Jong Chul Hana, Seungman Yuna, Chang Hwy Lima, Tae Woo Kimb, Ho Kyung Kima*

a School of Mechanical Engineering, Pusan National University, Busan 609-735, South Korea b Advanced Medical Engineering Laboratory, Vatech Co., Ltd., Giheung 449-904, South Korea* Correspondence: hokyung@pnu.edu

Feasibility study of CMOS detectors for mammographyPusan National University

Motivation•Many types of digital mammography detectors are currently used as clinical systems.•A photodiode array made by CMOS (complementary metal-oxide-semiconductor) process

in conjunction with a scintiallator has lately attracted considerable attention due to its unique advantages such as low read out noise, small pixel size, and high fill factor, etc.

•With the CMOS detector employing a thin phosphor screen as an x-ray converter the extra noise due to the direct absorption of x-ray photons unattenuated from the phosphor screen in the photodiode is white.- This additional white noise further raises the noise spectral density in the high spatial frequency

band, hence deteriorating the DQE performance.

Objective•To investigate the potential use of CMOS detectors employing a phosphor screen for

mammographic applications- The imaging performance of the detectors is characterized in the Fourier domain, such as

modulation-transfer function (MTF), noise-power spectrum (NPS) and detective quantum efficiency (DQE). - Componential analysis for various image noise sources due to the optical photons generated, the direct x-ray photons unattenuated from the phosphor screen, and the additive electronic noise charges is performed by experimental measurements.- The role of a fiber-optic plate (FOP) layer in the detector performance is also investigated.

Conclusions•The indirect-conversion detectors have an NPS, which

decreases with the spatial frequency, and the direct-conversion detectors have a nearly constant NPS with the spatial frequency.

•This explains that when a significant amount of x rays are not absorbed in the phosphor layer, then the additional absorption of x rays in the photodiodes with their white noise contributions degrades the total NPS performance.

•Although the introduction of the FOP layer reduces the sensitivity and MTF of the detector, it largely enhances the DQE performance because it plays as an efficient stopper for the direct x rays.

•The use of a combination of thicker columnar-structured CsI:Tl and FOP layers may provide a more efficient mammography detector with higher DQE performance but without the significant loss in the MTF performance.

AcknowledgementsThis work was supported by the Korea Research Foundation Grant (KRF-2008-313-D01339) and

the Korea Science and Engineering Foundation Grant (R01-2006-000-10233-0) funded by the Korea Goverment (MEST), and Vatech Research Grant (Year 2008).

CMOS detectors Cascaded model analysis

Results

•CMOS photodiode array - RadEyeTM, Rad-icon Imaging Corp.

CMOS photodiode array

MinR-2000TM phosphor screen

- Phosphor screen (MinR-2000TM, Carestream Healthcare, Inc.)• Mainly made up of terbium-doped gadolinium oxysulfide

(Gd2O2S:Tb) and a polyurethane elastomeric as a phosphor and binder

• 84 µm thickness & ~4 g/cm3 density

Property Description

Array format 512 × 1024 pixels

Pixel pitch 48 µm

Field of view 25 × 50 mm2

Dynamic range 85 dB (>14 bits)

ADC bit-depth 12 bits

Pixel fill factor 0.87

Saturation 2,800,000 electrons

Dark current < 104 electrons/sec

Read Noise (at 1 fps) < 200 electrons

•Modeling signal and noise characteristics using the cascaded linear-systems theory

•Theoretical DQE calculation

Parameter Description Parameter Description

q0 Incident x-ray fluence Tapert(ρ) MTF due to the aperture integration

αscn Quantum efficiency in the scintillator Tscn(ρ) MTF of scintillator

βscnQuantum amplification factor in the scintillator

Ipd Swank noise factor in photodiode

γpdLight quantum efficiency in the photodiode a Pixel aperature

αpd Quantum efficiency in the photodiode d Pixel pitch

βpdQuantum amplification factor in the scintillator Iscn Swank noise factor in the scintillator

σadd Additive electronic noise III(ρ) Sampling process

•Measured Characteristic curves of two different configurations

-The signal is expressed in the number of electrons (e–) considering the CMOS photodiode conversion gain.

- The introduction of FOP reduces the detector sensitivity (or the slope of the linear fit curve) by a factor of 2.- The measured detector sensitivities are extremely low compared to the reported data with an amorphous silicon-based detector.• Smaller pixel pitch of the detector used in this study maybe one

reason for this large differences.• The exact reason is unclear and will be further investigated.

