SPECT imaging with semiconductor detectors

Andy Bostonajboston@liv.ac.uk

Outline of presentation

• What is SPECT?• What detector technology can we

consider?• The ProSPECTus project & links to

fundamental research• The future prospects

What is SPECT?

Functional imaging modality

What SPECT Radionuclides?

141 keV

t1/2=65.94h

2.1105y

Mo9942

Tc9943

Ru9944

t1/2=6.01hm99 Tc

>99%

910-5%

stable

Tomographic Imaging• The sinogram is what we aim to measure

- Measure of intensity as a function of projection, θ and position, r - Often seen plotted as a 2d grey scale image

Underlying source distribution“Shepp-Logan Phantom”

Measured result – “Sinogram”(256 projections, 363 positions per projection)

Note : We measure from 0 to 180°

θ

r

p(r , θ)

x

y

f(x ,y)

SPECT : Problems/Opportunities

Technical• Collimator Limits Spatial Resolution & Efficiency• Collimator is heavy and bulky• Energy of radioisotope limited to low energy

• NaI:Tl Dominant for >40 Years...• MRI Existing PMTs will not easily operate

• Would like to be able to image a larger fraction of events.

Common radionuclides: 99mTc, 123I, 131I

Fraction

TrueScatter

Other

What are the detector requirements?

• Ideally would want:– Good energy resolution (Good light yield/charge

collection) < few%– High efficiency (High Z)– Position resolution– Timing resolution

• Detector materials:– Semiconductors (Si, Ge, CdZnTe) – Scintillators (LaBr3, CsI(Tl), NaI(Tl), BaFl, BGO)

ProSPECTus

Next generation Single Photon Emission Computed

TomographyNuclear Physics Group, Dept of Physics, University of Liverpool,

Nuclear Physics & Technology Groups, STFC Daresbury Laboratory, MARIARC & Royal Liverpool University NHS Trust

ProSPECTus: What is new?ProSPECTus is a Compton Imager

• Radical change No mechanical collimator• Utilising semiconductor sensors• Segmented technology and PSA and digital electronics

(AGATA)• Image resolution 7-10mm 2-3mm• Efficiency factor ~100 larger• Simultaneous SPECT/MRI

What’s new?Conventional SPECT Compton camera

• Gamma rays detected by a gamma camera

• Inefficient detection method• Incompatible with MRI

• Gamma rays detected by a Compton camera

• Positions and energies of interactions used to locate the source

Source

E0

Factors that limit the performance of a Compton Imager:Energy resolution, Detector position resolution, Doppler Broadening

1cm 2cm

2cm5cm

141keV

Si(Li) Ge

• Total Coincident ~3.49%

• SPECT ~ 0.025% (typical value)

• Factor of ~140

Event Type %

Single / Single 2.23Single / Multiple 0.33Multiple / Single 0.61Multiple / Multiple 0.04Not absorbed 0.28

System ConfigurationGEANT4 simulations L. Harkness

HPGe Germanium

• Excellent energy resolution • Medium Z (32)• Lithographic electrode segmentation

• Requires cooling to LN2• HPGe growth still presents challenges

• Technology drivers: large scale physics projects (AGATA/GRETA/GERDA/MAJORANA)

AGATA(Advanced GAmma Tracking Array)

4 -array for Nuclear Physics Experiments at European accelerators providing radioactive and high-intensity stable

beams Main features of AGATAEfficiency: 43% (M =1) 28% (M =30)today’s arrays ~10% (gain ~4) 5% (gain ~1000)

Peak/Total: 58% (M=1) 49% (M=30)today ~55% 40%Angular Resolution: ~1º FWHM (1 MeV, v/c=50%) ~ 6 keV !!!today ~40 keV

Rates: 3 MHz (M=1) 300 kHz (M

=30)today 1 MHz 20 kHz

• 180 large volume 36-fold segmented Ge crystals in 60 triple-clusters • Digital electronics and sophisticated Pulse Shape Analysis algorithms

allow• Operation of Ge detectors in position sensitive mode -ray tracking

Pulse Shape Analysisto decompose

recorded waves

Highly segmented

HPGe detectors

· ·

··

Identified interaction

points(x,y,z,E,t)i

Reconstruction of tracks

e.g. by evaluation of permutations

of interaction points

Digital electronicsto record and

process segment signals

1

2 3

4

reconstructed -rays

Ingredients of -Tracking

ProSPECTus

Next generation Single Photon Emission Computed

TomographyNuclear Physics Group, Dept of Physics, University of Liverpool,

Nuclear Physics & Technology Groups, STFC Daresbury Laboratory, MARIARC & Royal Liverpool University NHS Trust

