Applications in Nuclear Medicine and Radiology

Professor Bob Ott

Institute of Cancer Research and Royal Marsden Hospital

Topics

• Digital x-ray imaging

• Fast CT scanning

• SPECT scanners for small animals

• SPECT/CT scanning

• PET scanners for small animals

• New crystals for PET and SPECT

• Active pixel sensors in medical imaging

Imaging requirements• X-ray imaging of anatomy

energies between ~20keV and 140keV

performed in integrate mode

contrast between tissues often small• Single photon emission computed

tomography (SPECT) imaging tissue function

gamma ray energies between 80-364 keV• Positron emission tomography (PET) imaging

of tissue function

gamma ray energy 511 keV• PET and SPECT in pulse counting mode

Digital planar X-ray imaging

• Systems have been based on the use of:storage phosphor platesflat panel detectors such as amorphous

silicon or seleniumscanning slot devices with CCDsphosphors imaged with a CCD or CMOS devices

• Typically 70 microns spatial resolution is possible for breast imaging with ~100% photon detection at ~20keV

• 10 lp/mm possible compared to 15 lp/mm with film

Indirect flat panel sensor for x-ray imaging

Diagnostic X-ray CT scanning

• Historically detectors based on the use of CsI coupled to silicon diodes or Xe gas detectors

• More recent developments involve the use of CdWO4 or ceramic scintillators such as Yttrium Gadolinium Oxide which have ~2x the light output of CdWO4

• New fast ceramic detectors use gadolinium oxide (GDOS) have a short decay time and reduced afterglow (by 400 times).

• Can make fast images with 30% less radiation dose

Comparison of afterglow from scintillators used in CT

Diagnostic X-ray CT scanning• Main developments are in multi-slice imaging to

speed up scanning allowing heart scanning in 5 beats

Scanner Channels Axial length (mm)

Rotation speed (s)

GE Lightspeed VCT

64 x 0.625 40 0.35

Philips Brilliance 64

64 x 0.625 40 0.4

Siemens Sensation 64

64 x 0.6 28.8 0.37

Toshiba Aquilion 64

64 x 0.5 32 0.4

Curved View (1) and X-Section (2) views showing the calcified plaque on the LAD

X-ray CT future developments

• Toshiba have developed a new 256 x 0.5mm row detector array which is soon to be commercial

• GE and Siemens are developing flat panel detector CT systems which can be used for RT planning but are presently too slow for diagnostics

High resolution animal SPECT

• The HiSPECT system is an add-on to existing NaI(Tl)-based gamma cameras to give multi-pinhole aperture sensitivity and enhanced resolution.

• The Nano-SPECT system is a purpose built (Mediso) small animal imaging gamma camera system with a resolution of <0.8mm and with multi-pinhole sensitivity.

HiSPECT withmultipinholecollimator

HiSPECT images of mouse using Tc-99m tracers

Nano-SPECT system

Nano-SPECT images of mouse taken in helical mode

SPECT/CT scanning

• Following the development of PET/CT scanners several SPECT/CT scanners have now been developed

• Provide improved attenuation/scatter correction plus anatomy as well as function

• Siemens and Philips have just connected double headed gamma cameras to conventional CT scanners

• GE have produced a gamma camera gantry incorporating a low cost CT scanner

SPECT/CT images

The new LabPET system

Made with APDs coupled to individual scintillating crystals (LSO)

Properties of the LabPET systemSpecification LabPET 3.6 LabPET 7.2

Ring diam (cm) 15.6 15.6

Aperture (cm) 11 11

Axial FoV (cm) 3.6 7.2

# of APDs 1536 3072

Scint size (mm) 2 x 2 2 x 2

Linear spatial Resolution (mm)

1.1 1.1

Volume resolution (l)

2.4 2.4

Coinc time window (ns)

2-12 2-12

Images from the LabPET system

F-18-FDG F-18 fluoride

HIDAC MWPC PET system

HIDAC MWPC PET system

F-18 fluoride F-18 FDG

The new PETMOT system

• The system will combine phoswitch- PET and micro-lens array optical tomograph

• Optical lens system is 1cm2 block containing 100 x 1mm lenses

• Optical collimator used to reject non-orthogonal rays

• Coupled to photodiodes

Multi lens array assembly for a single block

Without (l) and with (r) optical collimator

PET-MOT system

Transaxial

With and without optical collimator

PET-MOT system

• Allows both optical and positron emission tomography simultaneously

• The optical system inside the PET array has little effect on the 511keV photons and is insensitive to them

• J Peter and W Semmler, German Cancer Centre, Heidelberg

An MR compatible PET system for small animals

• LSO multi ring PET system mounted inside the MR magnet with a purpose-built RF coil within PET ring

• 104 2mm x 3mm x 5mm crystals coupled to 2mm diameter optical fibres

• Fibres connected to MC-PMTs mounted in an RF screened box

• Ring diameter 75.5 mm• P Marsden et al at St Thomas’ Hospital

PET- MR system layout

PET scanner within MR boreOff-set concentric PET rings

PET – MR resolutions

spatial pulse height timing1.4-1.9mm ~45% in 1m 10.9ns

15cm

3.4m3.4m

3.4m

New crystals for PET/SPECTCrystal Rel. light

output1/e decay time (ns)

Peak (nm)

Refractive index

Density (g.ml)

NaI (Tl) 1.0 250 415 1.85 3.67

LaCl3(Ce) 0.7-0.9 28 350 ~1.9 3.79

BaF2 0.05/0.16 0.6/630 195/310 1.5 4.88

BGO 0.2 300 480 2.15 7.13

LSO 0.75 40 420 1.82 7.4

GSO 0.2 60 430 1.85 6.71

LaBr3(Ce) 1.3 26 380 ~1.9 5.29

LaBr3:Ce scintillation camera

• Pani et al have developed a small scintillation camera using this new scintillator coupled to a flat panel PSPMT

• Achieve an energy resolution of 6.5% and a spatial resolution of 1.1mm FWHM

• Efficiency at 140 keV is twice that of NaI(Tl) with a 6mm crystal

LaBr3:Ce TOF PET scanner

• Karp et al have developed a ring PET scanner using LaBr3:Ce crystals 4mm x 4mm by 30mm coupled via continuous light guide to PMTs

• Energy resolution (8.5%) is better than the equivalent LSO scanner (>20%) leading to a reduced scatter fraction (22% vs 42%)

• Peak NEC rates are better than the LSO scanners

• Timing resolution is ~315ps!!

Active Pixels Sensors in Medical Imaging

• Active pixel sensors are being developed under the MI-3 basic technology grant

• Will allow on-chip intelligence and ‘individual pixel/ROI’ read-out

• Applications include:High resolution (sub-mm) gamma camera imaging

Digital X-ray imaging

High resolution (<5) digital autoradiography

Summary

• Still plenty of mileage in new detectors for both NM and Radiology to:

improve image contrastimprove spatial resolutioncombine modalitiesreduce radiation dosepixel intelligence