Download Imaging Techniques Nuclear Medicine

Imaging Techniques
Nuclear Medicine
Patrícia Figueiredo
IST 2010-2011
The wide spectrum of
medical imaging techniques
(F. Deconinck, Vrije
University, Belgium).
The wide spectrum of
medical imaging techniques
(F. Deconinck, Vrije
University, Belgium).
Nuclear medicine imaging
General principles:
• Imaging of the biodistribution of a radiotracer:
• composed of a radionuclide incorporated into a molecule whose biodistribution
and /or metabolsim is to be studied;
• introduced into the body by inhalation, injection into the circulation, oral or
subcutaneous administration;
• measured by the detection of gamma photons resulting from the radioactive
decay of the radionuclide in the radiotracer.
Imaging modalities:
- Planar imaging:
Scintigraphy
- Tomography:
Single photon emission computed tomography (SPECT)
Positron Emission Tomography (PET)
Radioactivity and radionuclides
intrinsic property of isotopes with unstable nuclei (↓A/Z)
Radionuclide:
nuclear species of a radioactive isotope
(A-Z)
Radioactivity:
Radioactivity and radionuclides
Radioactive decay:
transition to stability by emission of some form of radiation
(from parent to daughter nuclide)
Activity [Ci or Bq]:
(# disintegrations/time)
Nb. parent nuclides:
Decay constant [s-1]:
Half life [s]:
Biological half life [s]:
Effective half life:
Radioactivity and radionuclides
Radioactive decay:
transition to stability by emission of some form of radiation
(from parent to daughter nuclide)
Types of radioactive decay
(most common):
1. α emission (210Po, 238Pu, 241Am, 242Cm)
2a. β+ or e+ (positron) emission: p→n+e++ν
2b.
or electron capture: p+e-→n+ν
3. β- or e- (electron) emission: n→p+e-+ν~
Radioactivity and radionuclides
Radioactive decay:
transition to stability by emission of some form of radiation
(from parent to daughter nuclide)
Types of radioactive decay
(most common):
2b. and 3. → intermediate metastable state
→ 4. Isomeric transition (electron reshuffling)
⇒ γ (X ray ) emission (~10-1000 keV)
or emission of Auger electron (~keV)
or internal conversion
Radioactivity and radionuclides
Radioactive decay:
transition to stability by emission of some form of radiation
(from parent to daughter nuclide)
Types of radioactive decay
(most common):
2a. β+ or e+ (positron) emission: p→n+e++ν
⇒ e+ e- annihilation: e+ + e- → γ + γ
⇒ 2 × γ emission
(511 keV,opposite directions)
Radioactivity and radionuclides
Interactions of γ rays with matter:
Radiation intensity attenuation:
Linear attenuation/absorption coefficient:
µl = µl (ρ, N0, Z , E) [cm-1]
Half value layer:
HVL = ln 2 / µl
Radioactivity and radionuclides
Radionuclides useful in diagnostic imaging:
- radioactive decay
⇒
monochromatic γ emission
- half life long enough
⇒
distribution within the body
- half life short enough
⇒
minimize radiation dose
- γ energy high enough (>100 keV)
⇒
HVL in tissue > object size
- γ energy low enough (<200 keV)
⇒
minimize scattered radiation
- minimal emission of α and β particles ⇒
- onsite production!
minimize radiation dose
Radioactivity and radionuclides
Radionuclides useful in diagnostic imaging:
Single photon emission (for planar imaging and SPECT):
Element
Radionuclide
Chemical Form
Half-life
γ Energy (keV)
Technetium
99m-Tc
various
6.02 h
140
Gallium
67-Ga
Gallium Citrate
3.2 d
93, 185, 300, 394
Thallium
201-Tl
Thallous chloride
3.0 d
68-82 (X-rays)
Xenon
133-Xe
5.3 d
81
Indium
111-In
Indium chloride
2.8 d
171, 245
Iodine
131-I
Sodium iodide
8.0 d
364
Positron emission (for PET):
Radioactivity and radionuclides
Radionuclide production:
- neutron capture (nuclear reactor)
- nuclear fission (nuclear reactor)
- radionuclide generator
- charged-particle bombardment (cyclotron)
Nuclear fission (nuclear reactor):
x, y:
~100 MeV
…
Chemical precursor of technetium
Radioactivity and radionuclides
Radionuclide generator (technetium generator):
A metastable radionuclide (99m-Tc, half life 6h) is produced by radioactive decay of a long-lived
parent (99-Mo, half life 66h) and extracted by flowing an eluting solution on the surface of the
ceramic column with the generator, usually with a periodicity of 12h, 24h or 48h.
