# Presentation: 521917

## Investigation of the LabPETTM Detector and Electronics for Photon-Counting CT Imaging

**Investigation of the LabPET****TM**** Detector and Electronics for Photon-Counting CT Imaging**

- _Philippe Bérard_a, J. Riendeaub, C. M. Pepina, D. Rouleaua,
- J. Cadorettea, R. Fontaineb, R. Lecomtea

***1st European Conference on Molecular Imaging Technology ***
***Marseille 9-12 May 2006***

- a Department of Nuclear Medicine and Radiobiology
- b Department of Electrical and Computer Engineering
- Université de Sherbrooke, Sherbrooke, Québec, Canada

## 2003 - 2004

- IEEE NSS/MIC Portland, October 2003 (2 x 2 x 10 mm3).
- Academy of Molecular Imaging, March 2004, Orlando FL (1 x 1 x 10 mm3).

- 1 x 1  mm2  single L(Y)SO pixel PET detector
- Standard NIM electronics

- Am241  59.5 keV 1mm3 source     74 MBq

**2003 - 2004**

- 1 week !

- 5 sec/proj

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- 40 sec/proj

- 30 sec/proj

## Main Purpose

**Main Purpose**

- 	Develop a combined multi-modality scanner capable of obtaining ***anatomical*** & ***functional**** ****data*** using a single apparatus relying on the ***same detection system.***

- 	Conduct successful follow-up investigations of gene expression, disease progression and therapeutic outcome in small animal models.

- See **poster # 191** Lecomte et al. Preliminary report on the LabPETTM, a high-performance APD-based digital PET scanner for small animal imaging.

**Investigation of photon-counting mode CT performed using LabPETTM detectors and digital electronics.**

## Dose issue in follow-up studies

_**Effects of radiation dose on small animals**_

- LD50/30 for mouse:  ~ 5 – 7.5 Gy
- Doses in the range 5 - 200 cGy : induce cell resistance to subsequent therapeutic doses of radiation (“adaptive response”*)
- Doses in the range 1 - 20 cGy : induce therapeutic effects on certain tumor cells

-  *G.P. Raaphorst and S. Boyden, “Adaptive response and its variation in human normal and tumor cells”, Int. J. Radiat. Biol. 75, pp. 865-873, 1999.

**PET/CT dose objective:  lower than 1 cGy**

- Lower spatial resolution: Dose ~ Δx-4,  PET scintillator ~ 2 x 2 mm2 pixels

- Micro CT typical dose ranges 10 - 40 cGy (100 μm resolution, σ~ 25 HU)**

- **Lee et al. A flat-panel detector based micro-CT system: performance evaluation for small-animal imaging, Physics in Medicine and Biology, no.48, 2003.
- Ford, N. L.,  Thornton, M. M.,  Holdswoth, D. W. Fundamental image quality limits for microcomputed tomography in small animals, Med. Phys., vol. 30, no. 11, pp. 2869-2877, 2003.

## Photon counting in CT

- Each event has equal weight independent of energy
***Elimination of weight factor proportional to energy of integration imaging***
***Closer to optimal weighting of E******-3******  ****
- Threshold detection allows discrimination of noise and scatter

*****	R.N. Cahn et al., “Detective quantum efficiency dependence on X-ray energy weighting in mammography”, *Med. Phys.* 26 (12), pp. 2680-2683, December 1999.

**Photon counting in CT**

- Detector: high light output scintillators to detect low-energy X-rays,
-                    fast fluorescence to avoid pulse-pile-up
-  count-rate > 106 - 107 counts/s, *the higher the better*

_**Requirements :**_

## Potential PET/CT Scintillators

|  | BGO | GSO | LSO | LYSO | YSO | NaI(Tl) | CsI(Tl) |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Decay Time (ns) | 300 | 60 | 40 | 41 | 70 | 230 | 1000 |
| Light Output (APD) | 30 | 40 | 80 | 100 | 105 | 100 | 210 |
| Peak Emission (nm) | 480 | 430 | 420 | 420 | 430 | 410 | 550 |
| 1/μ (mm) at 60 keV | 0.40 | 0.17 | 0.60 | 0.62 | 1.73 | 0.44 | 0.29 |
| Photoelectric probability (%) at 60 keV | 97 | 99 | 95 | 95 | 94 | 98 | 99 |
| 1/μ (mm) at 511 keV | 11.6 | 15 | 12.2 | 12.6 | 44 | 31 | 24 |
| Photoelectric probability (%) at 511 keV | 44 | 26 | 34 | 34 | 5 | 18 | 22 |

