# Presentation: 931485

## Physics & Instrumentation inPositron Emission Tomography

**Paul Vaska, Ph.D.**
- Center for Translational Neuroscience
- Brookhaven National Laboratory
- July 21, 2006

## Non-invasive Medical Imaging Techniques

** ****Anatomical**
**X-ray**
**CAT**
**MRI**
**Ultrasound**
** ****Functional**
**“****nuclear medicine” - SPECT, PET**
**Optical fluorescence, ...**

## Positron Emission Tomography

**Recent mainstream acceptance**
**relatively expensive**
**cyclotron for tracer production**
**detectors must stop high-energy gamma-rays**
**low resolution (>2 mm), limited counting statistics**
**BUT unique functional capabilities**
**Applications**
**Diagnosis of disease**
**cancer (WB), cardiac, ...**
**Research**
**brain function**
**animal studies**

## Technical Challenges in PET Imaging

- Radiochemistry - better tracers
- Imaging Physics - better images by
- Detector design
- Spatial resolution
- Sensitivity
- Image processing
- Corrections for physical effects
- Image reconstruction algorithms
- Data Analysis & Biological Modeling - better interpretation of images

## PET Imaging Overview

**Synthesize radiotracer**
**Inject radiotracer**
**Measure gamma-ray emissions from isotope (~20-60 min)**
**Reconstruct images of radiotracer distribution (nCi/cc)**

## Positron (+) Decay

**Positron (********+****) Decay**

- 18F-FDG

## + Decay

- Neutron-deficient isotopes can decay by emitting positrons

- anti-neutrino

- positron

- Net effect: one proton replaced by
-  neutron
-  anti-neutrino
-  positron

## Positron annihilation

- Annihilation gives
- 2x 511 keV gamma rays
- 180 degrees apart
- Line of response
- Positron range & gamma noncollinearity
- Scanner is  just a photon counter!
- Counts gamma-ray _*pairs*_ vs. single gammas
- Time window ~ 1 ns

- 511 keV

- 511 keV

- e+

- e-

## Raw Data & Image Reconstruction

**Raw Data & Image Reconstruction**

**0******** projection**

**90******** projection**

**image  reconstruction**

**“****sinogram”**

## Important Detector Properties

- Spatial resolution
- Directly controls spatial resolution in reconstructed image
- Currently ~ 1 - 5 mm
- Depth-of-interaction?
- Reduces “parallax”

## Important Detector Properties

- Detection efficiency (aka sensitivity, stopping power)
- Reduces noise from counting statistics
- Currently > ~ 30% (singles)

## Important Detector Properties

- Time resolution
- Affects acceptance of random coincidences
- Currently ~ 1 - 10 ns
- Time-of-flight (TOF)?
- c = ~ 1 ft/ns
- Need << 1 ns resolution

## Important Detector Properties

- Scatter and Attenuation

- 511 keV

- Energy resolution
- Scattered gammas change direction AND lose energy
- Affects acceptance of scattered coincidences
- Currently ~ 20%
- Deadtime
- Handle MHz count rates!

- 511 keV

- 400 keV

## Prototypical PET Detector

**Scintillation**
**Crystal**

**PMT**

**Pre-Amplifier**
**+ Electronics**

**Gamma photon converts to optical photons (proportional to gamma energy, typ. 1000’s)**

**photons are collected at the end of the crystal**

**light is converted to an electrical signal & amplified**

**Front-end electronics condition the signal for further processing**

**Prototypical PET Detector**

**Gamma Ray**

**Optical reflector**

## New Developments

- Detectors
- Multimodality imaging
- Specialized applications

## New Developments: Detectors

**New Developments: Detectors**

- Scintillators
- No perfect choice - tradeoffs
- Also practical qualities
- Rugged?
- Hygroscopic?
- Cost?

## New Developments: Detectors

- Photosensors
- Photomultiplier tubes
- Avalanche photodiodes
- Arrays, position-sensitive
- Compact but noisier
- Silicon photomultipliers
- Very new
- Best of both?

- APD array

- PMT

- SiPM

## New Developments: Detectors

- Solid-state detectors
- Direct conversion, no photodetector
- Great dE/E & spatial resolution
- Poorer timing & stopping power
- CZT

## New Developments: Detectors

- Pb converters & ionization

- HIDAC

- Pb-walled straws (50 cm long)

## New Developments: Detectors

- 3D gamma-ray event positioning
- Depth of interaction
- Reduces parallax problem

## New Developments: Detectors

- Time of flight using LaBr3

## New Developments

- Multimodality imaging
- PET/CT
- PET/MRI
- Specialized applications
- Brain, breast, prostate
- Small animal - microPET
- Arterial input function
- Humans - wrist scanner
- Animals - microprobe
- Awake rat brain - RatCAP

## RatCAP: Rat Conscious Animal PET

- Eliminate anesthesia in preclinical neuroscience using PET in order to:
- Remove confounding effects of anesthetic on neurochemistry
- Enable stimulation in animal PET
- Enable correlations of behavior and neuro-PET

## Architecture

- Detector blocks x12
- LSO 2.2 x 2.2 x 5 mm in 4 x 8 array
- 1:1 coupling to APD
- ASIC - single all digital output
- Timestamp & Signal Processing Module
- Programmable real-time logic (FPGA)
- 1 ns bins (debugging, now 10 ns)
- Data acquisition
- PCI card in standard PC
- Up to 70 MB/s = ~10 Mcps singles
- Offline software for coincidences, corrections, recon, ...

## Architecture

- RatCAP

- TSPM

-  LSO       APD

- ASICs

- all interconnections

- 38 mm FOV

- 72 mm OD

- optical links to PCI

- high voltage

- 194 g

- data, clock, power

- 18 mm axial FOV

## Performance

- Spatial resolution (FWHM @ CFOV)
- FBP: 	  			2.1 mm
- MLEM: 			           <1.5 mm
- Energy resolution: 			23% FWHM
- Time resolution: 			14 ns FWHM
- window = 30 ns
- Sensitivity (point @ CFOV):		0.7%
- Peak Noise Equivalent Count rate:	14 kcps @ 5 Ci/cc

- 1st prototype:  LLD = 150 keV average, variable

## Imaging Conditions

- Anesthetized 250-350 g rats
- Limited DAQ livetime >> long scans for statistics
- Artifacts

## F-18 Fluoride Bone Scan

- 1.3 mCi fluoride

- RatCAP

- microPET R4

## C-11 Raclopride

- 1.8 mCi raclopride

- In the RatCAP

## C-11 Methamphetamine

- Time-activity curve for striatum

## Thanks!

- DOE OBER funding