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Tomographic Imaging

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Tomographic Imaging. SPECT PET Hybrid. Spect. Conventional, Planar Imaging. Tomographic Imaging. Series of Projection images. Camera head(s) rotate about patient 360 o for most scans 180 o for cardiac scans Continuous acquisition or Step & Shoot Projection images acquired - PowerPoint PPT Presentation
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Tomographic Imaging SPECT PET Hybrid
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Page 1: Tomographic Imaging

Tomographic Imaging

SPECTPETHybrid

Page 2: Tomographic Imaging

SPECT

Page 3: Tomographic Imaging

Series of Projection images

Conventional, Planar Imaging Tomographic Imaging

Page 4: Tomographic Imaging

Data Acquisition

• Camera head(s) rotate about patient• 360o for most scans• 180o for cardiac scans

• Continuous acquisition or Step & Shoot• Projection images acquired• Images reconstructed

• Filtered Backprojection• Direct Fourier Transform• Iterative

• Multi-slice imaging

Page 5: Tomographic Imaging

Cardiac Scan

Myocardial perfusion studies are acquired with 180o arc.

Projection images from opposite 180o have poor spatial resolution & contrast due to greater distance & attenuation.

Page 6: Tomographic Imaging

Multi-Head SPECT Systems

A dual-headed gamma camera system (top).

Note that the camera heads can be placed in different orientations to provide 2 simultaneous views of an organ or the body (bottom). • Typically 180° for whole body SPECT,

90° for cardiac imaging

Sensitivity ↑ 2 angular projections acquired simultaneously 2-fold total # countsORSame # of counts acquired in ½ time

Page 7: Tomographic Imaging

Orbit Shape

Page 8: Tomographic Imaging

Image Reconstruction

2-D intensity display of set of projection profiles (sinogram)

Each row in display corresponds to individual projection profile, sequentially displayed from top to bottom.

Point source of radioactivity traces out a sinusoidal path in the sinogram

Page 9: Tomographic Imaging

1/r Blurring

A. Computer-simulation phantom

B. Sinogram of simulated data for a scan of the phantom

C. Image for simple backprojection of data from 256 projection angles. 1/r blurring is apparent in the object, and edge details are lost.

Page 10: Tomographic Imaging

Filter Kernels

Ideal Ramp Filter Ramp Filters w/ Roll-Off

Removes 1/r blurring, sharpening image detailAmplifies high frequency noise

Statistical noise (random nature of decay & photon interactions) dominates high frequencies roll-off will smooth image

Page 11: Tomographic Imaging

Iterative Reconstruction

More computationally intense than FBP1. Requires > 1 iteration / image

• Each iteration ≈ 1 FBP2. Algorithms often incorporate

characteristics of imaging device• Collimator & object

scatter• System geometry• Finite detector

resolution

Page 12: Tomographic Imaging

Iterative Example

Page 13: Tomographic Imaging

Matrix Size

Page 14: Tomographic Imaging

Sampling Effects

Linear Sampling of Projections # of Angular Samples

Δr (Sampling Interval) ≤ FWHM/3 Nviews ≥ π FOV/(2Δr )

Page 15: Tomographic Imaging

Sampling Coverage

Effects of angular sampling range on images of a computer-simulation phantom. Images obtained by sampling over 45°, 90°, 135°, and 180°.

Sampling over an interval of less than 180° distorts the shape of the objects and creates artifacts

Page 16: Tomographic Imaging

Detector Failure

Effects of missing projection elements on reconstructed image. Left, Sinogram of computer-simulation phantom. Right, Reconstructed image.

