alessio - pet/ct instrumentation 11/26/07 motivations for ...depts.washington.edu/imreslab/from old...

Alessio - PET/CT Instrumentation 11/26/07

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PET/CT Instrumentation Basics

1. Motivations for PET/CT imaging2. What is a PET/CT Scanner3. Typical Protocols4. CT Attenuation Correction5. Problems and Challenges with PET/CT6. Examples

Adam AlessioDepartment of RadiologyUniversity of Washington

[email protected]

11/27/072/36

Motivations for PET/CT Imaging

• Desire for aligned anatomical and functional images

CT image shows goodanatomical detail but no

functional information

PET image contains usefulfunctional information but

poor anatomical detail

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Software Based Image Fusion/Registration

Anatomy (CT) Function (FDG-PET)

kidneys

bladder

RegistrationSoftware

• Logistical Challenge (schedule 2 studies, image manipulation, …)• Error prone due to topology differences (and patient positioning, disease

progression between scans, internal motion)• Better to acquire scans at same time with same device

- Hardware Registration 4/36

What is PET/CT?

• A device with shared mechanical componentsproviding aligned anatomic (CT) and functional (PET)images.

PET CT “PET/CT”


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Aside: Why are PET images less visuallyappealing than CT?

A. Resolution is worse: PET limited by1. Positron Range2. Emitted photons are not exactly anti-parallel (slight angular spread)3. Intrinsic spatial resolution of detectors (high energy photons travel some

distance in detectors)4. Sampling of detectors

B. Signal to noise ratio worse: PET images are noisy/blurry because:1. Have much less good counts than CT2. Have more contributions of “bad” counts (scattered and random events)(Noisy images are then blurred for visual appeal leading to even worse resolution)

So why bother?

Answer: Sensitivity Typical whole-body PET system can detect ~2% of allemitted photons, thus we can image almost any biological compound.

In comparison CT looks at an energy-weighted density and MR (mostly) looksat proton (hydrogen) density

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What is a PET/CT scanner?

PET/CT Scanner consists of separate CT and PETComponents

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Some Background

Siemens/CTI/HitachiBiograph/Reveal(BGO or LSO)

GE Medical SystemsDiscovery LS, ST, STE

(BGO)

Phillips/ADACGemini (GSO)

Gemini TF (LYSO)

• First PET/CT scanner introduced in 1998 by University of Pittsburgh,NCI, and CTI PET Systems (Knoxville, TN)

• Combination of independent scanner components still standard

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Benefits of PET/CT?

• Primary: Mechanically Aligned Anatomical andFunctional images– “Fusion” - images not “fused”, simply overlaid - “alpha”

blending


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Multimodality Imaging

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Benefits of PET/CT?

• Primary: Mechanically Aligned Anatomical and Functionalimages– “Fusion” - images not “fused”, simply overlaid - “alpha” blending

• Secondary: CT scan provides attenuation correctioninformation for PET Reconstruction– Much faster than conventional PET transmission scan

(20 sec vs. ~20 min)– Less noise than conventional PET transmission scan– Shorter overall scan time

• Wholebody PET/CT ~ 30 min• Wholebody PET ~ 50 min

• Tertiary:– CT scan used for scatter correction– CT image could add in resolution recovery in PET image

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PET/CT scan protocol1. Scout scan

(5-20 sec)

CT PET

3 or 4. Whole-bodyPET (6-40 min)

CT PET

3 or 4. HelicalCT (1-2 min)

CT PET

2. Selectionof scanregion(1-2 min)

Scout scan image

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The most significant error in PET data:Attenuation

• Attenuation is mainly due to scatter• It is by far the most important effect for both noise (due to

reduced counts) and qualitative image appearance (greater thanscatter, randoms, deadtime, normalization…)

detectorring

FDG inpatient

dataacquisition

expectedcountsmeasured

counts

attenuation by afactor of 10 to 100

(!)

scannerFOV


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Effects of attenuation on qualitative appearance:Patient example

Transmission:tissue density

Emission: FDG(Attenuationcorrected)

Emission: FDG(not corrected

for attenuation)

There are striking changes in appearance with and without attenuation correction14/36

PET Attenuation Correction-Transmission Scan Methods

Coincident photon Ge-68/Ga-68 (511 keV)

Single photonCs-137 (662 keV)

X-ray (CT)(~30-140kVp)

high noise~12-45 min whole body scan

no biaspost injection contamination

lower noise~4-20 min scan time

some biaspost injection contamination

Virtually no noise~20 sec scan time

potential for significant biasno post injection contamination

Rod source

PET Detectors

Past: Siemens and GE PET Scanners andoriginal PET/CT scanners

Now: not available for AC on any newsystem

Past: Philips PET Scanners

Now: not available on any new systems

Past: not on PET Scanners

Now: only option with new PET/CTsystems

HISTORY:

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Why are PET Attenuation Correction Factors potentiallybiased when derived from CT Transmission scan?

