FPD – The next generation technology for fluoroscopic surgical C-arm

Digital revolution is evident in all spheres of life in today’s world. So, here comes the era of digital technology for fluoroscopic surgical C-arm; the FPD is here to stay and revolutionize your surgical imaging needs. But FPD has been around for some time now, is not it? This article will give you insights into what makes the modern FPD a must go to technology compared to yesteryears.

So, what is digital about FPD?
FPD uses a scintillator to convert X-ray energy to visible light, and then this visible light is read by photodiodes and stored as electrical charge. Each electrical charge is synonym to a pixel on the image.

The photodiodes are organized in an array of 1k×1k or 1.5k×1.5k in an area of 21 cm square for most of the surgical C-arms. A 30 cm square option exists too.

Technology options and comparison
Over a decade now, amorphous silicon (a-Si) flat-panel detectors (FPDs), based on active matrix thin-film transistor (TFT) arrays, have emerged as a technology for digital X-ray imaging. More recently, for fluoroscopic imaging, an indirect detection technology, based on a-Si and cesium iodide (CsI) scintillator has become common in clinical use. The main driver for X-ray imaging is to use very low dose and get high spatial resolution, and our technical fraternity is on job to achieve this.

The a-Si FPDs poses a notable limitation to get higher SNR at low X-ray dose (<0.01 mR per frame), and lot of research has gone in to improve the low-dose efficiency of this technology. One of such methods is to amplify the pixel gain through active pixel architecture. The amplification is achieved before the readout electronics and has helped in improving spatial resolution and overcome challenges in dark current management, fill factor, etc.

More recently, a superior technology has emerged using CMOS (complementary metal oxide semiconductor) sensor, which uses CsI (cesium iodide) as a scintillator to convert X-rays into visible photons and a photodiode to convert visible photons into electrons for readout circuit to consume. CMOS technology offers technology benefits over a-Si by offering lower dark current (improve SNR), higher charge mobility (high frame rate), and improved pixel pitch. The technology helps in providing a superior image quality and better performance over a-Si technology in terms of image noise, spatial resolution, DQE, and NEQ under low-dose conditions.

GADOX technology, will it disrupt the market?
Yet another technology is knocking at our door and has potential to disrupt the market. Gadolinium oxysulfide, commonly referred to as Gadox, is an excellent scintillator, which converts the X-ray energy into visible light (usually of green wavelength). The green light that emanates from the scintillator layer is in turn read by an array of photodiodes that converts light energy to electrons. The captured electrical signal (electrons) can be read and processed by a computer to yield an image.

The advantage of Gadox over the cesium iodide technology is that it offers slightly lower noise but on a slightly reduced signal. In effect, the SNR difference between these two technologies is less than 10 percent, cesium iodide being superior.

Another major difference between scintillators is how they are produced. Gadox is usually produced in a chemicalway whereas cesium is grown. Manufacturing Gadox scintillator is a relatively easy process that has simpler quality controls and lower variation from batch-to-batch.

Cesium on the other hand has dependency greatly on the concentration of the initial deposition, the time-temperature profile of the baking process, and the angle of crystal growth (if they grow even slightly diagonally, their efficiency significantly decreases). This does impact yield.

Outlook
FPD is the technology for your need for today and tomorrow, which gives you an edge over current II based system in terms of superior image quality with a lesser dose. The selection you need to make will directly depend on the investment you would like to commit. CMOS is the most superior if you are ready to invest but Gadox will need some more time to nurture and can be the technology of choice in future.