HU) • CT uses X-ray absorption to create images which differ in brightness depending on their physical density (atomic composition). • HU are defined with a reference to the value for water. • Higher densities will be brighter, lower densities darker.

  35. Hounsfield Unit (HU)

  36. CT Parameters • Spatial Resolution • Measure of the size of the smallest object that can be visualized in an image • Directly related to the slice thickness and reconstruction matrix • Best cardiac CT spatial resolution (x, y, and z-axes) is 0.6 x 0.6 x 3mm (nearly cubic/isentropic voxel) • Temporal Resolution • Precision of a measurement with respect to time • High temporal resolution needed to minimize coronary motion artifact • Determined by pitch, gantry rotation time, and the patient’s heart rate

  37. Radiation Dosimetery • 50% of a person’s lifetime radiation exposure is due to medical testing. • Three primary factors that go into radiation dosimetry: x-ray energy (kV), tube current (mA), exposure time. • ‘Effective dose’ is an estimate of the uniform, whole-body equivalent dose that would produce the same level of risk for adverse effects that results from the non-uniform partial body irradiation. • The unit for the effective dose is the milliSievert (mSv).

  38. Radiation Dosimetery

  39. Radiation Dosimetery • There are various choices with MDCT that can dramatically change the patient radiation dose. • Dose modulation was a large advancement. • Tube current (mA) is reduced during systole by 80%, then the full dose is used in diastole (40-80% R-R interval) • May reduce radiation exposure from 30-50% • Slower heart rates, less systole, reduction is less.

  40. Radiation Dosimetery • Other methods to decrease radiation: • Current (mA) can be increased/decreased, most often based on body habitus. • Increase the pitch (table speed/collimator width) • Lower pitch = more radiation • Slice thickness can be increased (faster scan). • Prospective reconstruction

  41. Articles to Review • Morin RL et al. “Physics and dosimetry in computed tomography” Cardiol Clin 21 (2003), pp. 515-520 • Budoff et al. Cardiac CT Imaging: Diagnosis of Cardiovascular Disease • Chapters 1 and 2 • CT Registry Review Program • Chapters 1, 2 and 3 • Budoff et al. “ACCF/AHA Clinical Competence Statement on Cardiac CT and MR” JACC Vol. 46, No. 2, 2005

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