**Potential PET/CT Scintillators**

**Photon-counting CT scintillator requirements:**

**Suitable for PET**

## PET/CT Detector

**PET/CT Detector**

- LYSO best combination of :
-  high attenuation coefficient
-  high light output
-  fast decay time
-                PET and CT imaging in photon-
-                   counting mode

|  |
| --- |
| Decay Time (ns) |
| Light Output (APD) |
| Peak Emission (nm) |
| 1/μ (mm) at 60 keV |
| Photoelectric probability (%) at 60 keV |
| 1/μ (mm) at 511 keV |
| Photoelectric probability (%) at 511 keV |

| LYSO |
| --- |
| 41 |
| 100 |
| 420 |
| 0.62 |
| 95 |
| 12.6 |
| 34 |

**The high photoelectric absorption ******** confines spatial resolution to the crystal of interaction**

- reflector

- scintillator

- APD

## PET/CT Configuration

- Oxford instruments

- 3. Normal microfocus x-ray tube with off focal spot geometrical configuration and reconstruction. **See TomXGam and PIXSCAN**

- 1. Rod anode microfocus

- 2. Carbon nanotube cold cathode

## Materials and Methods

- Stepping motors move X-ray source and phantom

- Data counts transferred by serial link

- Motor controller 115200 steps over 360o

- Acquisition with a limited number of detectors

- Step and shoot acquisition

## LabPETTM Electronics

- LabTEPTM 1    ‘fast track’ module

- See **poster # 180** Fontaine et al. Digital signal processing applied to crystal identification in positron emission tomography dedicated to small animals.

**Notes:**
16-channel low-noise charge sensitive preamplifier (CSP) along with individually adjustable 500 V regulators for APD biasing and differential outputs. The differential signals from the CSP were individually sampled by 8-bit, 45-MHz free-running analog-to-digital converters (ADCs) and the data processed in parallel by a Xilinx Virtex-II PRO field programmable gate array (FPGA) to extract the photon count rate of each individual detector. 
The digitized data stream from the free-running ADC of each detector channel was stored in a ring buffer memory and a moving average calculation was used to extract the baseline and detect local maxima of the signal. The estimated average
baseline is delayed and compared with the current sample by a configurable level detector to detect an event. When the
current sample exceeds the threshold energy and the detected maximum is below the cutoff energy, an event counter is
increased for this channel and the whole event data stream is sent to a FIFO. The maximum sample value detected in the event stream is also stored and used to build a pulse height histogram while the event counter records a measurement of
the count rate.

## Digitized CSP signal from X-ray source

- A veto with a duration of 8 samples was programmed in the FPGA after threshold crossing to detect maximum

- Resulting non-paralyzable dead-time of **156 ns** limits the **count rate to 5.6 x 10****6**** events/sec/channel**.

- Signal rise time ~90 ns (at 50% ADC maximum dynamic range)
- Fast baseline restorer to enable fast count rate

**Sample number**

**Sample value**

- ~ 55 keV event

- 65 kV, 20 A

- dxd = 23 cm

- 0.6 mm Cu

## Electronics count-rate

- _X-ray source and LabPETTM__ __detector_

- Non-paralyzable model

*m* = recorded count rate
*n* = true interaction rate
** = system dead time: 156 ns

**10 % dead time at 1.6 x 10****6**** counts/s**

**25 % dead time at 2.0 x 10****6**** counts/s**

- Paralyzable component from the detector !

- CSP saturation
- Charge integration time
- Scintillator decay time

## CT Image Acquisition

- _Operation characteristics_
- Voltage : 65 kV
- Current : 20 μA
- Filter : 0.6 mm Cu
- Distance x-ray source – phantom : 11.5 cm
- Distance phantom - detectors : 11.5 cm
- nb detectors : 16
- Linear sampling : 2 mm (detector width)
- 180 projections , 0.1 second/projection   25 minutes scan time
-                               1 second/projection   45 minutes scan time
- Parallel FBP reconstruction algorithm : Nyquist cutoff frequency

## Energy resolution of detector

- Energy spectrum used for image acquisition.