Page 17: Tomographic Imaging

SNR Comparison

Planar Imaging• SNR

• Npixel = # counts recorded for that pixel

Tomographic Imaging• SNR

• <Npixel> = average # counts recorded / reconstructed pixel

• npixels = total # of pixels

Stronger requirements on counting statistics for tomographic imaging as compared with planar imaging to achieve same level of SNR

Page 18: Tomographic Imaging

CNR Comparison

Planar Imaging• CNR

• |Cl| = Absolute contrast of lesion

• = • R counting rate over lesion • R0 = counting rate over background

• dl = Diameter of lesion• ID0 = Background info. density

(cts/cm2)

Tomographic Imaging• CNR SNRpixel

• |Cl| = Contrast of lesion• nl = # pixels occupied by lesion

• SNRpixel ↓ as compared to planar imaging

• Low-contrast lesion contrast ↑ as compared w/ planar imaging

For the same level of object contrast & total # of image counts (in absence of distance & attenuation effects), no intrinsic difference in CNR between planar & tomographic imaging

Page 19: Tomographic Imaging

Tomographic Imaging Advantages

• Detecting low-contrast lesions• Ability to remove confusing overlying structures that interfere w/

lesion detectability• e.g. ribs overlying lesion in lungs• Lesion shape & borders also become clearer

• Does not improve detectability of lesions by ↑ CNR• More accurate determination of radioactivity concentrations in

particular tissue volume

Page 20: Tomographic Imaging

Planar Vs. SPECT

Thoracic phantom images

↑Contrast & ↑visibility when overlying activity removed in SPECT

Page 21: Tomographic Imaging

SPECT Challenges

• Actual LOR resembles diverging cone rather than cylinder

• Signal recorded not exactly proportional to total activity w/in LOR• Signal from activity closer to detector more heavily weighted than deeper

lying activity due to attenuation of overlying tissue• Activity outside LOR contributes to signal

• Crosstalk due to scattered radiation• Septal penetration through collimator

• Most of discrepancies from ideal vary w/ γ-ray energy• Lead to artifacts & can seriously degrade image quality

Page 22: Tomographic Imaging

Divergence of Response Profile

Volumes of tissue viewed by a collimator hole at 2 different angles separated by 180°.

Differences in the volumes viewed results in different projections from the 2 viewing angles

Page 23: Tomographic Imaging

Attenuation Effects

Attenuation leads to further differences in these two projections, emphasizing activity that is close to the gamma camera compared with activity further away that has to penetrate more tissue to reach the gamma camera.

Values are shown for the attenuation of the 140-keV γ rays from 99mTc in water

Page 24: Tomographic Imaging

Conjugate Counting

Response profiles vs. source depth for single view projection of line source in air & H20 AIR: Degradation of spatial resolution w/ distance from collimatorH20: Degradation due to distance & attenuation

2 opposing view projections of line source in air & water arithmetically averagedAIR: No degradation w/ distanceH20: Only degradation due to attenuation

2 opposing view projections of line source in air & water geometrically averagedAIR: No degradation w/ distanceH20: No degradation w/ distance

Page 25: Tomographic Imaging

Attenuation & Photon Energy

𝑰=𝑰𝟎𝒆−𝝁 𝒙

Page 26: Tomographic Imaging

Attenuation Correction

If attenuation coefficient constant throughout tissue volume(reasonable assumption in brain, abdomen)

Flood source Line source

If attenuation coefficient not constant throughout tissue volume(reasonable assumption in thorax, pelvis)Transmission scan

Page 27: Tomographic Imaging

Attenuation Map

Attenuation map of the thorax reconstructed from the reference and transmission scans obtained with a moving line transmission source

Reference Scan: 1st scan acquired w/ no object in FOV

Transmission Scan: 2nd scan acquired w/ object of interest in FOV

Page 28: Tomographic Imaging

Scatter Correction

Dual energy windows used to simultaneously acquire SPECT & transmission scans

Patient equivalent phantom scanned to acquire scatter distribution in projections

Page 29: Tomographic Imaging

Partial Volume Effects

Each cylinder contains same concentration of radionuclide, but w/ ↓ diameter For sources/volume > 2 x FWHM, image

intensity reflects both the amount & concentration of activity w/in volume

For smaller objects that only partially fill voxel, total amount of activity still correct, but intensities of pixel no longer reflect concentration of activity

Spillover: when ROI has low tracer accumulation relative to surrounding tissues activity from these areas spills over to ROI

