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Bases techniques de la scintigraphie

CC Radiologie 4

PD Habib Zaidi, Ph.DDivision of Nuclear Medicine

Geneva University Hospital, SwitzerlandEmail: habib.zaidi@hcuge.ch

Web: http://dmnu-pet5.hcuge.ch/

STRUCTURECT MRI

Structural & Functional imaging

Nuclear Medicine is to physiology as Radiology is to anatomy

PET SPECTEEGFUNCTION

fMRI

Ce que l’on voit: Ce que l’on évalue:

Isotope TraceurTc-99m, F-18, … HMPAO, FDG,

…

RadiopharmaceutiquePerfusion cérébrale : Tc-99m HMPAO

Métabolisme cérébral : F-18 FDG

Chaque radiopharmaceutique est dédié àune instrumentation spécifique

Radiopharmaceutique

SPECT99mTc (6 heures) 140 keV201Tl (73 heures) 70 keV123I (13 heures) 159 keVSensibilité absolue: 10-4

Résolution: 6-10 mm

PET15O (2.07 min) 511 keV13N (9.96 min) 511 keV11C (20.4 min) 511 keV18F (109.8 min) 511 keVSensibilité absolue: 10-2

Résolution: 3-5 mm

Radioisotopes SPECT & PET

SPECT:Tc-99m: ECD, HMPAO

I-123: IMP, Iomazenil

Xe-133

PET:F-18: FDG, DopamineC-11: CholinesteraseO-15: H2O, CO2

Cyclotron + Hot Labs + modules

CYCLOTRONIBA CYCLONE 18/9protons: 18 MeV deuterons: 9 MeV8 target ports Weight: 23 tonnesNegative ions Dual irradiation

Blindage

Patient

Collimateur

Cristal scintillateur

PMTsGuide de lumière

Caméra à scintillations - Anger

Display

Electronique

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CT scanning

sinogram

distance

view

Image reconstruction

SPECT- single photon emission computedtomography

Coincidence?

Line o

f resp

onse

PET- positron emission tomography

Detector block concept

PMT's

Scintillation crystals

C

Crystal cuts formlight guides

DBA

First PET device (1952)Gordon L Brownell, MGH 1st PET device

1952

PET III1975

ECAT II1977

NeuroECAT1978

ECAT 9311985

EXACT HR+1995

HRRT LSO2000

e.cam duet1” crystal imaging technology

ECAT ARTPartial-ring

Advance NxiFull-ring

ECAT ACCELLSO technology

Biograph PET/CT function + anatomy

Evolution of PET Instrumentation

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Whole-Body C-PET GE Discovery LS PET/CT

PEM-Flex

Courtesy of Naviscan PET Systems

The source of FDG PET signal is different from x-rays

PET scanners dedicated to breast imaging

MicroPET (FWHM<1.3 mm)

Concorde Microsystems

eXplore Vista

GE Healthcare

MOSAIC

Philips MedicalSystemsHamamatsu Photonics

KK

SHR 7700

Oxford PositronSystems

HIDAC (~ 1 mm)

Small-animal PET imagingRaytest GmbH

ClearPET™ (~1.5 mm)

Example of linear image registration applied to the fusion of thoracic CT with whole-body F-18 FDG PET. Both scans were acquired with similar breathing patterns.

Courtresy of P. Slomka

Software image coregistration ‘Inherent’ Image Registration

but “Breathing” Image Is Better Registered

“Breath-hold” CT Shows Better Definition in Lung

Dual-modality imaging - PET/CT

Anatomy(CT)

Fusion(PET/CT)

+

=

Function(PET)

Spiral CT

Whole-body PET

(1-8 min total)

(6-40 min total)

CT PET

CT PET

Dual-modality imaging - PET/CT

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Dual-modality imaging - PET/CT Dual-modality imaging - PET/CT

Illustration of standard PET/CT scanning protocol. The patient is positioned on a common specially designed patient table in front of the combined mechanical gantry. First, a topogram is used to define the co-axial imaging range (orange). The spiral CT scan (grey box) preceded the PET scan (green box). The CT data are reconstructed on-line and used for the purpose of attenuation correction of the corresponding emission data (blue box). Black and blue arrows indicate acquisition and data processing streams, respectively.

Discovery LS Biograph 16Discovery ST GEMINI GXL

Dual-modality imaging - PET/CT

120 lbs – 1 min/bed 160 lbs - 2 min/bed

7 minute exam 12 minute exam 17 minute exam

PET/CT – Image quality

250 lbs - 3 min/bed

Whole-body phantom+ lungs + 22Na lesions

Lesion detectability

Transaxial PET images of8-mm diameter lesion in right lung at target-to-background ratio of 43:1

lesion

( 2D BGO Resolution)

History. Patient is a 52 year-old male with squamous cell tonsillar cancer with a history of a 4 cm positive node. Patient underwent pre-surgery chemo, right tonsillectomy and radical neck dissection. Removal of positive nodes in the neck and 45 additional nodes were previously allnegative.

PET/CT Findings. The follow-up PET showed diffuse band of activity while the HI-REZ PET/CT imaged the same day resolved individual nodes. Individual nodes were only visualized with the HI-REZ PET imaging on the 16 slice PET/CT

(3D LSO HI-REZ) (HI-REZ PET/CT)

Courtesy of the University of Tennessee Medical Center, Knoxville, TN

2D vs 3D PET imaging

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s

φ

s

Pµ(x,y)

f(x,y)

p(s,φ)

Radon Transform (1917): inversion problem of recovering a two-dimensional (2D) image ƒ(x,y) from the set of its one-dimensional (1D) projections p(s,φ).