The SmartPET DSGSD detectors

Detector Specification• Depletion at -1300V,

Operation at -1800V• 12 x12 Segmentation,

5mm strip pitch• 1mm thick Aluminium

entrance window

• Warm FET configuration, 300mV/MeV pre-amps

• Average energy resolution ~ 1.5keV FWHM @ 122keV

Am-241 AC x-y surface intensity distribution

• The results are presented for 60 keV with 2 minutes of data per position.

AC01 AC12DC12

DC1

PSA techniques developed through characterisation measurements

Calibration of variation in detector pulse shape response with position

Image Charge

Real Charge

Parameterisation of these pulse shapes provides increased position sensitivity

Pulse Shape Analysis

SmartPET detector depth response

AC signalsDC signals

DC signals AC signals

“superpulse” pulse shapes for 137Cs (662 keV) events versus depth

Image Reconstruction• Sensors have excellent energy &

position information. • Uniformity of sensor response• Optimise existing:

– Analytical– Iterative– Stochastic

• Requirement for GPU acceleration

Compton Imaging

Typical measurements:• 10μCi 152Eu• 6 cm from SPET 1• Source rotated• Zero degrees in 15º steps

up to 60º• Detector separation• 3 – 11cm in 2cm steps• Gates set on energies• 2 sources 152Eu and 22Na

at different x and y

Use of the SmartPET detectors in Compton Camera configuration

E1

E2

E1

E2

212

2 111cosEEE

cme

o Compton Cones of Response projected into image space

Compton Imaging

E1

E2

E1

E2

212

2 111cosEEE

cme

o Compton Cones of Response projected into image space

Compton Imaging

E1

E2

E1

E2

212

2 111cosEEE

cme

o Compton Cones of Response projected into image space

Compton Imaging

E1

E2

E1

E2

212

2 111cosEEE

cme

o Compton Cones of Response projected into image space

Compton Imaging

E1

E2

E1

E2

212

2 111cosEEE

cme

o Compton Cones of Response projected into image space

Compton Imaging

FWHM ~ 8mm

6 cm source to crystal

30 mm crystal to crystal

E = 1408 keV, 30 keV gate

Compton Camera measurements (Ge/Ge)

No PSA (5x5x20)Iterative reconstruction

Compton Imaging~7º Angular Resolution FWHM, central position

2cm source

separation

152Eu E = 1408 keV 22Na E = 1274 keV152Eu

Multi-nuclide imaging

No PSA (5x5x20)Cone back projection

• Test existing gamma-ray detector in an

MRI scanner

• Does the detector cause distortions in

the MRI image? No

• Does the MRI system degrade the

detector performance? In certain

positions (which can be minimised)

• Encouraging results!

• ProSPECTus final construction stage

• System in ~6 months

MRI compatibility & Status

What are the next steps?

• Immediate priorities• We (almost) have an integrated Compton

Gamma camera optimised for <500keV• Demonstrate sensitivity with phantoms• Commence trials including clinical evaluation• For the future:• Consider electron tracking Si scatterer• Possible use of large CZT analyser (requires

large wafer material with 1cm thickness)

Patient benefits:• Earlier and more effective diagnosis of tumours (higher

probability of effective treatment).• Higher sensitivity offering the scope for shorter imaging time

(more patients through one machine per day) or lower doses of radio pharmaceuticals.

• Cardiac and brain imaging• Image larger patientsSPECT/MRI:• Functional/Anatomical• Image co-registration

ProSPECTus : The Implication

Credit

STFC Daresbury Laboratory, Daresbury, WA4 4AD, UKDepartment of Physics, University of Liverpool, L69 7ZE,

UKMARIARC, University of Liverpool,

RLUH NHS Trust,UK Industries

Funding agencies STFC, EPSRC, MRC

Many people have made significant contributionsLots of UK PhD’s and Post Docs

Laura HarknessUniversity of Liverpool2010 Shell and Institute of PhysicsVery Early career Woman Physicist of the Year