Results in ↓ contrast & under or over-estimation of tracer concentrations

Page 30: Tomographic Imaging

SPECT Collimator Design

Parallel Hole Fan Beam

Page 31: Tomographic Imaging

Spatial Resolution

water

Co 57 line sources

In general the spatial resolution in SPECT is slightly worse than in planar imaging.• Camera head farther from patient• Spatial filtering used to reduce noise reduces resolution• Short time/view lower resolution collimator to obtain adequate numbers of counts

Page 32: Tomographic Imaging

SPECT vs. Planar

Planar• Radioactivity in tissue in front of & behind tissue/organ of interest ↓ contrast

• Non-uniform pattern of radioactivity superimposes on activity distribution of tissue of interest

• Structural noise

SPECT• ↑ contrast & ↓ structural noise by eliminating counts from activity on

overlapping structures• If iterative reconstruction implemented:

• partially compensate for effects of scattering photons in patient• collimator effects

• ↓ spatial resolution with ↑ distance from camera• septal penetration

• When attenuation is measured with sealed source or CT data, can also partially correct for patient attenuation

Page 33: Tomographic Imaging

SPECT QC

• X & Y Magnification Factors• Multi-Energy Spatial Registration• Center of Rotation

• Mechanical COR must coincide w/ COR defined for each projection• If detector sags/wobbles as it rotates, artifacts result

• Additional blurring or ring artifacts• Uniformity

• Even very small non-uniformities can lead to major artifacts unlike planar imaging)• Rings or arcs in images

• Flood field uniformities <1% desirable• To achieve 1% uncertainty, Poisson stats dictate 10,000 counts per pixel• For 64x64 matrix ~ 41 million counts

• Camera Head Tilt

Page 34: Tomographic Imaging

SPECT ARTIFACTSAttenuation Center of Rotation Uniformity Stray Magnetic Field Effects Motion

Page 35: Tomographic Imaging

Truncation Artifact

Portion of the imaging volume falls outside the gamma camera FOV during a portion of the acquisition arc.

Page 36: Tomographic Imaging

Bulls Eye Ring Artifact

Insufficient gamma camera uniformity

Page 37: Tomographic Imaging

COR Artifact

Ideally, the center of rotation is aligned with the center, in the x-direction, of each projection image.Misalignment can be• Mechanical• Camera head not exactly centered in gantry• Electronic

Will cause loss of resolution in imagesPoint sources can appear as doughnuts

Page 38: Tomographic Imaging

Jaszczak SPECT Phantom

Page 39: Tomographic Imaging

SPECT APPLICATIONSMyocardial PerfusionCerebral PerfusionOncologyInfection / InflammationLiver & Kidney Function

Page 40: Tomographic Imaging

Applications

• Myocardial Perfusion• Assess CAD & heart muscle damage following infarct• Gated or Non-gated

• Cerebral Perfusion• Cerebral vascular disease• Dementia• Seizure Disorders• Brain tumors• Psychiatric Disease

• Oncology• Accumulation of cancer cells in both primary & metastatic lesions

• Infection/Inflammation• Liver/Kidney Function

Page 41: Tomographic Imaging

Cardiac Perfusion

SPECT images showing perfusion in the heart muscle of a normal adult using 99mTc-sestamibi as the radiopharmaceutical. The image volume has been re-sliced into 3 different orientations as indicated by the schematics on the left of each image row. SPECT data were acquired over 64 views with a data acquisition time of 20 sec/view. Images were reconstructed with filtered backprojection onto a 128 × 128 image array.

Page 42: Tomographic Imaging

Brain Perfusion

Transaxial SPECT images showing perfusion in the brain of a normal adult following injection of 890 MBq of 99mTc-HMPAO. Data were acquired on a triple-headed gamma camera with low-energy, high-resolution fan-beam collimators. One hundred twenty projection views were collected in 3-degree increments (40 views per camera head) with an imaging time of 40 sec/view. Total imaging time was approximately 30 minutes, with acquisition commencing 50 minutes after radiotracer injection. 