•Measured MTFs of two different configurations of the detector with respect to various energies

- The MTF curves are quite similar to one another for a range of energy from 40 – 80 kVp. - Due to erroneous MTF results for 26 kVp, the MTF of 40 kVp was used for DQE calculations.- Comparing the graph (a) and (b), the FOP largely degrades the MTF performance over the entire spatial frequency region.

•Measured NPSs and DQEs of two different detector configurations with respect to various dose

- In the graph (a), • The use of FOP reduces the level of noise spectral densities over

the entire spatial frequency regions.• The configuration of the detector without the FOP layer shows a

more flat shaped spectrum, due to the contribution of direct x-ray absorption noise.- In the graph (b),

• DQE of the detector without the FOP is almost independent on the dose level.

• Although the FOP layer reduces the detector sensitivity and degrades the MTF performance, it greatly improves the DQE performance because it effectively prevents direct x-ray photons from the absorption within the photodiode.

•Componential analysis of noise spectral densities

- In the graph (a), in the case of detector without FOP• From the measurements, the scattered and direct x rays significantly

contribute to the noise spectral densities, especially in the high spatial frequency region because of the white noise characteristics. - In the graph (b), in the case of detector with FOP

• The contributions due to the scattered and direct x rays are negligible. - Although the cascade models developed in this study do not consider

the scattered x rays, the cascade models reasonably describe the measurements in both cases.

•DQE comparison between the measurement and theoretical calculation, and DQE(0) as a function of fluence for two detector configurations

- In the graph (a),• The discrepancy of the model for the detector with FOP is mainly

due to the incomplete model of the FOP layer. • The discrepancy of the model for the detector without FOP is mainly

due to incompleteness in the model.- In the graph (b),• The range of dose considered in this study is indicated as a hatched

box.• From the calculation results, maximally achievable DQE(0) is about

30% with the CMOS detectors investigated in this study.

Experimental

X-ray quality

IEC HVL Measured HVL

IEC Tube voltage

Adjusted voltage IEC q Measured

qX-ray quality

mmAlmmAl kVpkVp #/mm2/µGy#/mm2/µGy

W/Al 0.83 0.82 28 26 6575 6500

•Componential analysisDetector configurationsDetector configurations Noise sources

Detector without FOP

dark image σadd2

Detector without FOP

phosphor + photodiode σopt2 + σdirect2 + σscatter2 + σadd2Detector without FOP phosphor + black paper + photodiode σdirect2 + σscatter2 + σadd2

Detector without FOP

phosphor + air gap (+ carbon plate) + black paper +photodiode σdirect2 + σadd2

Detector with FOP

dark image σadd2

Detector with FOP

phosphor + FOP + photodiode σopt2 + σscatter2 + σadd2

Detector with FOP

phosphor + black paper + FOP + photodiode

σscatter2 + σadd2Detector with FOP

phosphor + air gap (+ carbon plate) + FOP + black paper + photodiode σadd2

The subscripts opt, direct, scatter, and add denote each corresponding component contributing to noise including optical photons, direct x rays, scattered x rays, and additive electronic noise, respectively.

•Source information - X-ray tube (UltraBright TM, Oxford Instrument)• Tungsten (W) target, 245 µm beryllium exit window, and DC type

•Standard radiation qualities and characteristics for the

mammographic application

•Blackout paper (Flock Paper #40, Emond Optic Inc.)- For the measurement of direct x-ray signals without lights- 100 µm thickness

•FOP (fiber optic plate: Income Corp)- 3 mm thickness & 4 g/cm3 density- To prevent the direct x-rays from the absorption within the

photodiode array

Cascaded model due to optical photon

Cascaded model due to direct x-ray absorption

•Measured DQE calculation

(a) (b)

DQE(ρ) = q0G2MTF2(ρ)NPS(ρ)

the optical quantum correlated noise term

NPS due to the direct x-ray absorption NPS due to the additive electron noise charges

WU =a2d 2q0 (1− α scn )α pdβ pd

2

I pdWadd = d

2σadd2

WQ (ρ) = a4q0α scnβscnγ pd 1+ γ pdβscn

Iscn−1

⎛⎝⎜

⎞⎠⎟Tscn2 (ρ ± k

d)

⎡

⎣⎢

⎤

⎦⎥sinc2 πa(ρ ± k

d)⎧

⎨⎩

⎫⎬⎭k=0

∞

∑

DQE(ρ) =q0 α scnβscnγ pd + (1− α scn )α pdβ pd⎡⎣ ⎤⎦

2Tscn2 (ρ)sinc2 (πaρ)

WQ (ρ) +WU +Wadd

SPIE Medical Imaging 2009Paper #: 7250-40

BiomedicalMechatronics Lab

BML