Photon Counting Energy Dispersive Detector Arrays for X-ray Imaging

Abstract

The development of an innovative detector technology for photon-counting in X-ray imaging is reported. This new generation of detectors, based on pixellated cadmium telluride (CdTe) and cadmium zinc telluride (CZT) detector arrays electrically connected to application specific integrated circuits (ASICs) for readout, will produce fast and highly efficient photon-counting and energy-dispersive X-ray imaging. There are a number of applications that can greatly benefit from these novel imagers including mammography, planar radiography, and computed tomography (CT). Systems based on this new detector technology can provide compositional analysis of tissue through spectroscopic X-ray imaging, significantly improve overall image quality, and may significantly reduce X-ray dose to the patient. A very high X-ray flux is utilized in many of these applications. For example, CT scanners can produce ~100 Mphotons/mm(2)/s in the unattenuated beam. High flux is required in order to collect sufficient photon statistics in the measurement of the transmitted flux (attenuated beam) during the very short time frame of a CT scan. This high count rate combined with a need for high detection efficiency requires the development of detector structures that can provide a response signal much faster than the transit time of carriers over the whole detector thickness. We have developed CdTe and CZT detector array structures which are 3 mm thick with 16×16 pixels and a 1 mm pixel pitch. These structures, in the two different implementations presented here, utilize either a small pixel effect or a drift phenomenon. An energy resolution of 4.75% at 122 keV has been obtained with a 30 ns peaking time using discrete electronics and a (57)Co source. An output rate of 6×10(6) counts per second per individual pixel has been obtained with our ASIC readout electronics and a clinical CT X-ray tube. Additionally, the first clinical CT images, taken with several of our prototype photon-counting and energy-dispersive detector modules, are shown.

Fig. 1

A photograph a CdTe crystal with a 16×16 pixel anode structure at a pixel pitch of ~1 mm on one side (shown on left) and a continuous cathode on the other side (shown on right).

Fig. 2

A pulse shape measured for a detector pixel.

Fig. 3

Spectrum taken with a single detector pixel in response to Co-57 source using a short shaping time. Electronic noise is represented by pulses injected from an external generator (pulser peak).

Fig. 4

Channel architecture.

Fig. 5

Principle schema of the amplifier.

Fig. 6

Principle schema of discriminator.

Fig. 7

The full ASIC architecture.

Fig. 8

Measured ENC [e- rms.] versus input capacitance.

Fig. 9

Gain versus channel-number.

Fig. 10

Output count rate taken from a single detector pixel versus X-ray tube anode current. A straight line represents a linear fit to data taken at low count rates.

Fig. 11

Mono-energetic images reconstructed at 60, 75, and 100 keV (from left to right), respectively. The arrows show the left internal carotid artery. Note: the HU of the iodinated left internal carotid artery decreases with increasing energy.

Fig. 12

Axial images at identical neck level. The left-side image shows contrast-enhanced neck arteries. The right-side image is the result of the un-enhanced process (iodine is identified and removed).

Fig. 13

(A) 3-D, (B) curved oblique, (C) coronal MIP, (D) volume rendering.

Fig. 14

Photographs of anode and cathode sides of CZT detector array with parallel drift structures.

Fig. 15

A pulse shape from a drift detector pixel.

Fig. 16

Co-57 spectra taken with bias voltage (V2) of 0 V (top spectrum), 100 V (middle spectrum) and 200 V (bottom spectrum) applied to the steering electrode.

Fig. 17

A pulse from a drift detector array pixel processed by amplifier using a delay line shaper.

Fig. 18

Co-57 spectrum taken with a drift detector array pixel.