Radioactivity and radionuclides
Radionuclide generator (technetium generator):
N1: 99-Mo
N2: 99m-Tc
N3: 99-Tc
equilibrium
→ equilibrium
Radioactivity and radionuclides
99m-Tc radiopharmaceuticals (planar imaging and SPECT):
Radiopharmaceutical
Short form
Clinical Use
Sodium Pertechnetate
Na299mTcO4
Thyroid, Salivary Gland and Gastric Scans
99mTc Sulphur Colloid
99mTcS/C
Liver, Spleen and Bone Marrow Scan
99mTc Macro Aggregated Albumin
99mTcMAA
Pulmonary Blood Flow (Lung Scan)
99mTc Diethylene Triamino Penta Acetic Acid
99mTcDTPA
Renal Blood Flow, Function and Excretion (Kidney Scan)
99mTc Methylene DiPhosphonate
99mTcMDP
Skeletal Studies (Bone Scan)
99mTc Red Blood Cells
99mTcRBC
Cardiac Function and Blood Pool Scans
99mTc Sestamibi
99mTc Tetrofosmin
99mTcMIBI
99mTcTETRO
Myocardial Perfusion (Heart Muscle Blood Flow)
99mTc Hexa Methylene Propylene Amine
Oxime
99mTc
HMPAO
Brain Scan and Scans for Infection
A chemical ligand is chosen to have high selectivity for the organ of interest with minimum
distribution in other tissues and is characterized by:
- binding to blood proteins (HSA)
- lipophilicity and ionization (membrane transport)
- means of body excretion (liver, kidneys)
Radioactivity and radionuclides
Radionuclide production through charged-particle bombardment (cyclotron)
D-shaped electromagnets ⇒ axial magnetic field
⇒ negative ions (1 proton, 2 electrons) of hydrogen (~10 MeV) or deuteron (~5 MeV)
⇒ accelerated in spiral trajectory
⇒ beam is extracted from cyclotron by passing it through a thin carbon foil
⇒ electrons are striped off, leaving protons to bombard the stable nuclei of the target
Radioactivity and radionuclides
Positron emitting
radiopharmaceuticals (PET):
Structural analogues of a
biologically active molecule
Instrumentation
The gamma camera
Instrumentation
The gamma camera: collimator
Instrumentation
Collimator:
To mechanically confine direction of
photons reaching each detector by
restricting the acceptance angle, to
allow spatial localization of source
while minimizing contributions from
Compton-scattered photons.
Hexagon pattern (most common):
d/2
t
Instrumentation
Collimator:
Parallel-hole collimator (most common):
Spatial resolution:
z = distance from source to collimator
⇒ R depends on depth in the body z!
⇒ Geometric distortions!!
