CT physics and instrumentation

1. CT physics and instrumentation Lecture (7) Multi-Slice Computed Tomography (MSCT) And Clinical application RSSI 471 Prepared by Mr. Essam Mohammed Alkhybari

2. Staff contact information: • Mr. Essam Mohammed Alkhybari • Radiological science and Medical Imaging Department • Lecturer in Nuclear Medicine stream • E-mail: e.alkhybari@sau.edu.sa

3. Objective: • Background • MSCT principle • Dual source CT scan • Clinical application of MSCT scan

4. Background: the evolution of MSCT scanners, including the DSCT scanner • 1989: Single slice spiral/Helical CT scanner= 1 slice per revolution • 1992:Dual slice CT scanners= 2 slice per revolution • 1997: Four slice CT scanners= 4 slices per revolution • 2000: 8-16 and 32-40 slice CT scanner=8,16,32, and 40 per revolution • 2004: 64 slice scanners= 64 slice per revolution • 2006: Dual source CT scanner= 2 x-ray Tubes coupled 2 detectors arrays • 2006/2007: 320 and 256 slice CT scanners 320 & 256 slice per revolution • Future: ?

5. Background: • Slip ring scanners and helical (spiral) CT were rapidly adopted and became the standard of care for body CT • However, a significant problem became evident: helical CT was very hard on x-ray tubes. • As a result, the more rotation of x-ray tube, creates huge amounts of heat which effects the ability of the scanner.

6. Background: • A straightforward solution to this heat issue, of course, is to develop x-ray tubes with a higher heat capacity • Another approach is to more effectively use the available x-ray beam: if the x-ray beam is widened in the z-direction (slice thickness) and if multiple rows of detectors are used, then data can be collected for more than one slice at a time • This approach would reduce the total number of rotation and therefore the total usage of the x-ray tube needed to cover the desired anatomy. This is the basic idea of MSCT.

7. MSCT Detectors: • The primary difference between single-slice CT (SSCT) and MSCT hardware is in the design of the detector arrays • In MSCT, each of the individual, monolithic SSCT detector elements in the z-direction is divided into several smaller detector elements, forming a 2-dimensional array Rather than a single row of detectors encompassing the fan beam, there are now multiple, parallel rows of detectors

8. Multi-Slice Spiral/Helical CT: • This types of scanner referred to as volume CT (VCT) Systems because covering entire body sections is easily accomplished in a sing breath holds • The advantage of MSHCT include isotropic viewing, longer anatomic coverage, multiphase studies , faster examination times, and improved spatial resolution • The advancement of VCT, with increasing larger detectors arrays, promises to provide unique clinical opportunities in diagnostic medicine

9. MSCT CONCEPTS: differences between MSCT AND SSCT Slice Thickness: Single Detector Array Scanners: • The slice thickness in single detector array CT systems is determined by the physical collimation of the incident x-ray beam with two lead jaws. Slice Thickness: Multiple detector Detector Array Scanners: • The slice thickness of multiple detector array CT scanners is determined not by the collimation, but rather by the width of the detectors in the slice thickness dimension

10. Cone beam and Fan beam geometry: • A cone beam geometry produce more beam divergence along the z-axis direction compared with fan beam geometry • For this reason increasing the number of detectors rows in MSCT crates a need for a different approach to the interpolation process because the rays that contribute to the imaging process are more oblique • Additionally, the number of detectors rows plays an important role in slice thickness selection and volume coverage

11. Dual-source CT scanner • This dual source CT scanner (DSCT) that features two x-ray tubes and two detectors specifically intended for imaging cardiac patients in a very short time

12. Dual-source CT scanner: Major Technical components: • Tow data acquisition system (DAS) offset by 90 degree • Two x-ray tube coupled to two separate detector system • Det A which covers the entire scan FOV(50 cm in diameters) • Det B which covers smaller central scan FOV (26 cm in diameter) • Each x-ray tube is STARTON type that uses the z-flying focal spot technique and cone beam geometry to image 32-slices combined to produce 64 slices per revolution • Each x-ray tube can be operated separately, so the scanner can perform dual energy imaging, where one tube operate at 80 kVp and other at 140 kVp

13. Fast + Poor Image quality Fast + Improved Image quality Dual Source CT Single source CT

14. Clinical application of MSCT scan: The new helical scanning CT units allow a range of new features, such as : • CT angiography • CT fluoroscopy, where the patient is stationary, but the tube continues to rotate • multisliceCT, where up to 64 (128 - 256) slices can be collected simultaneously • 3-D CT and CT endoscopy • Cardiac image acquisition during relevant heart phases (ECG pulsing synchronization)

15. CT angiography (CTA): • CT angiography defines as CT imaging of blood vessels opacified by contrast media • Images can be captured when vessels are fully opacified to demonstrate either arterial or venous phase enhancement through the acquisition of both data sets ( arterial and venous).

16. CTA

17. CT Fluoroscopy: • Real Time Guidance(up to 8 fps) • Great Image Quality • Low Risk • Faster Procedures • Approx. 80 kVp, 30 mA

18. CT Fluoroscopy: CT fluoroscopy is based on three advances in CT technology: Continuous scanning mode made possible by spiral/helical scanning principle Fast image reconstruction made possible by special hardware performing quick calculations and a new image reconstruction algorithm Continuous image display by use of cine mode at frame rates of two to eight images per second.

19. CT Endoscopy-Virtual Reality(VR) Imaging: • VR is a branch of computer science that immerses users in a computer-generated environment and allows them to interact 3D scenes. • A virtual endoscope is a graphic based software system used for simulating endoscopic exploration inside a 3Dimage. • In virtual endoscopy, a 3D image acts as ‘copy’ or virtual environment, representing the scanned anatomy

20. Types of Coronary CT imaging: • MSCT (multislice CT) : A new form of cardiac imaging. This is a way to measure obstruction similar to a cardiac catheterization. It is NOT a functional test. • Coronary Calcium Scoring: calcium scoring for risk assessment. This is for asymptomatic patients and is not yet recommended as a routine screen. (CCS can be normal in 5% of patients who have myocardial infarcts)

21. MSCT Coronary Angiography: Details • Initial studies 5-10 years ago used 4-slice MSCT. Then came 16-slice (we still use these in some centers) and now 64-slice MSCT is arriving in just the last few years. • The 64-slice CT is the current standard (approved in 2004), can handle faster heart rates • 256-slice CT angiograms are just starting to be evaluated.

22. 64-Slice CT Scanner • More coverage (volume) with each heart beat • Entire heart imaged in 5-15 seconds • Less contrast required • No increase in rotation speed, but with overlapping slices, can use segments from different heart beats to improve temporal resolution

23. 3-D Volume Rendered Image

24. Questions…!