Four or More: Understanding the Basics of Multidetector-Row CT

Four or More: Understanding the Basics of Multidetector-Row CT Took 28 minutes going very fast….

Four or More: Understanding the Basics of Multidetector-Row CT 6th Annual International Symposium on Multidetector-Row CT San Francisco, CA June 23-26, 2004 Sandy Napel, Ph.D. Department of Radiology Stanford University School of Medicine Stanford, California USA http://www-radiology.stanford.edu/

Outline • Detector and data acquisition basics • Helical mode • Pitch • Image reconstruction Goal: Provide insight into how to use MDCT

Outline • Detector and data acquisition basics • Helical mode • Pitch • Image reconstruction

SDCT 10 mm z Detector

SDCT 10 mm z 20+ mm Detector MDCT

z Detector How to slice it? Uniform or Matrix Array N elements Length = Nd d

z Detector How to slice it? Uniform or Matrix Array N elements Length = Nd d Non-uniform or Adaptive Array

z Detector How to slice it? Uniform or Matrix Array N elements Length = Nd d Non-uniform or Adaptive Array Hybrid Array { thinner inner elements

16 x 1.25 mm Detector 4-Slice Scanners 20 mm 5 2.5 1.5 1 1 1.5 2.5 5 32 mm 4 x 0.5 mm

4-Slice Scanners Collect 4 Simultaneous Channels of Data How do we use all 16 elements? Detector 16 elements 4 DAS channels

4 x 1.25 mm Detector Configuration X-ray Tube Focal Spot X-ray Beam Collimator Detector Switching Array

5 2.5 1.5 1 1 1.5 2.5 5 Detector Do I get unequally-sized slices from an MDCT scanner with a non-uniform detector array? NO!

5 2.5 1.5 1 1 1.5 2.5 5 Detector X-ray source collimation 2 x 0.5 mm slices

Detector X-ray source collimation 4 x 1 mm slices 5 2.5 1.5 1 1 1.5 2.5 5

Detector X-ray source collimation 4 x 2.5 mm slices 5 2.5 1.5 1 1 1.5 2.5 5

Detector X-ray source collimation 4 x 5 mm slices 5 2.5 1.5 1 1 1.5 2.5 5

8-Slice Scanners Collect 8 Simultaneous Channels of Data How do we use all 16 elements? Detector 16 elements 8 DAS channels

Detector 16-Slice Scanners 3 basic detector designs

16 x 0.625 mm 4 x 1.25 mm 4 x 1.25 mm 20 mm Detector 16-Slice Scanners

16 x 0.625 mm 4 x 1.25 mm 4 x 1.25 mm 20 mm 16 x 0.75 mm 4 x 1.5 mm 24 mm 4 x 1.5 mm Detector 16-Slice Scanners

16 x 0.625 mm 4 x 1.25 mm 4 x 1.25 mm 20 mm 16 x 0.75 mm 4 x 1.5 mm 24 mm 4 x 1.5 mm 16 x 0.5 mm 12 x 1 mm 12 x 1 mm 32 mm Detector 16-Slice Scanners

Detector c. 2004 Multislice Scanners 4 basic detector designs

64 x 0.625 mm 40 mm Detector c. 2004 Multislice Scanners A: • 64 slice scanner: • 64 0.625 mm slices over 40 mm • Thicker slices available via software

40 mm 40 x 0.625 mm 6 x 1.25 mm 6 x 1.25 mm Detector c. 2004 Multislice Scanners B: • 40 slice scanner: • 40 0.625 mm slices over 25 mm • 32 1.25 mm slices over 40 mm • Thicker slices available via software

32 mm Detector c. 2004 Multislice Scanners C: 64 x 0.5 mm • 32 slice scanner: • 32 0.5 mm slices over 16 mm • 32 1.0 mm slices over 32 mm • Thicker slices available via software

4 x 1.2 mm 4 x 1.2 mm Detector c. 2004 Multislice Scanners D: 32 x 0.6 mm 29 mm • 64 slice scanner: • 32 0.6 mm slices over 19 mm • 24 1.2 mm slices over 29 mm • 64 0.6 mm slices over 19 mm via focal spot shift

Detector c. 2004 Multislice Scanners D: .

Detector c. 2004 Multislice Scanners D: ~ 0.3 mm at isocenter (~ 50% overlap) .

4 x 1.2 mm 4 x 1.2 mm Detector c. 2004 Multislice Scanners D: 32 x 0.6 mm 29 mm • 64 slice scanner: • 32 0.6 mm slices over 19 mm • 24 1.2 mm slices over 29 mm • 64 0.6 mm slices over 19 mm via focal spot shift • Thicker slices available via software

Detector When M ≠ N, there may be multiple ways to make slices of a given thickness

least PVA 4 x 1.2 mm 4 x 1.2 mm Detector Example: Make 3.6 mm thick slices with this detector D: 32 x 0.6 mm 29 mm • Sum 6 0.6 mm slices: • 0.6 mm slices available • Coverage/rotation = 19 mm • Sum 3 1.2 mm slices: • 0.6 mm slices NOT available • Coverage/rotation = 29 mm

table travel per rotation (mm) table travel per rotation (mm) Pitch = Pitch = slice thickness (mm) beam width (mm) Helical Pitch • Single-Slice Helical CT: slice thickness = x-ray ( beam ) width

slice thickness <=> D = detector channel width table travel per rotation (mm) Pitch = M • D (mm) where M = no. of simultaneous slices Helical Pitch • Multi-Slice Helical CT: 1.25 mm }

slice thickness <=> D = detector channel width table travel per rotation (mm) Pitch = M • D (mm) where M = no. of simultaneous slices Helical Pitch • Multi-Slice Helical CT: } 2.5 mm

slice thickness <=> D = detector channel width table travel per rotation (mm) Pitch = M • D (mm) where M = no. of simultaneous slices Helical Pitch • Multi-Slice Helical CT: } 3.75 mm

slice thickness <=> D = detector channel width table travel per rotation (mm) Pitch = M • D (mm) where M = no. of simultaneous slices Helical Pitch • Multi-Slice Helical CT: } 5 mm

table travel per rotation (mm) Pitch = M • D (mm) Helical Pitch With this definition, intuition regarding artifacts and dose as a function of pitch are independent of M.

table travel per rotation (mm) Pitch = M • D (mm) Helical Pitch On a 4-slice scanner (M=4): At pitch = 0.75, table travel per rotation is: = 0.75 • 4 • D = 3 • D At pitch = 1.5, table travel per rotation is: = 1.5 • 4 • D = 6 • D

table travel per rotation (mm) Pitch = M • D (mm) !! (D=.625mm 6 cm/rot) Helical Pitch On a 4-slice scanner (M=4): At pitch = 0.75, table travel per rotation is: = 0.75 • 4 • D = 3 • D On an 64-slice scanner (M=64): At pitch = 1.5, table travel per rotation is: = 1.5 • 64 • D = 96 • D

Reconstruction Approaches • z-interpolation • Single-slice rebinning (ASSR) • Cone-beam approximations (Feldkamp)

The Need for Cone Beam Reconstruction X-ray Tube Focal Spot X-ray Beam Collimator z isocenter Detector

Four or More: Understanding the Basics of Multidetector-Row CT