tomographic imaging

Tomographic Imaging

SPECTPETHybrid

SPECT

Series of Projection images

Conventional, Planar Imaging 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

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.

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

Orbit Shape

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

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.

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

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

Iterative Example

Matrix Size

Sampling Effects

Linear Sampling of Projections # of Angular Samples

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

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

Detector Failure

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

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

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

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

Planar Vs. SPECT

Thoracic phantom images

↑Contrast & ↑visibility when overlying activity removed in SPECT

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

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

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

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

Attenuation & Photon Energy

𝑰=𝑰𝟎𝒆−𝝁 𝒙

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

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

Scatter Correction

Dual energy windows used to simultaneously acquire SPECT & transmission scans

Patient equivalent phantom scanned to acquire scatter distribution in projections

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

SPECT Collimator Design

Parallel Hole Fan Beam

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

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

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

SPECT ARTIFACTSAttenuation Center of Rotation Uniformity Stray Magnetic Field Effects Motion

Truncation Artifact

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

Bulls Eye Ring Artifact

Insufficient gamma camera uniformity

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

Jaszczak SPECT Phantom

SPECT APPLICATIONSMyocardial PerfusionCerebral PerfusionOncologyInfection / InflammationLiver & Kidney Function

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

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.

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. 

PET

Positron-Electron Annihilation

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

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

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

MicroPET

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

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

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

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

Resolution ↓ w/ ↑Distance from Center

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

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

Attenuation Correction: Transmission Measurement

rod source-Ge68-Ga68 β+ emitter

Attenuation Methods

Ge68-Ga68 positron source

Cs-137 gamma ray source

120 kVp x-ray source

With/Without Attenuation Correction

transmission (attenuation) image

18FDG uptake with attenuation correction

without attenuation correction

Image Reconstruction

Filtered Backprojection

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

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

Time-Of-Flight

TOF

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

TOF Images

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

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

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

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

HYBRID MODALITIESPET/CTPET/MRISPECT/CT

SPECT/CT

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

Attenuation Correction

Uncorrected SPECT scan

Attenuation correction factors

Attenuation corrected SPECT

Bilinear Model of CT attenuation Correction

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

PET/CT Artifacts

Respiration

Contrast Agent

Truncation

PET/MRI

PET/CT vs. PET/MRI

Dose

Modality Effective Dose (mSv)

PET/CT-10 mCi Dose

PET 7

CT diagnostic 16

CT nondiagnostic 4

Effective Doses

85

END