We have previously proposed SAPHIRE (scintillator avalanche photoconductor with high resolution emitter readout), a novel detector concept with potentially superior spatial resolution and low-dose performance compared with existing flat-panel imagers. The detector comprises a scintillator that is optically coupled to an amorphous selenium photoconductor operated with avalanche gain, known as high-gain avalanche rushing photoconductor (HARP). High resolution electron beam readout is achieved using a field emitter array (FEA). This combination of avalanche gain, allowing for very low-dose imaging, and electron emitter readout, providing high spatial resolution, offers potentially superior image quality compared with existing flat-panel imagers, with specific applications to fluoroscopy and breast imaging. Through the present collaboration, a prototype HARP sensor with integrated electrostatic focusing and nano- Spindt FEA readout technology has been fabricated. The integrated electron-optic focusing approach is more suitable for fabricating large-area detectors. We investigate the dependence of spatial resolution on sensor structure and operating conditions, and compare the performance of electrostatic focusing with previous technologies. Our results show a clear dependence of spatial resolution on electrostatic focusing potential, with performance approaching that of the previous design with external mesh-electrode. Further, temporal performance (lag) of the detector is evaluated and the results show that the integrated electrostatic focusing design exhibits comparable or better performance compared with the mesh-electrode design. This study represents the first technical evaluation and characterization of the SAPHIRE concept with integrated electrostatic focusing.

1. DAVID A. SCADUTOa, ANTHONY R. LUBINSKYa, JOHN A. ROWLANDSa,
HIDENORI KENMOTSUb, NORIHITO NISHIMOTOb, TAKESHI NISHINOb, KENKICHI TANIOKAc, WEI ZHAOa
aDept. of Radiology, Stony Brook University, Stony Brook, New York 11794-8460, bNanoX Japan, 2-1, Kanda-Ogawamachi, Chiyoda-ku, Tokyo 101-0052 Japan, cTokyo Denki University, 5 Senju Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan
Investigation of spatial resolution and temporal performance of SAPHIRE
(scintillator avalanche photoconductor with high resolution emitter readout)
with integrated electrostatic focusing
BACKGROUND
 Scintillator coupled to amorphous selenium (a-Se) high-
gain avalanche rushing photoconductor (HARP)
 High-resolution electron beam readout achieved with field-
emitter array (FEA)
 Potentially superior imaging performance for low-
dose/high-resolution imaging
 New integrated electrostatic focusing improves resolution
METHODS: Prototype Sensor
 Electrostatic focusing: integrated to each FEA pixel;
compatible with large-area sensors
 Accelerates electrons to reduce lateral
spread: high spatial resolution
0 20 40 60 80 100 120
0.1
1
10
100
1000
Dependence of X-ray to Charge Conversion Gain on Electric Field
RelativeSignal
Electric Field (V/µm)
Integrated electrostatic focusing (E = 27.8 keV)
External mesh-electrode focusing (E = 27.8 keV)
Hunt et al. 2002 (E = 40.9 keV)
METHODS: Imaging Performance
 Operational conditions: Focusing potential, electric field
of a-Se (ESe) varied, 40 kVp W/Al spectrum
 Sensitivity: X-ray to charge conversion gain and
avalanche gain versus ESe
 Spatial resolution: MTF measured using slanted-edge
method
 Lag: Residual signal measured after irradiation termination
RESULTS: Sensitivity and Lag
X-ray to Charge Conversion Gain vs. Electric Field
Lag Dependence on Focusing and Mesh Potentials
RESULTS: Spatial Resolution
Integrated Electrostatic Focusing
External Mesh-electrode Focusing
CONCLUSIONS
 Spatial resolution achieved by integrated electrostatic
focusing comparable to external mesh-electrode
performance
 Temporal performance of integrated electrostatic focusing
exceeds mesh-electrode focusing
 Results suggest FEA with integrated electrostatic focusing
practical approach for large-area sensor fabrication
 Future work: further improvements to focusing electrode
geometry, real-time lag clearance procedure
Figure 2. (Right) Sensors
with integrated electrostatic
focusing and with external
mesh-electrode focusing.
(Below) Photograph of
assembled sensor.
Integrated Electrostatic Focusing
External Mesh Electrode
Figure 3. Avalanche
gain (>10×)
demonstrated for
ESe > 80 V/μm allows
for x-ray quantum-
noise limited
operation at low
exposures.
D.C. Hunt et al., Med. Phys.
29(11), 2464, (2002).
Figure 4. First frame
lag smaller for
integrated
electrostatic focusing
than external mesh-
electrode. Further
improvements may be
made with real-time
clearance procedure.
Figure 6. Increasing external mesh-electrode voltage improves spatial
resolution. Maximum achieved MTF is 20% at Nyquist frequency.
Figure 1. SAPHIRE (Scintillator
Avalanche Photoconductor with
High Resolution Emitter
readout): CsI scintillator optically
coupled to amorphous selenium
HARP with avalanche gain. A
field emitter array provides high
resolution readout.
0 20 40 60 80 100 120
0.1
1
10
100
1000
RelativeSignal
Electric Field (V/µm)
Integrated electrostatic focusing (E = 27.8 keV)
External mesh-electrode focusing (E = 27.8 keV)
Hunt et al. 2002 (E = 40.9 keV)
0 2 4 6 8 10 12 14 16 18 20
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Lag
Frame Number
Integrated electrostatic focusing, VF
= 50 V
Integrated electrostatic focusing, VF
= 200 V
External mesh-electrode focusing, VM
= 300 V
Figure 5. Increasing focusing voltage generally improves spatial resolution.
Maximum achieved MTF is 20% at Nyquist frequency, similar to external mesh-
electrode case (below). Further improvements may be achieved by optimizing
focusing electrode geometry.
0 1 2 3 4 5 6 7 8 9 10
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
ESe
= 30 V/µm
MTF
Spatial Frequency [cycles/mm]
VF
= 50 V
VF
= 100 V
VF
= 150 V
VF
= 200 V
0 1 2 3 4 5 6 7 8 9 10
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
ESe
= 100 V/µm
MTF
Spatial Frequency [cycles/mm]
VF
= 50 V
VF
= 100 V
VF
= 150 V
VF
= 200 V
0 1 2 3 4 5 6 7 8 9 10
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
ESe
= 30 V/µm
MTF
Spatial Frequency [cycles/mm]
VM
= 200 V
VM
= 300 V
VM
= 400 V
0 1 2 3 4 5 6 7 8 9 10
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
ESe
= 100 V/µm
MTF
Spatial Frequency [cycles/mm]
VM
= 200 V
VM
= 300 V
VM
= 400 V