Technical Note: A novel interdigital transparent thin-film detector for medical dosimetry

A new type of thin-film interdigital detector (TFID) for medical dosimetry is investigated. The focus of this study was to characterize the detector response as a function of detector geometry in an attempt to optimize it and to understand the underlying radio-electrical effects leading to signal formation.

We characterize the detector response to kilovoltage x-ray beams used in fluoroscopy and computed tomography. Each element (pixel) of the detector is composed of conductive intercombing digits deposited on a thin-film dielectric substrate by nanofabrication or using a printing process. The detector is practically transparent to x-ray radiation, yet it generates sufficient signal for many types of medical dosimetry and quality assurance tasks. The thin-film detector has negligible surface mass density (about 2.5 mg/cm² for a 1-μm-thick Cu TFID on 12.5-μm-thick Kapton substrate) and it is conformable to curved geometries found in the medical x-ray equipment or on patient skin surface. The prototype detectors were made using glass and Kapton substrates with copper-copper and copper-aluminum interdigits. Although in principle the detector can be operated without any external bias voltage when the digits are made of disparate materials (e.g., Cu-Al), we also characterized the detector properties under small electric fields via its current-voltage curve (IV curve).

Using 120 kVp, 25 mA x-ray beam with 10V external bias, the Cu-Cu detector response was about 0.2 nA/cm² . We also measured a one-dimensional transmitted dose profile for a phantom under fluoroscopic x-rays and found relatively good agreement with a commercial photodiode (XR R12-0191, IBA Dosimetry).

We demonstrated the potential of TFID detectors for kilovoltage dosimetry and we defined its optimal geometry. For digits made of the same material and for digit width equal to the separation between them, we found that the thin-film detector has optimal performance when the distance between the digit centers is about 1 mm, while in the fixed digit width cases we observed that the signal is higher when their edge-to-edge separation is as small as possible.