Page 43: Tomographic Imaging

PET

Page 44: Tomographic Imaging

Positron-Electron Annihilation

Page 45: Tomographic Imaging

Detectors

• Scintillation crystals coupled with PMTs in pulse mode

• Signal characteristics identified:• Position in detector• Energy deposited

• Energy discrimination used to reduce mispositioning due to scatter

• Time of interaction• Coincidence detection

Page 46: Tomographic Imaging

Detector Characteristics

• Emit light promptly (Small decay constant)• True coincidence distinguishable from random coincidence• Reduce dead time losses at high interaction rates

• High linear attenuation coefficient for 511-keV γ• Max counting efficiency

• High conversion efficiency• More precise event localization• Better energy discrimination

Page 47: Tomographic Imaging

Comparison of Scintillators

Scintillator Decay Constant (ns) Attenuation Coefficient, 511 keV (cm-1)

Conversion Efficiency Relative to NaI

NaI(Tl) 250 0.343 100

BGO 300 0.964 12-14%

GSO(Ce) 56 0.704 41%

LSO(Ce) 40 0.870 75%

BGO being replaced by LSO, LYSO or GSO

Page 48: Tomographic Imaging

MicroPET

Page 49: Tomographic Imaging

PET Radionuclides

Nuclide Half-Life (min)

Max Positron Energy (keV)

Max Positron Range (mm)

C-11 20.4 960 4.2

N-13 10.0 1198 5.4

O-15 2.0 1732 8.4

F-18 110 634 2.4

Rb-82 1.3 3356 17

Page 50: Tomographic Imaging

FDG

Radioactive tracers manufactured to mimic naturally occurring substances already used by the body.

As the body incorporates the radioactive tracers into its systems, PET scan can monitor their progress and examine specific bodily processes that use the tracer

Page 51: Tomographic Imaging

Potential Coincidences

Ratio of Random to True coincidence ↑ as activity ↑ & ↓ as time window ↓• Difference in arrival time of true

coincidence photons from edge of FOV

• Decay constant of scintillation in detector

Scatter coincidences depends on amount of scatter material

• Less in head than body• Energy discrimination circuits can

reject some of scatter• Many 511 keV photons interact in

detectors by Compton scattering• Deposit less than their entire

energy in detectors

Page 52: Tomographic Imaging

Spatial Resolution

• Whole Body Scanners• ̴W 5 mm FWHM of LSF at center of ring

• 3 factors limit spatial resolution• *Intrinsic resolution of detectors*• Distances traveled by positrons before annihilation• Annihilation photons are not exactly 180o apart

• Organ Motion also ↓resolution• respiration

Page 53: Tomographic Imaging

Resolution ↓ w/ ↑Distance from Center

Page 54: Tomographic Imaging

Resolution Factors

positron

electron

range travelled before annihilation

positron

electron

vβ+

γ

γ

γ

γvβ+≠ 0vβ+≈ 0

Resolution ↓ as β+ energy ↑ (range ↑) If β+ velocity at time of annihilation > 0, γ’s not emitted at 180o

Page 55: Tomographic Imaging

Attenuation Correction

511 keV annihilation γ

511 keV annihilation γ

dd-x

xProbability of both γ’s escaping patient without interaction independent of where annihilation occurred

Page 56: Tomographic Imaging

Attenuation Correction: Transmission Measurement

rod source-Ge68-Ga68 β+ emitter

Page 57: Tomographic Imaging

Attenuation Methods

Ge68-Ga68 positron source

Cs-137 gamma ray source

120 kVp x-ray source

Page 58: Tomographic Imaging

With/Without Attenuation Correction

transmission (attenuation) image

18FDG uptake with attenuation correction

without attenuation correction

Page 59: Tomographic Imaging

Image Reconstruction

Page 60: Tomographic Imaging

Filtered Backprojection

Page 61: Tomographic Imaging

2D vs. 3D Acquisition

A

B

CA

B

C

A: Activity outside FOV REMOVEDB: Scattered Photon REMOVEDC: Valid Coincidence REMOVED

A: Activity outside FOV INCREASEDB: Scattered Photon INCREASEDC: Valid Coincidence ACCEPTED

Page 62: Tomographic Imaging

Coincidence Detection Efficiency

Coin

ciden

ce d

etec

tion

efficie

ncy

Position along axis of PET

2D

3D

2DEfficiency nearly constant along axial length of detector rings

3DEfficiency ↑ linearly from ends of rings to center 3D whole body acquisitions accomplished by discontinuous motion, greater overlap of bed positions necessary

Page 63: Tomographic Imaging

Time-Of-Flight

Page 64: Tomographic Imaging

TOF

Ability of PET scanner to accurately measure time between 2 γ-interactions from 1 annihilation is defined as TOF capability.