z
t
R
d
Front
face
L
Geometric efficiency:
↓t ⇒ ↑G ; ↑ d ⇒ ↑G
k = constant determined by geometry
(parallel; diverging; converging; pinhole)
Back
face
L>>t (L~24 mm, t~0.15 - 0.40 mm)
z~10 cm ⇒ R~1 cm; G~0.015%
Instrumentation
The gamma camera: scintillation detector
Instrumentation
Scintillation detector:
Scintillation crystal coupled with a photomultiplier tube (PMT)
Instrumentation
1 large thallium-activated sodium iodine,
NaI(Tl), crystal
coupled to an array of ~ 50-100 PMTs
PMT
Scintillation detector:
NaI(Tl)
Scintillation crystal:
emission of 415 nm photons, 4 eV energy
13% conversion efficiency
Hexagonal PMTs:
Efficient packing +
PMT centers are equidistant
from their neighbors
(d ~ 25 – 30 mm)
d
Instrumentation
Scintillation detector:
Intrinsic spatial resolution R:
-Crystal thickness: ↑δ ⇒ ↑ R, ↑ sensitivity
(optimal δ ~ 0.6 cm in NaI(Tl) detector)
Light Spread Function
δ
-Statistical variability of crystal scintillation:
-Number of PMTs: ↑NPMT ⇒ ↓ R
(NPMT ~ 50-100 in NaI(Tl) detector)
-PMT non-linearities:
PMT energy responses vary and are non-uniform across face of scintillation camera
⇒ intensity inhomogeneity (corrected by calibration) + geometric distortions
Instrumentation
The gamma camera: Anger position network and PHA
-Position network: combination of PMT
signals to produce 4 output signals:
(X+, X-, Y+, Y-) and their sum – z signal.
-Pulse height analyzer (PHA): selection of z
within predefined range (~FWHM photopeak)
Instrumentation
Energy resolution (photopeak FWHM):
Ability to discriminate between unscattered and scattered γ rays
Energy spectrum for radionuclide/scintillation crystal used (e.g., 99m-Tc / NaI(Tl)):
-Photo peak: photoelectric effect in body (140 keV, FWHM ~14 keV, 10%)
-Iodine escape peak: photoelectric effect in crystal (28.5 keV ⇒ 140-28.5 = 111.5 keV)
-Lead X ray peak: photoelectric effect in collimator - lead (75, 88 keV ⇒ 140-88 ~ 52 keV)
-Compton scatter region: Compton scatter in crystal (peak ~ 90 keV ⇒ 140-90 ~ 50 keV)
Pulse height analyzer (PHA) threshold (~15%)
lead X ray
peaks
Compton
region
photo
peak
iodine
escape peak
FWHM
Broadening due to Compton scatter in body:
Câmara gama
The gamma camera: CRO or computer!
-Cathode ray oscilloscope (CRO): light flash
on screen in position related to scintillation
Scintigraphy
General principles:
Scintigraphy
General principles:
- Isotropic photon emission from the body
- Mechanical collimation to position (x,y)
- Crystal scintillation and light generation in a disc centered in (x,y) with a certain width,
with intensity proportional to energy of incident photon and number of photons
- Scintillation light divided among PMTs according to spatial proximity to (x,y)
- Conversion of light to electric pulse at each PMT, proportional to intensity of light
- Combination of pulses in position network and energy selection by Pulse Height
Analyser (PHA)
- Pulse intensities for each (x,y) position are stored to produce a 2D projection image
(projection axis perpendicular to plane of collimator)
Scintigraphy
Image characteristics:
Spatial resolution R:
- Rcollimator: depends on collimator geometry and depth within the body
- Rdetector: depends on crystal thickness, photon energy, number of PMTs
- RCompton: depends on photon energy, depth in body, collimator and PHA
- kfilter:
depends on post-acquisition low-pass filtering
(typically, R ~1-2 cm at large depth; R ~5-8 mm near the surface)
PSF
FWHM
tail
detector +
collimator
geometry
Compton
scattering
x
Scintigraphy
Image characteristics:
Signal-to-noise ratio SNR:
Poisson distribution ⇒
- radiopharmaceutical dose and organ specificity (limited by safety)
- scanning time (limited by radiotracer half life and patient comfort)
- tissue attenuation (depth within the body, photon energy)
- gamma camera sensitivity (crystal thickness and collimator geometry)
- gamma camera dead time
(~20% loss)
N is the true count rate
n is the observed count rate
- post-acquisition low-pass filtering (to reduce noise levels)
Contrast-to-noise ratio CNR:
no background signal ⇒ CNR ~ SNR
- some degradation due to Compton scattering
- further degradation due to partial volume effects (depend on R)
- further degradation (or improvement) as a result of low-pass filtering
Scintigraphy
Applications:
Part of the Body
Example Radiotracer
Brain
99mTc-HMPAO
Thyroid
Na99mTcO4
Lung (Ventilation)
Lung (Perfusion)
Liver
Spleen
133Xe
gas
99mTc-MAA
99mTc-Tin
99mTc-Damaged
Colloid
Red Blood Cells
Pancreas
75Se-Selenomethionine
Kidneys
99mTc-DMSA
Scintigraphy
Applications:
Renogram:
Use of a radiotracer that is
excreted through the kidneys
(e.g., 99m-Tc-DTPA) and
acquisition of a series of
images to observe excretion
through the kidneys.