Image reconstruction

Corresponding sinogram, with 256 pixels per rowand 256 angles equally spaced between 0° and359°. Each row of sinogram is projection of slice at given angular position of detector

Shepp–Logan phantom slice (256 x 256 pixels)

Analytic - FBP

Iterative vs analytic reconstruction

Iterative - ML-EM

Limitations of quantitative PET

True coincidences:Both annihilation photons escape withoutscatter and are detected.

Random coincidences:Two uncorrelated photons from separateemission strike the detectors at the sametime.

Attenuated coincidences:Absorption + Scatter

Scatter coincidences:One or both annihilation photons scatter in the patient body.

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Source depth (cm)

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tons 69 keV

140 keV247 keV364 keV511 keV

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H2OBone

With attenuation correction

Uniform cylinder

Without attenuation correction

Photon attenuation

Attenuation correction:• improves localization• no effect on lesion detection???

no atten. corr. with atten. corr. attenuation mapAC or not AC? That is the question!

Example of FDG uptake with and without attenuation correction:

Bai et al. J Nucl Med (2003)

Difficult to mathematically predict the specific appearance of PET images not corrected for attenuation.

Possible to predict some common artifacts (e.g. enhanced activity in pulmonary regions (hot lungs) and negative tracer concentrations in mediastinal regions in uncorrected images).

AC or not AC? That is the question!

A. A harmatoma (arrow) is visible in the transmission image, also called an attenuation mapB. It is also visible in the FDG-PET image with attenuation correctionC. But is not visible in the FDG-PET image without attenuation correction!

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Spectral distribution of scattered photons

BrainSF: 30-35%

ThoraxSF > 50%

18F-FDG Brain

18F-FDG Thorax

SCATTER CORRECTEDNOT CORRECTED

Relevance of scattercompensation

SF: 30-35%

SF> 50%

Grandeurs et unités dosimétriquesActivité (Bq)

désintégrations par seconde (1 Ci = 3.7x1010 Bq)

Exposition (C/Kg)ionisations produites dans l’air par les photons

Dose absorbée (Gy)déposition d’énergie dans la matière (1 Gy = 1 J/kg)

Equivalent de dose (Sv)mesure des dommages biologiques (WR=1 pour γ ou Rx)

Dose efficace (Sv)somme des doses équivalentes à tous les organes

La justification des pratiquesLes bénéfices de l’exposition aux rayonnements ionisants doivent être supérieurs à ses inconvénients

L'optimisation de la protectionALARA (As Low As Reasonably Acheviable): l'expression d'optimisation de la radioprotection adoptée par la CIPR.

La limitation des doses individuellesLa limitation des doses individuelles proposée par la CIPR dans sa publication 60 procède d'une approche fondée sur l'appréciation du degré de tolérance d'une exposition. Une exposition peut être: inacceptable,

lé bl bl

Principes de radioprotection

Quels sont les moyens de protection contre les rayonnements ionisants?

Principes de radioprotection

Temps: s’exposer le moins possible!Dose ~ temps d’exposition

Distance: se tenir le plus loin possible! Dose ~ 1/(distance)2

Blindage: interposer un blindage entre le manipulateur et la source!

Dose ~ exp(-µd)µ: coefficient linéaire d’atténuation

d: épaisseur de l’atténuateur

Quelle est la limite de dose au corps entier imposée par l’ORAP pour les travailleurs?

Radioprotection opérationnelle

La CIPR a proposé d'ajouter à l'ancienne limite, toujours valable, de 50 mSv par an, une nouvelle limite de 100 mSvpour 5 ans, soit, 20 mSv par an.

Cette limite correspond à un objectif de dose pour la vie de 1 Sv pour une durée de vie professionnelle de 50 ans.

Quelle est la limite de dose aux extrémités par l’ORAPpour les travailleurs?

500 mSv par an (ORAP 1994).

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Ao

r1

r2

r3

_ _ _

AdministeredActivity

Modèle de calcul - MIRD

D rk( )A0

= D rk ← rh( )h∑ = τhS rk ← rh( )

h∑

Paramètres biologiquesParamètres physiques

Medical Internal Radiation Dose Committee (MIRD)

Modèles anatomiques (fantômes)Modèles mathématiques Modèles numériques

Zubal phantom

Reference man

NCAT phantom

VIP-phantom

La dose absorbée en méd nucl

X-ray CT: dépend de plusieurs paramètres (région explorée, mAs, … )valeurs typiques pour un CT scan thoracique: 5 à 10 mSv

Cerveau 99mTc-HMPAO: 3.6 mSv (400 MBq)Myocarde 201Tl: 23 mSv (100 MBq)Transmission (153Gd): 4 µSv (cerveau ); 14 µSv (myocarde)

PET: dépend de l’isotope et du protocole d’imagerie18F-FDG: 4.3 mSv (225 MBq)11C-Benzotropin: 1.70 mSv (225 MBq)Transmission (68Ge): 54 µSv (cerveau ); 154 µSv (myocarde)

SPECT: dépend de l’isotope et du protocole d’imagerie

Effective Dose in Mx8000 IDT 16 Slice scanner

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