**Landmark for small animals : 20-30 keV mean energy.**

## Spatial Resolution

- PSF : FWHM 1.03 mm
-           FWTM 2.50 mm

- 250 μm rod

- PSF

- PSF

- FWHM

- FWTM

## Spatial Resolution

**Spatial Resolution**

- G. T. Barnes, M. V. Yester, M. A. King, *Optimizing computed tomography (CT) scanner geometry*, SPIE Vol. 173 Application of Optical Instrumentation in Medicine VII (1979)

- MTF10% sys exp = 0.71 lp/mm

**MTF**

## Spatial Resolution

**Spatial Resolution**

- MTFsys = **MTF****foc**  MTFdet  MTFsam  MTFalg

- G. T. Barnes, M. V. Yester, M. A. King, *Optimizing computed tomography (CT) scanner geometry*, SPIE Vol. 173 Application of Optical Instrumentation in Medicine VII (1979)

**MTF**

- MTF10% sys exp = 0.71 lp/mm

**a = 50 ********m**

## Spatial Resolution

**Spatial Resolution**

- MTFsys = MTFfoc  **MTF****alg**  MTFdet  MTFsam

- G. T. Barnes, M. V. Yester, M. A. King, *Optimizing computed tomography (CT) scanner geometry*, SPIE Vol. 173 Application of Optical Instrumentation in Medicine VII (1979)

**MTF**

- MTF10% sys exp = 0.71 lp/mm

## Spatial Resolution

**Spatial Resolution**

- MTFsys = MTFfoc  MTFdet  **MTF****sam**  MTFalg

- G. T. Barnes, M. V. Yester, M. A. King, *Optimizing computed tomography (CT) scanner geometry*, SPIE Vol. 173 Application of Optical Instrumentation in Medicine VII (1979)

**MTF**

- MTF10% sys exp = 0.71 lp/mm

## Spatial Resolution

**Spatial Resolution**

- MTFsys = MTFfoc  MTFdet  MTFsam  **MTF****alg**** **

- G. T. Barnes, M. V. Yester, M. A. King, *Optimizing computed tomography (CT) scanner geometry*, SPIE Vol. 173 Application of Optical Instrumentation in Medicine VII (1979)

- MTF10% sys theo = 0.73 lp/mm

**MTF**

- MTF10% sys exp = 0.71 lp/mm

## Noise

- 3.5 cm diameter cylinder filled with water

- Ring artifacts !!!

- 1 sec/projections

**Noise**

## Noise

| Biological Tissue | HU |
| --- | --- |
| Bone | 100 to 1000 |
| Soft tissues | 0 to 80 |
| Adipose tissues | -50 to -100 |
| Kidney | 30 |
| Liver | 40 to 50 |
| Blood | 40 |

**σ**** ****~ D****-0.5**

**Noise**

- GORE, J. C., TOFTS, P. S. _Statistical limitations in computed tomography_, *Phys. Med. Biol.,* vol. 23, no. 6, pp. 1176-1182, 1978.

## Contrast phantom

**Contrast phantom**

|  | Average (HU) | Standard 
deviation (HU) | Density (g/cm3) |
| --- | --- | --- | --- |
| 1. Teflon | 303.6 | 20.2 | 1.2 x 10-3 |
| 2. Polyethylene | -126.6 | 14.2 | 1.06 |
| 3. Polystyrene | -86.6 | 18.2 | 0.93 |
| 4. Air hole | -996.2 | 51.7 | 2.25 |
| 5. Polycarbonate | 50.6 | 18.9 | 1.15 |
| 6. Nylon  | 55.5 | 16.3 | 1.2 |
| Plexiglas | 117.1 | 19.2 | 1.19 |

- 1

- 2

- 3

- 4

- 5

- 6

- 3.5 cm diameter Plexiglas phantom filled with 5 mm diameter rods

**Several tissue-like materials can be discriminated with sufficient accuracy.**

- Measured dose (TLD): 0.7 mGy

## Conclusion

- APD-based inorganic scintillator detectors and fast digital multi-channel pulse processing electronics, as developed for the LabPETTM, are suitable to ***detect***** **and ***count*** individual X-rays in the diagnostic energy range.

-  Resolution : 1.03 mm
-  Noise : 6.8 HU
-  Dose : 0.7 mGy

## Future Work

-  Higher resolution : magnification, higher sampling, smaller detector pixel size

-  Image artefact : CT adapted image reconstruction algorithm

-  Add more detector, more analog and digital electronics

- Objectives :
- ~ 1 μl PET resolution, < 500 μm CT resolution

## Acknowledgments

**Acknowledgments**

- Thanks to:
- Murray Davies and Henri Dautet of PerkinElmer Optoelectronics, Vaudreuil, QC for providing the detectors used in this study

## Thank You for your attention ! 

- Thank You for your attention !