Single photon emission computed tomography (SPECT)
Configuration:
Rotating gamma camera
- Rotation around 360º
-Circular or elliptical orbit
(to minimize depth in body)
- Continuous or in discrete steps
Single-head gamma camera
(to optimize efficiency vs blurring)
-Single or multiple head:
single-head, two-head, three-head
↑ nb cameras ⇒ ↑ sensitivity, ↓ R
- A 3D image is obtained
Dual-head gamma camera
Single photon emission computed tomography (SPECT)
Basic principles
-Application of tomographic techniques to produce 2D cross-sectional images
using a gamma camera to detect the emission of a radiopharmaceutical (as in planar
imaging) at different positions around the patient.
-Radionuclides and radiopharmaceuticals identical to planar scintigraphy (99m-Tc)
-Scintillation detectors identical to planar scintigraphy (NaI(Tl)-PMT)
-Mechanical collimation similar to planar scintigraphy, but different beam shape
-Image characteristics differ: ↑ CNR, ↓ SNR, ↑ R…
Single photon emission computed tomography (SPECT)
Collimator:
Parallel hole collimator:
Focused, converging
(pinhole) collimator:
↑ sensitivity,
but more difficult
reconstruction
Single photon emission computed tomography (SPECT)
Sensitivity and line spread function (LSF):
Sensitivity depth dependence:
Solid angle of detection ∝ 1/r2
Volume of sensitive area ∝ r2
d
r
⇒ Sensitivity is ~ invariant with distance between source and detector
Cone beam: Convergent axially and transaxially
d=0.5 cm
r = 10 cm
Ω=0.015 %
Fan beam: Convergent transaxially
Single photon emission computed tomography (SPECT)
Image reconstruction
Image function = spatial distribution of radiopharmaceutical activity:
y
y’
Projection at position φ:
x’
pφ(x’)
φ
Filtered backprojection:
x
Filtered projection:
A(x,y)
Single photon emission computed tomography (SPECT)
Image reconstruction
Image function = spatial distribution of radiopharmaceutical activity:
y
j
Projection at position φ:
i
piφ
φ
Filtered backprojection:
x
Filtered projection:
Aij
Single photon emission computed tomography (SPECT)
Attenuation correction:
Attenuation ⇒ artifacts due to gamma ray attenuation between source and detector
Including attenuation µ(x,y):
Including attenuation µij:
Weighted filtered backprojection:
Attenuation weighting factor:
Attenuation correction methods:
-assume µ is spatially uniform and estimate physical dimensions of object (using
CT or MRI)
- good for brain, not chest.
