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CT Scanning: Dosimetry and Artefacts

CT Scanning: Dosimetry and Artefacts. Dr. Craig Moore Medical Physicist & Radiation Protection Adviser Radiation Physics Service CHH Oncology. Operator Controlled Variables and their effects on Image Quality and Patient Dose. &. Imaging Performance. Spatial Resolution Significance

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CT Scanning: Dosimetry and Artefacts

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  1. CT Scanning:Dosimetry and Artefacts Dr. Craig Moore Medical Physicist & Radiation Protection Adviser Radiation Physics Service CHH Oncology

  2. Operator Controlled Variables and their effects on Image Quality and Patient Dose &

  3. Imaging Performance • Spatial Resolution • Significance • Factors affecting resolution • Z-sensitivity • Significance • Factors affecting z-sensitivity

  4. In-plane Spatial Resolution • Maximum that can be achieved is about 20 lp/cm (but usually less) • Typical matrix size is 512 x 512 • In the scan plane limited by pixel size • Pixel size = FOV/matrix • i.e. if FOV = 40cm and matrix = 512 then pixel size = 0.8 mm • Pixel size determines limit of resolution but there are other factors that affect resolution on a scanner

  5. In-plane Spatial Resolution • Other factors affecting resolution: • Filter used for back-projection • Size of focal spot, geometry of the scanner and size of detectors • Sampling frequency (number of times the x-ray beam is sampled as it rotates around the patient) • Spatial resolution the same for axial and helical scanning

  6. z-plane Resolution • Spatial resolution in the z-direction (parallel to patient) dependent on pitch • The greater the pitch the lower the resolution

  7. Summary of spatial resolution • In-plane spatial resolution and z-resolution are usually thought of as different parameters • In fact, z-resolution is an extension of spatial resolution in third dimension • Multi-slice scanning is moving CT away from slice based medium to a truly 3D modality • Structural features are 3D, so resolution should be equal in all dimensions

  8. Image Noise

  9. Sources of noise (1) • Quantum Noise: • Randomness of photon detection • This type of noise is the most dominant in CT

  10. FOV

  11. Sources of Noise (2) • Structure noise: • Affected by back-projection filters

  12. Sources of Noise (3) • Electronic noise – small compared to other sources

  13. Increased contrast Pixel number Increased noise Image Contrast & Noise • Contrast is equivalent to the difference in CT number between an object and its surrounding tissue • When viewing objects which have CT numbers close to background noise can mask detail

  14. Low Contrast Resolution: • Measure of how well a system can differentiate between an object and its background having similar attenuation coefficients Low Medium High

  15. Low contrast resolution is important when small contrast differences are crucial for early detection of disease • Low contrast exams account for approximately 90% of CT scans • Affected by all parameters that influence noise • Minimum detectable contrast is <0.5% Low contrast of a few HU

  16. Principles of CT Dosimetry

  17. Dose in CT • One of the highest dose techniques used in medical imaging • Within the UK it has been shown to contribute to 40% of the total dose attributable to medical exposure • But only 4% of the total number of exams

  18. Radiation Units • Absorbed dose in CT • CT Dose Index (CTDI) (mGy) • Radiation risk in CT • Dose Length Product (DLP) (mGy cm) • Effective dose (mSv)

  19. Dose Distribution in CT • Absorbed dose not a single value • Dose values vary with position in patient in scan plane and along z axis

  20. Dose Distribution • Depends on • Filtration • Beam shaping • Scanner geometry • Size of patient • More uniform for • Higher filtration • Optimised beam shaping • Smaller patient • Periphery:centre • Body 2:1 • Head 1:1

  21. CT Dose Index (CTDI) • Measure of dose in the scan plane from a single rotation • CTDI defined as: • Dose at position z, Dz is integrated over the complete dose profile and divided by slice thickness T

  22. Dose Profile along z patient

  23. Measurement of CTDI • Routinely measured with air filled pencil ion chambers • Use PMMA phantoms to simulate patient • Single rotation with chamber: • At the centre • At the edges • Calculate CTDIcentre and CTDIedge

  24. Complete cross section of dose

  25. HIGHER DOSE ‘MEDIUM’ DOSE LOWER DOSE

  26. Pitch typically between 1 and 2

  27. HIGHER DOSE HIGHER DOSE LOW DOSE (mA typically 100 to 200)

  28. HIGH DOSE LOW DOSE MUST INCREASE mA and/or s kV typically between 80 and 140 kV

  29. For the same pitch

  30. Effective Dose • To estimate the stochastic risk to the patient, must consider scan length (DLP) and anatomical location of scan • Effective dose used • This is the equivalent whole body dose

  31. x 7

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