www.le.ac.uk

Evaluation of a CdTe detector for medical imaging

Layal K. Jambi

Bioimaging UnitSpace Research CentreSupervisor: Dr John Lees and Professor Alan Perkins

Outline

• Introduction

• Main research area

• XRI-UNO CdTe detector

• Performance specification

• Further work

Radiation detectors

• Radiation detectors are the sensing element in nuclear measurement systems.

• Detection of radiation is related to the absorption of radiation and how it interacts with matter.

• There are several types of radiation detectors.

Scintillation detectors

• The main purpose of scintillation in detectors is that the scintillator material converts higher energy incident photons into several lower energy photons.

Figure - taken from the Idaho state university Radiation Information Network.

Scintillation detectors

• There are two types of solid state scintillators:– Inorganic scintillatorse.g. NaI and CsI.NaI(TI) is the most frequently used scintillation crystal.

– Organic scintillatorse.g. Plastics composed of aromatic hydrocarbons.

Semiconductor detectors

• Semiconductors are based on a more direct approach which converts photons directly into an electronic signal.

Figure - taken from NSEP Nuclear Safeguards Education Portal.

Nuclear Medical Imaging

Small field of view (SFOV) cameras in medical imaging

• Pinhole collimator has been used with SFOV offering high spatial resolution.

• In medical imaging SFOV used for a small organ dedicated system.

Compact Gamma Camera (CGC)

Image taken from (Bugby, S.L, 2014)

Hybrid Gamma Camera (HGC)

CdTe X-ray and -ray detectors for imaging system

• Hybrid CdTe pixel detector arrays for breast screening to replace mammography.

• CdTe MediProbe for sentinel lymph nodes (SLNs) to replace the gamma probe.

Main research area

• Evaluate the performance of the XRI-UNO CdTe detector in comparison with CGC.

XRI-UNO system

Physical specification

Dimensions (W x L x H)

138mm x 172mm x 34mm

Active area 14.08mm x 14.08mm

Pixel size / # Pixels 55 µm x 55 µm / 65.536 pixels

Semicoducting material

1mm Cadmium Telluride

Initial investigation• A 0.45mm diameter cannula tube, 18mm length

filled with 0.63 MBq 99mTc solution.

A. With high resolution parallel hole

collimator (1mm )

B. With low resolution parallel hole

collimator (2mm )

C. Without collimator

A B C

No. of frames 999 frame, each frames 100ms

Performance specification

1. Intrinsic spatial resolution

2. System spatial resolution

3. Intrinsic spatial uniformity

4. Intrinsic sensitivity

5. Count rate capability

1. Intrinsic spatial resolution

• It is the full width at half maximum (FWHM) of a line spread function (LSF) or of a point spread function (PSF) without a collimator.

1. Intrinsic spatial resolution

345MBq of Cd-109 at 112mm distance from the 3mm width slit

1. Intrinsic spatial resolution

0 20 40 60 80 100 120

0

100

200

Cou

nts

Pixels

Edge Response Function (ERF) for Cd-109

1. Intrinsic spatial resolution

Modulus of LSF with fitted Gaussians

0 50 100

0

10

20

Counts Cumulative Fit Peak

Der

ivat

ive

of c

ount

s

Pixels

1. Intrinsic spatial resolution

Intrinsic spatial resolution

XRI-UNO CdTe 0.45 mm

CGC 0.63 mm

XRI-UNO is better in intrinsic spatial resolution

2. System spatial resolution

• The FWHM of a LSF or of a PSF with the imaging collimator in place.

2. System spatial resolution

345MBq of Cd-109 at 17mm away from the collimator with 9.5mm Perspex

2. System spatial resolution

FWHM (black) and FWTM (red) for Cd-109 by using 0.5mm pinhole collimator

5 10 15 20 25 30 35 40 45 500

2

4

6

8

10

12

14

Cd-109 FWHM FWTM

Spa

tial R

esol

utio

n (m

m)

Perspex Thickness (mm)

2. System spatial resolution

System spatial resolution

XRI-UNO CdTe 1.61 mm

CGC 1.28 mm

XRI-UNO has poorer system spatial resolution

3. Intrinsic spatial uniformity

• Describe the variation in counts per pixel.