-measure µ spatial distribution (using a transmission scan of a reference activity tube)
and use to calibrate - easier with iterative algorithms, but these are slower…
Single photon emission computed tomography (SPECT)
Scatter correction:
Scatter ⇒ significant contributions due to (inevitable) absence of source collimation
Dual-energy window detection method:
-main window: around photopeak with width Wm
– photoelectric + Compton contributions:
Cm = Cprim + WmCCompton
-sub-window: in Compton region with width Ws
– Compton contributions only:
Cs = Ws CCompton
⇒ Primary radiation contribution (nb counts):
Cprim = Cm - Cs Wm/Ws
The window width ratio Wm/Ws can be optimized by maximizing the SNR after scatter
correction
(e.g. for 99m-Tc, Wm = 28 keV (~20%) around 140 keV,
Ws = 32 keV (~7%), around 120 keV)
Single photon emission computed tomography (SPECT)
Image characteristics
Similar to planar scintigraphy – additionally:
Spatial resolution R
Approximately Gaussian PSF, with FWHM ~ 1 cm, data matrix ~64x64, 128x128
(14-17 mm for single-head systems, 6-10 mm for three-head systems)
↑ Data matrix ⇒ ↓ R
Signal-to-noise ratio SNR
Signal = mean nb counts / Noise ~ standard deviation of photopeak
(typically, ~500.000 counts in brain, ~100.000 counts in myocardium)
↑ Data matrix ⇒ ↓ SNR
Contrast-to-noise ratio CNR
Ratio of peak nb counts in “hot spot” to mean nb counts in the background
(~ 5-6 x greater than planar scintigraphy, due to separation of superimposed sources)
Single photon emission computed tomography (SPECT)
Applications
Part of the Body
Example Radiotracer
Brain
99mTc-HMPAO
Thyroid
Na99mTcO4
Lung (Ventilation)
Lung (Perfusion)
Liver
Spleen
133Xe
gas
99mTc-MAA
99mTc-Tin
99mTc-Damaged
Colloid
Red Blood Cells
Pancreas
75Se-Selenomethionine
Kidneys
99mTc-DMSA
Measurements: Spatial distribution; Extent of uptake;
Rate of uptake; Rate of washout
Single photon emission computed tomography (SPECT)
Applications
Brain perfusion: using 99m-Tc-HMPAO (Ceretec) or Neurolite, which pass BBB
⇒ rupture of BBB leads to radiotracer accumulation in diseased tissues
⇒ spatial distribution of radiotracer is proportional to rCBF
⇒ radiotracer accumulates in regions of high rCBF (tumor, epilepsy, stroke,
schizophrenia, dementia, …)
Single photon emission computed tomography (SPECT)
Applications
Cardiac function: using radiotracers that accumulate in myocardium (Tc or Tl-based)
(most common,
myocardial perfusion
– stress test,
to diagnose
myocardial ischemia
and infarct)
Positron Emission Tomography (PET)
Radioactive decay by positron emission
Positron emitting isotopes
Positron Emission Tomography (PET)
Positron emitting radionuclides
Major elemental
constituents of
biological tissues
1H analog
Positron Emission Tomography (PET)
Basic imaging principle:
External source:
Uncollimated
Collimated
Positron Emission Tomography (PET)
Basic imaging principles:
External source
Internal source
Positron Emission Tomography (PET)
Basic imaging principles:
Internal source:
Mechanical collimation
Electronic collimation
SPECT
PET
Positron Emission Tomography (PET)
Basic imaging principle:
- Detection of 2 anti-parallel γ within a certain time window (simultaneity) and along a
line of response (LOR) (colinearity – electronic collimation): annihilation
coincidence detection (ACD) – to localize annihilation (positron emission) point.
Positron Emission Tomography (PET)
Basic system configurations:
Translations + Rotations:
- easily to fulfill uniform sampling requirements
- coincidence detection from opposite banks
⇒ limited efficiency
Rotations only:
- insufficient linear sampling, resolved by
new sampling schemes (e.g., wobbling)
-improved efficiency
Positron Emission Tomography (PET)
Basic system configurations:
Complete stationary ring of detectors
defines the Field-of-View, FOV)
Positron Emission Tomography (PET)
Basic system configurations:
Each detector element is connected by a coincidence circuit to a set of opposite
detector elements (both in plane and axial, ~ half the total number of detectors N/2):
⇒ N/2 fan beam projections per detector
acceptance
angle
Positron Emission Tomography (PET)
Basic system configurations:
Multilayer scanners: stacks of rings of detectors in spherical or cylindrical arrangements
Instrumentation
Basic components:
-Scintillation detectors (crystals coupled with PMTs, in rings)
-Annihilation coincidence detection circuitry (PHA + coincidence detector)
(+ cyclotron!)
