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Cardiac CT Basic Principles

Cardiac CT Basic Principles. David M. Whitaker, MD. Outline. Steps to get a CT Types of CT Acquisition Brief history of multislice CTs ECG Gated Reconstruction ECG Synchronization Patients with fast heart rates Radiation Exposure & ECG modulation. CT Basics. Steps involved

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Cardiac CT Basic Principles

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  1. Cardiac CT Basic Principles David M. Whitaker, MD

  2. Outline • Steps to get a CT • Types of CT Acquisition • Brief history of multislice CTs • ECG Gated Reconstruction • ECG Synchronization • Patients with fast heart rates • Radiation Exposure & ECG modulation

  3. CT Basics • Steps involved • Data acquisition • Image reconstruction • Post-processing • Evaluation • Reporting & Communication

  4. CT Basics • Data Acquisition • X-ray source produces a collimated beam in the shape of a fan • As it passes through object – photons are either absorbed or scattered • Attenuation = reduction in X-ray transmission • Depends on anatomic composition & density of tissue & energy of the photons • After passing through patient – X-rays collected by detectors – converted to electrical/digital signals • Since the tube and detectors rotate around patient, a large number of projections are collected from a large number of angular positions

  5. CT Basics

  6. CT Basics • Image Reconstruction • Must be pre-processed to correct for beam hardening and scattered radiation • Raw data then filtered by convolution kernels • Because the pt moves through gantry during scan, X-ray measurements acquired at different longitudinal positions • To reconstruct a slice at a particular position, attenuation measurements from > 180o of the X-ray tube rotation are required • A back-projection technique then used to calculate density values within the plane

  7. CT Basics • Image Processing • Digital axial source images can be converted to electrical signals and shown in gray-scale on a screen • For CCT however, usually requires some post-processing prior to evaluation.

  8. CT Basics • Data Storage • A single CT slice is approximately 500kB • A complete cardiac study (containing more than 200 slices) will require at least 500MB

  9. CT Basics • Image Display • CT density values are expressed as Houndsfield Units (HU) • Range from –1024 to +3071 HU • Theoretically the entire range could be displayed in a sliding scale from black to white but human eye cannot distinguish such fine detail in the middle of the range • Therefore the images must be displayed with settings that provide optimal contrast between structures

  10. CT Basics

  11. Types of CT Acquisition • Cardiac Sequential CT • Can do ECG-triggered acquisition • After an R wave the acquisition is initiated and performed during the predicted diastolic phase based on the R-R interval of the previous cycles. Table then moves and waits for next R wave • Image quality of triggered multi-slice CT regarded inferior compared to spiral, but the radiation is used more efficiently and thus often used for calcium scoring in the preventative setting

  12. Types of CT Acquisition • Electron Beam CT (EBCT) • A non-mechanical sequential CT scanner • No mechanically rotating parts • Temporal resolution is 100ms • Acquisition time is about 50ms • Can be triggered by ECG • Can get a single or up to 3 acquisitions of a single slice – thus can chose the best data set

  13. Types of CT Acquisition • Spiral CT • Table moves at a constant speed while the detectors continuously acquire data • Larger sections can thus be scanned in same amount of time • Slip ring technology made this possible as cord wrap around is not an issue

  14. Types of CT Acquisition

  15. Types of CT Acquisition • Sequential CT • Shoot and move • Time inefficient & only limited coverage can be achieved in the same amount of time

  16. Types of CT Acquisition • Multislice CT • Instead of a single detector row, have several parallel detector rows • Allows simultaneous acquisition of several slices • Again, larger sections scanned in shorter time

  17. Evolution of Multislice CT • 1998 – Four-slice scanners • Rotation time of 500ms • Slice thickness varying from 0.5 to 1.25mm • Using partial scan algorithms the reconstruction time reduced to about 250 ms • This time proved sufficient to visualize coronary arteries without motion artifact during diastole

  18. Evolution of Multislice CT • 2002 – 16-slice scanners • Rotation time of newer scanners was reduced below 400 ms • Slice thickness between 0.5 and 0.75mm • A complete scan could be completed in less than 20 seconds

  19. ECG Gated Reconstruction • Imaging the heart requires acquisition that is synchronized to the motion of the heart • Sequential scanners acquire slices prospectively triggered by the pt ECG • Spirals acquire continuous, overlapping data throughout the cardiac cycle • The recorded ECG is used afterwards to select spiral data acquired during the same cardiac phase to reconstruct slices • Reconstruction of any cardiac phase can be performed

  20. Patients with Fast Heart Rates • Qualitative assessment of lumenal narrowing is most accurate when HR is less than 60-65 bpm • Temporal resolution is limited, thus much effort put into developing technology with faster rotation speeds • Also referred to as “pitch”

  21. Patients with Fast Heart Rates

  22. Patients with Fast Heart Rates • Use oral or IV beta blockers to achieve slower HR unless contraindicated • If unable to use BB • Alternative scanning & reconstruction methods are available • The effectiveness of these alternative algorithms varies with the actual HR in relation to the rotation time of the scanner

  23. Patients with Fast Heart Rates

  24. ECG Synchronization • Cardiac phase with minimal coronary displacement is mid- to end- diastole • Reconstructions can be performed at time positions relative to the previous or upcoming R-wave • Or a position as a percentage of the R-R cycle can be selected

  25. ECG Synchronization

  26. ECG Synchronization Pitfalls • Requires a good ECG signal • Avoid electrical interference from underlying musculature etc… • Software detects the up-slope or R wave and call this T=0 – Leads I and II usually the most useful • If abnormal rhythm or unusual ECG pattern, other leads should be used

  27. Radiation Exposure • Determinants of total exposure • Scanner Parameters • Scanner Type • X-ray output (kV and mA) • Pitch • Collimation: slice number and width • Scan Range • ECG triggered X-ray tube modulation • Patient Gender, Size, Heart Rate

  28. ECG Gated Reconstruction • Radiation exposure can be reduced by modulating the X-ray tube prospectively • Guided by the ECG, output is decreased during the systolic phase while full output occurs during an interval within the diastolic phase • Total radiation exposure can be reduced by 30-40% depending on the heart rate

  29. ECG Gated Reconstruction

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