CT

Multi-Slice CT

Third Generation CTSingle or Multislice

Patient

Z-axis orientation perpendicular to page

Single Slice Thickness Determined by Collimation

Detector

Z

Single-Slice DetectorsMany detectors rotate

around patientSingle row in z-directionSlice thickness

determined by collimation

Z-Axis

Single Slice CT: Slice Thickness

Z-Axis

Collimated Beam Thickness

Z-Axis

Collimated Beam Thickness

Thin slice

Thick slice

Multi-slice CT

Detectors

Post-collimator

Tube

What’s Different for Multislice CT?

Multislice CTMultiple rows

of detectorsOpen

collimators in “Z” direction

http://www.veterinary-imaging.com/images/MSS_CT.gif

1 2 3 4

Multi-slice CTDeveloped in late

1990’sDetector 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

Z-Axis

Collimated Beam Thickness Single slice detector

Multislice detector

Multi-Slice DetectorsMany detectors going around patientMany detector rows in z-directionSlice thickness determined by

Collimationelectronic detector selection

“Z” Direction

Single Multi

Multi-slice CTSize & distribution of detectors in non-

axial direction similar to previous CT’sSimilar spatial & contrast resolution

Distribution of detectors in axial direction varies with manufacturer

All detectors same width

“Z”Direction

Variable widths

Multi-slice CTUniform Detector ThicknessMultiple detectors in

axial directionSize must accommodate

thinnest sliceDetector signals can be

usedIndividuallyIn groups

1 2 3 4

Four thin slices

1 2 3 4

Four thicker slices

“z” direction

Detectors vs. Channels# Physical Detectors not necessarily equal

to # of possible SlicesMaximum # slices limited by Digital

Acquisition System (DAS) channelsElectronic counters

Imaging speed bottleneckHow fast data can be received from

detector arrays

Detectors vs. Channels Example

16 detectors

4 channels

Multi-Slice Detector Example16 Detector Rows, 4 Channels

Detectors vs. Channels4 X 1.25 mm

Beam collimated to 4 detector rows

1 detector row per DAS channel

Effective Detector

Detectors vs. Channels4 X 2.5 mmBeam collimated to 8

detector rows2 detector rows per

DAS channel

Effective Detector

Detectors vs. Channels4 X 3.75 mm

Beam collimated to 12 detector rows

3 detector rows per DAS channel

Effective Detector

Detectors vs. Channels4 X 5 mm

Beam collimated to 16 detector rows

4 detector rows per DAS channel

Effective Detector

Capture EfficiencyFraction of detector area

that is active detector

Equal-width Detectors Disadvantage

Many gapsGaps are dead space

Reduce capture efficiency

Multi-slice CT“Adaptive Array Detectors”

Some scanners use detectors of various widths

Post-collimators used to partially block wider elements for thinner slices

“z” direction

1 2 3Three thicker slices

1 2 4

Four thinner slices

3

Post-collimators

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 PitchDefined only for Multi-slice scanners

table motion during one rotation Beam Pitch = ---------------------------------------

Beam thickness

Beam thickness

Beam PitchDefined only for Multi-slice scanners

Beam Pitch = 1Beam Pitch > 1

CT Beam Pitch

Example

table motion during one rotation Beam Pitch = ---------------------------------------

Beam thickness

5 mm slices 4 simultaneous

slices Beam pitch = 1 1 revolution / sec. Beam thickness? Table speed?

Beam Thickness

table motion during one rotation Beam Pitch = ---------------------------------------

Beam thickness

5 mm slices 4 simultaneous

slices Beam pitch = 1 1 revolution / sec.

Beam Thickness = 5 X 4 = 20 mm

Table Speed

table motion during one rotation Beam Pitch = ---------------------------------------

Beam thickness

5 mm slices 4 simultaneous

slices Beam pitch = 1 1 revolution / sec. 20 mm beam

thickness

Table speed = 20 mm rotation (1 sec) = 20 mm / sec

Slice Thickness Defined at Rotational Center

TubeRotational

Center

Detector Field must be Larger than Slice Thickness at Rotational Center

RotationalCenter

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)

CauseDivergent beam

Other considerationsTendency to cover more volume (anatomy)Better availability of equipment

Other Reasons for High CT Doses

Repeat ExamsNo adjustment of technique factors for

different size patientsNo adjustment for different areas of body

Multislice CT Advantage?Speed!

Single slice / Multislice Images about the same!

Speed = PowerSpeed enables new applications

How do we spend our new speed?

Multi-slice CT ImagingClinical Advantages

1. Thinner slices for improved z-direction resolution

2. Same acquisition in shorter time3. Scan larger volumes in same time

Multi-slice CT ImagingClinical Advantages

Thinner slicesImprovement in CTA of neck, aorta, renal

vesselsBetter reconstructions

Sagittal, coronal, oblique 3-D

Fundamental Trade-off“z” axis resolution vs. image noise

Multi-slice CT ImagingClinical Advantages

Improved x-ray tube utilizationReduced 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 benefitsCT endoscopyDiagnosis of pulmonary embolism

Multi-slice CT ImagingClinical Advantage: Angiography

Simplifies contrast bolus timing Continuous observation of target vesselCan reduce amount of contrast requiredCoverage from aorta to lower extremities

Runoff

Continuous CT ImagingInterventional ProceduresBiopsy & drainage

NeuroChestAbdominalSpineCatheter and tube placement

Helps operator avoid critical structures near path of biopsy needle

Better visualizing of moving structuresRespiration

Functional CTBrain perfusion

Multi-Slice Compared to Single-slice helical

Much FasterNo significant image quality differencesEquivalent Patient DoseRef:

Willi Kalender, Ph.DInstitute of Medical PhysicsUniversity of Erlanger, Germany

Organ Coverage Time Depends On

1. Beam Pitch2. Rotation Time3. Detector

Acquisition Length

64 Slice CT Typical Coverage Times

Heart & coronary arteries / brain / lungs5 seconds

Whole body coverage for blood clot search30 seconds

Philips

64-Slice Commercial Cardiac CT

IGE Philips Siemens (1 tube)

Siemens (2 tube)

Toshiba

Min. Rotation Time (s) .35 .53 .33 .33 .4

Detector length (mm) 40 40 19.2 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

Philips256 Slicer

Toshiba320 Slicer

Implications of 256+ SlicesFull organ coverage in single rotation

12-16 cm coverageImproved temporal resolutionReduced artifacts

Whole-organ function (perfusion) studiesFunctional data perfectly registered to

anatomic data

The Future

More slicesFlat panel area detectors???

Multi-slice challenges: More SlicesComputer issues

More archival capacityRequires faster computer systemsRequires faster communications for remote

viewingRadiologist responsible for all images

AcknowledgementMany drawings obtained from

www.impactscan.org website