Objectives:
→ optimize spatial resolution (localization accuracy)
→ optimize sensitivity (nb detected annihilations)
Instrumentation
Scintillation detectors:
Desired characteristics for PET:
- High density
- High Z
- Short scintillation decay time
⇒
⇒
⇒
↑ detection efficiency
↑ detection efficiency
↑ SNR
- Emission wavelength ~ 400 nm
- Refraction index ~ 1.5
- High relative light output
(photon yield per keV)
⇒
⇒
⇒
↑ sensitivity of PMT
↑ light transmission crystal to PMT
↓ complexity and cost
- Non-hygroscopy
⇒
↑ compacticity, ↓ complexity and cost
-Small crystal size (~1cm)
⇒
↓R
Instrumentation
Scintillation detectors:
Instrumentation
Scintillation detectors:
Instrumentation
Scintillation detectors:
Block detector of BGO crystals
1 crystal block: 8x8=64 grooves, 4 PMTs
~6.5 mm
~6.5 mm
4
Instrumentation
Scintillation detectors:
Block detector of BGO crystals
Instrumentation
Scintillation detectors:
Block detector of BGO crystals
Quadrant sharing: each PMT straddles over 4 quadrants of 4 different blocks
⇒ larger PMTs
⇒ ↓ number of PMTs
⇒ ↓ R
⇒ ↑ dead time
Instrumentation
Detector rings:
Blocks of detectors arranged in 1 or more rings (up to 48)
(e.g., 288 blocks in 4 rings of 72 blocks) ⇒ Millions of LORs are measured
Axial coverage
~ 15 cm (brain)
~170-198 cm
(whole body)
Diameter
~ 80-90 cm
Instrumentation
Detector rings:
Each detector element is connected by a coincidence circuit to a set of opposite
detector elements (both transaxially and axially:
~ half the total number of detectors N/2 ⇒ N/2 fan beam projections per detector
Instrumentation
Annihilation coincidence detection circuitry
Detector: PMT electric pulse → PHA logic pulse of length τ → coincidence detector:
Summed signal from 2 detectors > thr. ⇒ record line integral between the 2 detectors
2τ = coincidence
resolving time
(~12-20 ns)
Instrumentation
Annihilation coincidence detection circuitry
Image reconstruction
Filtered backprojection
Object function:
Projection:
= total nb of coincidence events detected
by the pair of detectors defining line L
Filtered backprojection:
Projection
L
Image reconstruction
Filtered backprojection:
Fan beam geometry
Image reconstruction
Multi-slice: 2D
Image reconstruction
Multi-slice: 2D
N direct planes
N-1 cross planes
2N-1 planes
Image reconstruction
Multi-slice: 3D
3D vs 2D: ↑↑↑ sensitivity, ↑scatter (due to absence of absorbing septa)
2N-1 planes
N2 planes
Reconstrução de imagem
Multi-slice: 3D
Reconstrução de imagem
Multi-slice: 3D vs 2D: L-6-[18F]fluoroDOPA study: ↑SNR
Corrections
Total number of observed coincidences for detector pair i,j:
Cijtrue number of true coincidences
Cijscatter number of scatter coincidences
Cijaccidental number of accidental coincidences
DTL
dead time losses
T
acquisition time
Fij distribution of detector efficiencies
Corrections
Normalization of detector efficiency:
exposing uniformly all detector pairs to a 511 keV photon source (e.g., 68 Ge source),
without a subject in the field of view:
Normalized counts for detector pair i,j:
Normalization factor:
Amean = average counts for detector pairs in the same plane
Corrections
Multiple coincidence:
Combination of a true coincidence with one or more unrelated events ⇒
data need to be discarded ⇒
Dead Time loss (DTL) = Cijmultiple
(depends on the electronics)
Correction method:
- multiply Cijtotal by correction factor:
Corrections
Random or accidental coincidence:
2 unrelated annihilation photons are emitted from 2 independent sources and are
detected within the coincidence resolving time ⇒ incorrectly assigned line integral
resulting from the finite resolving time of the coincidence circuitry
Rate of random coincidences for detector pair i,j:
Ri, Rj = single count rates of detectors i, j
2τ = coincidence resolving time
Correction methods:
-subtract Cijrand from Cijtotal before reconstruction
-subtract a delayed coincidence measurement
(using additional timing circuitry) before reconstruction
Corrections
Scatter coincidence:
1 or both photons resulting from one event suffer Compton scattering with a small
enough energy loss / scatter angle, so that they are detected within the photopeak
Corrections
Scatter coincidence:
1 or both photons resulting from one event suffer Compton scattering with a small
enough energy loss / scatter angle, so that they are detected within the photopeak
⇒ incorrect position information
⇒ broader tail in line spread function
Correction method:
-measure scatter outside patient + extrapolate
-tighter inter-slice collimation in multi-ring systems
Corrections
Attenuation correction
The count rate observed
is independent of the position
of the source along the LOR:
it depends on the diameter alone.