3. Intrinsic spatial uniformity

Flood image of a 2 MBq Co-57 placed 107mm away from the detector

3. Intrinsic spatial uniformity

Intrinsic spatial uniformity

Co-efficient of variation XRI-UNO CdTe 0.38

CGC 1.58

Differential uniformity XRI-UNO CdTe 5.17

CGC 0.6

XRI-UNO is less uniform

4. Intrinsic sensitivity

• The proportion of photon flux incident on the detector that is recorded within the photopeak energy window being used.

• Tested by placing various width of scattering medium (Perspex) between the source and the detector.

4. Intrinsic sensitivity

0 10 20 30 40 50 60 70

0.020

0.025

0.030

0.035

0.040

0.045

Co

un

ts p

er

seco

nd

pe

r in

cid

en

t co

un

ts

Perspex Thickness (cm)

Cd-109 placed at 420mm away from the detector with increasing layers of Perspex

4. Intrinsic sensitivity

Intrinsic sensitivity

XRI-UNO CdTe 28839

CGC 62300

XRI-UNO is less sensitive

5. Count rate capability

• The ability of the detector to linearly measure counts.

5. Count rate capability

176 MBq of 99mTc placed directly on top of the un-collimated detector

0 2000 4000 6000 80000.0

2.0x106

4.0x106

6.0x106

8.0x106

1.0x107

1.2x107

1.4x107

Me

asu

red

co

un

ts

Incident counts

5. Count rate capability

Count rate capability

XRI-UNO CdTe 8134

CGC 1200

XRI-UNO has higher count rate capability

Summary

Performance specification XRI-UNO CdTe CGC

Intrinsic spatial resolution Better

System spatial resolution poorer ✓Intrinsic spatial uniformity Less uniform ✓Intrinsic sensitivity Less sensitive ✓Count rate capability Higher

Conclusion

• The XRI-UNO CdTe detector exceeds the CGC in areas such as intrinsic resolution and count rate capability.

• The XRI-UNO CdTe detector would not be able to replace the CGC due to low sensitivity.

Further work

• No more semiconductors.

• Try to use different type of scintillators such as Gadolinium Oxysulfide (GOS) ceramic scintillator.

Acknowledgement • University of Leicester

Dr. John Lees, Sarah Bugby, Mohammed Alqahtani, Dr. Simon Lindsay, Bahadar Bhatia and William R McKnight

• University of Liverpool

Sean Tipper

• University Hospitals Nottingham

Prof. Alan Perkins and A K Ng

• Leicester Royal Infirmary

Helen Hill and David Monk

Further information

• Bugby, S.L., J.E. Lees, B.S. Bhatia, and A.C. Perkins, Characterisation of a high resolution small field of view portable gamma camera. Phys Med, 2014. 30(3): p. 331-9.

• Bhatia, B.S., S.L. Bugby, J.E. Lees, and A.C. Perkins, A scheme for assessing the performance characteristics of small field-of-view gamma cameras. Phys Med, 2015. 31(1): p. 98-103.

Radioactive Sources used

Source Activity (MBq) Energy (keV) Diameter (mm)

Cadmium-109 345 22 8

Cobalt-57 2 122 6

Technetium-99m 229 140 NA

Comparison TableDetector CGC CdTe

Field of view (mm) 40 x 40 14.08 x 14.08

Intrinsic spatial resolution FWHM (mm) 0.63 0.4555

FWTM (mm) 1.06 0.7985

System spatial resolution FWHM (mm) 1.28 1.61

FWTM (mm) 2.35 8.00

Intrinsic spatial uniformity Integral uniformity (%) 8.5 100

Spread of differential uniformity (%) 0.6 5.174

Co-efficient of variation (%) 1.58 0.38

Intrinsic Sensitivity Counts per second 62300 28839

Count rate capability Maximum counts (incident counts per MBq)

1200 1668

Contrast to noise ratio CNR No data 12

Equations

• COV=

GOS Advantages

• High sensitivity, short decay time, short afterglow.

• Eco-friendly with no hazardous materials contained.

• Resistant to humidity.

• High x-ray shielding capability to protect light receiving element.

• Toshiba also has line-up of green and red light emitting high sensitivity scintillators, although with inferior decay time, afterglow characteristics.

http://www.toshiba-tmat.co.jp/eng/list/sc_cera.htm