ct basics “pure” ct - aapm: the american … = ~ 1 mm effects of recon filters on resolution...

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CT BasicsDianna Cody, Ph.D.

Professor & Chief, Radiologic PhysicsU.T. M.D. Anderson Cancer Center

“Pure” CT

• Information regarding attenuation correction with CT AND

• Information regarding how CT is partneredInformation regarding how CT is partnered with PET

• Will be covered later in the workshop

Axial Platforms

first generation second generation

Axial/Helical Platforms

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CT X-ray Tube DesignEvacuated glass or metal envelope

Oil for insulation and heat dissipation

Lead housing absorbs unwanted x-rays

Port for useful beam

X-ray Beam Characteristics

• Polyenergetic beam– Bremsstrahlung

radiationCh i i– Characteristic radiation

• Max photon energy depends on kVp

• Min photon energy depends on filtration

Beam Collimation• Pre-patient

collimators define width of beam in z (all systems)

• “Detector” collimators reduce scatter at detectors (some CTs)

Beam Filtration

• Removes low energy x-rays from beam– Low E photons contrib to dose, not image– Filter reduces beam-hardening artifacts

• Shapes energy distribution across beam– Removes more low energy from edges– Results in more uniform beam hardening after

passing through filter and patient

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Beam Filtration

X-ray fan beam

Filter

Shaped beamShaped beam

Patient

Uniform output

Detector Characteristics

• Efficiency• Response time• Dynamic range• High reproducibility• Electronic stability

Solid State Detectors

• Photodiode multipliers (no PMT)

• CdWO4 crystalsCdWO4 crystals99% conversion and

capture efficiency• Ceramics

99% absorption, 3X conversion

Back Projection – 1st Generation CTObject = Rod in air

Beam direction black arrow 1 (angle 1)

Tube detector scans across

Image Reconstruction Process

Tube-detector scans across (red arrows)

Data (Profile 1) recorded w/ detector position

Repeat for Angle 2 to get Profile 2, etc

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Backprojection Reconstruction Filters

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3 4

1 Object

2 Projection data

3 Recon filter

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4 Backprojection of filtered data

5 Backprojection of filtered data for two angles

Filtered Backprojection

Filtered profileFiltered Backprojection

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Spatial ResolutionAbility to detect a small object easily distinguished

from background• Display Field of View (DFOV) size• Reconstruction filter (algorithm, kernel)• X-ray tube focal spot sizey p• Image thickness (blurs edges of objects)• Pitch (blurs edges of objects)• Patient motion• Image zoom Voxel size = DFOV/512

512 50 cm DFOV

X-Y Voxel Size

512 pixels

pixelsPixel = ~ 1 mm

Effects of Recon Filters on Resolution & Noise

Std Recon Soft Recon


Std Recon Bone Recon

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Std Recon Detail Recon


Std Recon Edge Recon

Effects of Recon Filters on NoiseRecon Filter Std Dev

Water ImgSoft 3.8

Standard 4.7Lung 19.6Detail 6.5Bone 18.8Edge 35.8

Bone Plus 27.0

Contrast Resolution Ability to see a small object not easily distinguished

from background (NOISE)

• Effective mAsmA * time / pitch

• Image thickness• Image thickness• Patient size• Reconstruction filter• Viewing conditions

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ACR Phantom - Low Contrast Section

120 kVp, 1600 mAs 120 kVp, 192 mAs

Viewing Conditions - Contrast

• Distance• Ambient (room) lighting

Cannot see the stars in the daytime– Cannot see the stars in the daytime• Monitor brightness• Reflections• Viewing angle (flat screens)• [Age of eyeballs…]

Pixels and Image Matrices Pixels and Image Matrices

222 220 200 146 103

200 158 127 96 73

207 131 103 82 86

202 126 112 124 133

Pixel Values (HU)

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CT Number

• Pixel bit-depth of 212 = 4096 values• Contrast scale

HU = Constant (µ µ ) / µHU = Constant (µm – µwater) / µwater– CT number for water = 0 at all energies– CT number range –1024 to +3072

• CT number affected by kVp– Reduce kVp, increase contrast

Typical CT Numbers• Air• Lung• Fat

-1024~ -700

~ - 120 to ~ -80• Water• Brain• Soft Tissue• Bone• Metal

0 +/- 5~ 40

~ 40 to ~ 100200 to > 600

> 1000

Select CT#’s with WW WL Slip-ring

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Pitch for Single-Slice CT

• Image and beam width are same for conventional CT

• Pitch = table travel ÷ beam width

• Typical pitch values are 0.7 to 1.5

Conventional Helical CT Detectors

Image width determined by beam thicknessbeam thickness

Pitch = table mm / beam mm

z

PitchImagine a CT Scanner with a spray paint can in place of the x-ray tube.