Correction methods:
-assume µ is spatially uniform and estimate physical dimensions of object (using
CT or MRI) - good for brain, not chest.
-calibrate data using transmission spatial distribution obtained from external source
(ring of positron emitters, e.g. 68-Ge)
-estimate µ spatial distribution using X ray transmission and use to calibrate
Corrections
Attenuation correction
Image reconstruction
using transmission scan
calibration for
attenuation correction
Corrections
Attenuation correction using X-ray transmission
Corrections
Dual-modality imaging: PET-CT
Patient table:
translation
Corrections
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.
Corrections
Iterative reconstruction
Corrections
Iterative reconstruction
A
B
Image characteristics
Sensitivity:
number of counts per unit time detected per unit of activity present in a source.
Electronic collimation:
Detector A
coincidence detection records
events from a volume defined by
the column joining the two detectors
Detector B
r
True coincidence count:
(empirical formula derived from phantom)
k = constant
ρ = activity concentration [µCi / cm3]
α = probability of photon scattering within the object volume [cm-3]
η = detector efficiency [counts / sec]
h = slice thickness [cm]
d = object diameter [cm]
D = ring diameter [cm] (↑D ⇒ ↓ sensitivity)
Image characteristics
Signal-to-noise ratio SNR:
- radiopharmaceutical dose and organ specificity
- scanning time
- tissue attenuation (depth within the body)
- detector sensitivity
- post-acquisition low-pass filtering
-noise equivalent count rate:
Contrast-to-noise ratio CNR:
- no background signal ⇒ CNR ~ SNR
- some degradation due to Compton scattering
- further degradation due to partial volume effects (depend on R)
Image characteristics
Noise equivalent count rate:
After random and scatter coincidence correction:
Doses which lead to concentrations greater than 1 µCi/cc give no improvement in the
number of events recorded by the scanner and serve only to increase the dose to the subject.
Image characteristics
Spatial resolution
-Rdetector = intrinsic resolution of scintillation detector
Rdetector~ crystal diameter/2
- Rpositron = finite distance between positron emission and annihilation
Rpositron~rms~1-2 mm (“sphere of uncertainty”: increases with energy)
- Rnoncol = non-colinearity due to slight deviation from nominal angle of 180º →
statistical distribution around 180º with FWHM ~ 0.25º
⇒ Rnoncol ~ 1.6 mm (D = 60 cm), 2.6 mm (D = 100 cm).
-krecon = degradation due to reconstruction algorithm (up to 1,5 depends on the method)
Image characteristics
Spatial resolution
Positron range and non-colinearity
Image characteristics
Partial volume effects
As the resolution degrades, the smaller cylinders progressively show an underestimation
of the concentration due to partial volume effects.
Applications
Oncology
Neurotransmitters
References
• • • • Webb, “Introduction to Biomedical Imaging”, Wiley 2003.
Cho, “Foundations of Medical Imaging”, Wiley 1993.
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