Page 65: Tomographic Imaging

TOF Images

Page 66: Tomographic Imaging

PET QC

Test Description Frequency

Uniformity Uniform Scan of positron emitting source

Daily

Tomographic Uniformity

Scan of uniform cylindrical source Periodically

Normalization Measure efficiencies of all detector LOR & update stored normalization factors

Quarterly or if any uniformity test reveal non uniformity

Absolute Activity If quantitative measurements are to be used

Quarterly

Systems Test:Spatial resolutionStatistical noiseCount rate performanceSensitivityImage quality

Annually

Page 67: Tomographic Imaging

Quantitative Imaging

• Desire: Pixel value α to # of nuclear transformations • Physiological Model developed in 1970’s

• Rate of local tissue glucose utilization calculated from amount of FDG that accumulates in tissue

• SUV: Standardized Uptake Value• g/cm3

• Attempts to normalize for:• Administered activity• Radioactive decay• Body mass

𝑺𝑼𝑽=𝒂𝒄𝒕𝒊𝒗𝒊𝒕𝒚 𝒄𝒐𝒏𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒐𝒏 𝒊𝒏𝒗𝒐𝒙𝒆𝒍𝒔𝒐𝒓 𝒈𝒓𝒐𝒖𝒑𝒐𝒇 𝒗𝒐𝒙𝒆𝒍𝒔

𝒂𝒄𝒕𝒊𝒗𝒊𝒕𝒚 𝒂𝒅𝒎𝒊𝒏𝒊𝒔𝒕𝒆𝒓𝒆𝒅 /𝒃𝒐𝒅𝒚𝒎𝒂𝒔𝒔

Page 68: Tomographic Imaging

Factors Limiting Accuracy

• Assayed activity accuracy• Extravasation of activity during administration• Accuracy of attenuation correction• Correct recording of elapsed time• Accuracy of patient body mass• Physiological state• Body composition• Size of lesion• Motion• ROI selection

Page 69: Tomographic Imaging

PET ARTIFACTSAttenuation Correction Motion Stray Magnetic Fields Module Loss, Block Loss or Mis-calibration Coincidence Timing

Page 70: Tomographic Imaging

HYBRID MODALITIESPET/CTPET/MRISPECT/CT

Page 71: Tomographic Imaging

SPECT/CT

CT data can be used to correct for tissue attenuation in the SPECT scans on a slice-by-slice basis.

Page 72: Tomographic Imaging

Attenuation Correction

Uncorrected SPECT scan

Attenuation correction factors

Attenuation corrected SPECT

Page 73: Tomographic Imaging

Bilinear Model of CT attenuation Correction

Page 74: Tomographic Imaging

PET/CT & SPECT/CT Advantages

• Superior Attenuation Correction• High photon flux reduces statistical noise• Imaging time reduced• Post injection CT scans can be made• Eliminates need for (consumable) transmission source

• Anatomic CT images fused with functional SPECT scan• Functional anatomic maps

Page 75: Tomographic Imaging

PET/CT Artifacts

Page 76: Tomographic Imaging

Respiration

Page 77: Tomographic Imaging

Contrast Agent

Page 78: Tomographic Imaging

Truncation

Page 79: Tomographic Imaging

PET/MRI

Page 80: Tomographic Imaging

PET/CT vs. PET/MRI

Page 81: Tomographic Imaging

Dose

Modality Effective Dose (mSv)

PET/CT-10 mCi Dose

PET 7

CT diagnostic 16

CT nondiagnostic 4

Page 82: Tomographic Imaging

Effective Doses

Page 83: Tomographic Imaging

85

END


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