Pitch Definition

• Pitch = distance table travelswidth of x-ray beam

• Pitch = distance table travelswidth of spray paint

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PitchHelical Interpolation

Collect data (black dots)

Rebin to estimate the 180° data (blue squares)

I t l t t ti tInterpolate to estimate image between collected and rebinned data

Helical CT needs fast computers

Multi-Detector Concept

• Acquisition of multiple images per scan• Electronic post-patient collimation• Faster volume acquisition times• Better bolus tracking and thin slices for 3DBetter bolus tracking and thin slices for 3D

MSCT detectors

z

64 x 0.625 mm

General Electric 4 & 64 & 16 channel detectors

1.25 mm

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Channels (or data channels) Detector Configuration

detector

4 x 1.25 mm 4 x 2.5 mm

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4 x 3.75 mm4 x 5 mm

MSCT Detectors

z z

chan

nel

chan

nel

chan

nel

chan

nel

z

THIRD Gen.

MSCT Detectors

Image width determined by output channel

Pitch = table mm / beam mmPitch table mm / beam mm

Pitch = table mm / n * T where n = no. of channels and T = channel thickness

z

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MSCT Faster ScanningDetector Beam Thick.

(mm)# rotations Total scan

time (sec)1 x 1.25 1.25 160 128

4 x 1.25 5 40 32

8 x 1.25 10 20 16

16 x 1.25 20 10 8

64 x .625 40 5 4

1.25mm images and 20cm scan length at 0.8sec rotation and 1.0 pitch

Artifact Sources• Scanner

– Detector imbalance– Obstruction of beam– Pitch and detector configuration

• Patient– Motion– Implants (dental, prosthetics, etc.)– Non-uniformity of normal “ingredients”

Ring Artifact3rd generation and MSCT

Detector imbalance

Material in beam IV contrast

‘gunk’ factor

0.625mm image

Axial acquisition

16 x 0.625

16-slice CT

Image number 7of group with 16 images

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Axial acquisition

16-slice CT


Axial acquisition

16-slice CT


Recall Helical Interpolation

Collect data (black dots)

Rebin (blue squares)

Interpolate for image

Multislice CT

• Helical non-planar data• Data from multiple channels

1 2 3 41st rotation 2nd rotation

1 2 3 43

direct data

complementary data

Longitudinal direction

00

1800

3600

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MSCT Ring Artifact

• Imbalance of detector causes ring in those axial images that are same width as detector element

• Images from “binned” detector elements may not h ishow ring

• Helical MSCT images have arc instead of ring

• Arc artifact might not show in images thicker than the element size (depends on pitch and recon alg)

0.625mm image

Helical scan

16 x 0.625

Pitch = 0.562

16-slice CT

Pitch 0.562


Helical scan


Helical scan


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Helical scan


MSCT Arc Artifact

• Might not be visible in each image due to overlying anatomy

• Easiest to find when viewing images inEasiest to find when viewing images in “stack” mode

• Lower pitch, longer arc• Visibility affected also by WW/WL

1.25mm image

Pitch = 1.375

(16 x 0.625)

16-slice CT

Position of arc inferred from other images in series

Visibility depends on local anatomy

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Arc visible, but faint

Arc visible, but faint

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Helical or Windmill Artifact

• Occurs at high subject contrast interface– Bone and soft tissue (ribs, skull)– Air and soft tissue– Air and Ba contrast– i.v. contrast in tubing

• Varies with – Angle of interface w.r.t. scan plane– Pitch and image width (combined)

MSCT Helical Artifact

High-contrast objects at angle to scan planeReducing Helical Artifact

• Increase z-axis sampling• Change pitch, if possible• Change detector configuration, if possible

• For Prospective study with thin retros – check all image thicknesses at several pitches

(scan a phantom)– choose optimal pitch for all desired image

thicknesses

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Prospective images at 5mm

Scanner: 16-channel

Detector: 8 x 2.5

Pitch = 0.875

Retrospective images at 2.5mm

Same as patient study

Pitch: 0.875, Detector: 8×2.5mm, Beam: 20mm

SE 2, IM 2, 5mm SE 3, IM 3, 2.5mm

Change detector (incr. Z sampling), retain beam widthPitch: 1.375, Detector: 16×1.25mm, Beam: 20mm

Effective mAs = 109 (decreased from 171)SE 10, IM 2, 5mm SE 11, IM 3, 2.5mm

Z-axis Sampling Summary

• In general, use smallest detector spacing possible!

• More powerful than decreasing pitch to reduce helical artifactsreduce helical artifacts

• Beam width may change with detector configuration

• Changes in beam width and/or pitch will affect total scan acquisition time

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End

• Please score this section on evaluation sheetsheet.

• Thanks!!

ct basics “pure” ct - aapm: the american … = ~ 1 mm effects of recon filters on resolution...

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