CT Multi-Slice CT

Third Generation CT Single or Multislice Z-axis orientation perpendicular to page Patient

Single Slice Thickness Determined by Collimation Z Detector

Single-Slice Detectors Many detectors rotate around patient Single row in z-direction Slice thickness determined by collimation Z-Axis

Single Slice CT: Slice Thickness Collimated Beam Thickness Collimated Beam Thickness Thin slice Thick slice Z-Axis Z-Axis

Multi-slice CT Tube Post-collimator Detectors

What’s Different for Multislice CT?

Multislice CT Multiple rows of detectors Open collimators in “Z” direction 4 3 2 1 http://www.veterinary-imaging.com/images/MSS_CT.gif

Multi-slice CT Developed in late 1990’s Detector array segmented in z-direction Simultaneous acquisition of multiple slices http://www.ctisus.com/gallery/images/multidetector/multislice_ct.jpg

Single Slice vs. Multislice Detector Collimated Beam Thickness Single slice detector Multislice detector Z-Axis

Multi-Slice Detectors Many detectors going around patient Many detector rows in z-direction Slice thickness determined by Collimation electronic detector selection “Z” Direction Single Multi

Multi-slice CT Size & distribution of detectors in non-axial direction similar to previous CT’s Similar spatial & contrast resolution

Distribution of detectors in axial direction varies with manufacturer All detectors same width Variable widths “Z” Direction

Multi-slice CT Uniform Detector Thickness Multiple detectors in axial direction Size must accommodate thinnest slice Detector signals can be used Individually In groups “z” direction 1 2 3 4 Four thin slices 1 2 3 4 Four thicker slices

Detectors vs. Channels # Physical Detectors not necessarily equal to # of possible Slices Maximum # slices limited by Digital Acquisition System (DAS) channels Electronic counters Imaging speed bottleneck How fast data can be received from detector arrays

Detectors vs. Channels Example

Multi-Slice Detector Example 16 Detector Rows, 4 Channels

Detectors vs. Channels 4 X 1.25 mm Beam collimated to 4 detector rows 1 detector row per DAS channel Effective Detector

Detectors vs. Channels 4 X 2.5 mm Beam collimated to 8 detector rows 2 detector rows per DAS channel Effective Detector

Detectors vs. Channels 4 X 3.75 mm Beam collimated to 12 detector rows 3 detector rows per DAS channel Effective Detector

Detectors vs. Channels 4 X 5 mm Beam collimated to 16 detector rows 4 detector rows per DAS channel Effective Detector

Capture Efficiency Fraction of detector area that is active detector

Equal-width Detectors Disadvantage Many gaps Gaps are dead space Reduce capture efficiency

Multi-slice CT “Adaptive Array Detectors” “z” direction Some scanners use detectors of various widths Post-collimators used to partially block wider elements for thinner slices 1 2 3 Three thicker slices Post-collimators 1 2 3 4 Four thinner slices

Variable Width Detectors Center detectors thinner Thicker detectors can function as thinner ones using collimation Thinner detectors can function a thicker one by combining signals

Single Slice Pitch Definition table motion during one rotation Slice Pitch = --------------------------------------- slice thickness

Beam Pitch Defined only for Multi-slice scanners table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness Beam thickness

Beam Pitch Defined only for Multi-slice scanners

CT Beam Pitch

Example 5 mm slices 4 simultaneous slices Beam pitch = 1 1 revolution / sec. Beam thickness? Table speed? table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness

Beam Thickness 5 mm slices 4 simultaneous slices Beam pitch = 1 1 revolution / sec. table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness Beam Thickness = 5 X 4 = 20 mm

Table Speed 5 mm slices 4 simultaneous slices Beam pitch = 1 1 revolution / sec. 20 mm beam thickness table motion during one rotation Beam Pitch = --------------------------------------- Beam thickness Table speed = 20 mm rotation (1 sec) = 20 mm / sec

Slice Thickness Defined at Rotational Center Tube

Detector Field must be Larger than Slice Thickness at Rotational Center Diverging Beam Rotational Center

Beam Divergence More of a Problem for Multi-Slice Rays diverge No longer essentially parallel Leads to Cone Angle Artifact Significant for 16, 32, 64 … data channels Requires use of special reconstruction algorithms to compensate

Multislice CT Doses Can be 10-30% higher than for single slice units (ICRP #47) Cause Divergent beam Other considerations Tendency to cover more volume (anatomy) Better availability of equipment

Other Reasons for High CT Doses Repeat Exams No adjustment of technique factors for different size patients No adjustment for different areas of body

Multislice CT Advantage? Speed!

Single slice / Multislice Images about the same!

Speed = Power Speed enables new applications

How do we spend our new speed?

Multi-slice CT Imaging Clinical Advantages Thinner slices for improved z-direction resolution Same acquisition in shorter time Scan larger volumes in same time

Multi-slice CT Imaging Clinical Advantages Thinner slices Improvement in CTA of neck, aorta, renal vessels Better reconstructions Sagittal, coronal, oblique 3-D Fundamental Trade-off “z” axis resolution vs. image noise

Multi-slice CT Imaging Clinical Advantages Improved x-ray tube utilization Reduced x-ray tube loading 4 slices acquired with same tube loading previously used for 1 Less need to pause of tube cooling Reduced wear & tear Other anticipated benefits CT endoscopy Diagnosis of pulmonary embolism

Multi-slice CT Imaging Clinical Advantage: Angiography Simplifies contrast bolus timing Continuous observation of target vessel Can reduce amount of contrast required Coverage from aorta to lower extremities Runoff CTA ~ 13% of all CT procedures

Continuous CT Imaging Interventional Procedures Biopsy & drainage Neuro Chest Abdominal Spine Catheter and tube placement Helps operator avoid critical structures near path of biopsy needle Better visualizing of moving structures Respiration Functional CT Brain perfusion

Multi-Slice Compared to Single-slice helical Much Faster No significant image quality differences Equivalent Patient Dose Ref: Willi Kalender, Ph.D Institute of Medical Physics University of Erlanger, Germany

Organ Coverage Time Depends On Beam Pitch Rotation Time Detector Acquisition Length

64 Slice CT Typical Coverage Times Heart & coronary arteries / brain / lungs 5 seconds Whole body coverage for blood clot search 30 seconds Philips

64-Slice Commercial Cardiac CT IGE Philips Siemens (1 tube) Siemens (2 tube) Toshiba Min. Rotation Time (s) .35 .53 .33 .4 Detector length (mm) 40 19.2 32 Time to cover heart (120 mm) (s) 5.3 6.3 10.3 5.1 7.5 www.impactscan.org

What’s Next? Slice Wars Toshiba 320 Slicer Philips 256 Slicer

Implications of 256+ Slices Full organ coverage in single rotation 12-16 cm coverage Improved temporal resolution Reduced artifacts Whole-organ function (perfusion) studies Functional data perfectly registered to anatomic data

The Future More slices Flat panel area detectors ???

Multi-slice challenges: More Slices Computer issues More archival capacity Requires faster computer systems Requires faster communications for remote viewing Radiologist responsible for all images

Acknowledgement Many drawings obtained from www.impactscan.org website