• Source for CT x-rays (tube) is1. at a lower energy level and2. a distribution of energies

• For any given material a stream of 70keV photons will beattenuated differently than 511keV photons– Most materials fairly straightforward to convert from 70 keV to

511keV– Contrast agents complicate conversion

E (keV)30 120 511 662

IntensityI0(E)

X-ray source positron source γ-ray source

0

spectra

PET emission data attenuated at 511 keV16/36


Linear attenuation coefficients for bone and muscle tissue in the range of 10 to1000 keV. Photoelectric absorption is the main contributor to the attenuation ofphotons at lower energies whereas Compton scattering dominates at higherenergies.


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Bias in AC Factors

Illustrates that mass attenuation coefficient of iodine is appreciably different frombiological tissue at CT energies while similar at PET 511keV energy.

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Converting CT Numbers toPET Attenuation Values

• Linear Scaling Method maps Hounsfield Units to Attenuation correctioncoefficients at 511keV

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Flowchart of typical PET/CT Operation

Attenuation correction factors can be obtained from either (A) CT-based or(B) conventional PET transmission source (rare on recent systems)


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Comparing PET/CT Scanners*

190

182

182

160

160/200

co-scanlength(cm)

430

450

450

400

400

max bedload (lbs)

14 x 24

15 x 25

15 x 25

14 x 24

14 x 24

room size(feet)

501 (CT)60 (PET)

501 (CT)60 (PET)

501 (CT)58.5(PET)

501 (CT)70 (PET)

50 (CT)55 (PET)

FOVdiam(cm)

X-ray137Cs

X-ray

X-ray

X-ray

X-ray68Ge

TXModes

70 (CT)63 (PET)

3DPhilipsMX8000 Dual orBrilliance 16

Allegro(GSO)

PhilipsGemini

703DSOMATOM(6 or 16 slice)

wide-boreAccell(LSO)

SiemensHIREZ

703DSOMATOM(2 or 16 slice)

wide-boreAccel(LSO)

Siemens*** PICO

702D/3DLightspeed Plus(4,8, or 16slice)

wide-boreNXi (BGO)

GEH ST

70 (CT)tapered to59 (PET)

2D/3DLightspeed Plus(4,8, or 16slice)

NXi (BGO)GEH LS

Patientport (cm)

PETMode

CTComponent**

PETComponent

Scanner

*Based on published specifications - OUT OF DATE**CT sub-system options are changing rapidly***CPS products sold through CTI as Reveal and Siemens as Biograph 22/36

Challenges & Problems with PET/CT

1. Functional and Anatomical Image Alignment– Possible Movement between scans– Respiratory Motion (breath hold protocol?)

• Several organs deviate in volume and position (liver, spleen,…)– Calibration of CT gantry, PET gantry, and bed

2. CT Based Attenuation Correction(in my order of least to most important)– Truncation Errors from arms down in FOV ⇒ Artifacts in

corresponding regions in PET image– Biases in AC factors from conversion of X-ray energies to PET

energies ⇒ Incorrect Values in PET Image• Contrast agents, Prostheses, implants

– Movement between Scans– Respiratory Motion ⇒ Can cause artifacts near dome of liver (in

cardiac PET, causes defects in cardiac values)

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CT Based AC Problems(Bias in AC Factors)

General Rule:• If one over attenuation corrects (uses higher (”more dense”) than

true correction factors for a region), will get artifactually high PETvalues

– Example: Contrast agents in CT image, when scaled to PET energies,are higher than should be leading to artifactual focal hot spots.

• If one under attenuation corrects (uses lower than true factors fora region), will get artifactually low PET values

– Example: Respiratory motion can replace diaphragm in PET imagewith lung space in CT image. Causes photopenic region (bananaartifact) along diaphragm in PET image.

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Artifact Examples…

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