Chapter 5- TFT and Flat-panel array image acquisition

what is a flat panel detector in radiography, and what are its two main types

A flat-panel detector is a digital x-ray detector that captures images without a cassette. Two main types are direct conversion detectors (using amorphous selenium [a-Se]) and indirect conversion detectors (scintillator + amorphous silicon/photodiode).

how does the indirect conversion FPD convert x-rays into digital image

X-rays hit a scintallator (CsI or GD2O2S), producing light; this light is then converted to an electrical signal by photodiaodes; then with thin flim transistor array and ADC it becomes digital

how does direct conversion differ? what is the advantage

direct conversion uses a layer of a-Se that directly converts x-ray photons to charge, without scintillator. advantage less light spread, better spatial resolution, sharper image

what is a TFT array, and what role does it play 

TFT array is an electrically addressable grid of elements that collects the charges generated by photodiodes (indirect) or directly by the a-Se (direct), controlling readout row by row 

what is the role of the scintillator in an indirect FPD Name two common materials

the scintillator converts x-ray photons into light photons. common materials: Cesium Iodide and Gadolinium Oxysulfide

what factors in FPD influence spatial resolution and what are the trade-offs

pixel size, fill factor (proportion of pixels sensitive area), thickness of scintillator, and charge diffusion. trade-offs: smaller pixel size improves resolution but increase noise and data size; thick scintillator improves sensitivity but lowers sharpness

what is detective quantum efficiency (DQE), and why is it important for FPD

DQE is a measure of how efficiently a detector converts incoming x-ray signal (and noise) into a useful image. Higher DQE means better image quality for lower dose 

compare dynamic range of FPD vs. PSP

FPDs also have wide dynamic range, similar or better than PSP in many systems, allowing tolerance for exposure variation. however, specific implementation and electronics affect the usable range

how does fill factor affect detector sensitivity and image quality

higher fill factor means more of each pixel is active area receiving signal (not electronics), increasing sensitivity and reducing noise; but increase fill factor can make physical design more complex/costly

list three advantages of FPD over CR (PSP) 

1. Immediate image acquisition (no cassette handling or scanning) 

2. Generally better spatial resolution (particularly with direct detectors)

3. Improved workflow/faster turnaround; reduced possibility of plate damage or handling artifacts

list some limitation or challenges of FPD systems 

• Higher initial cost 

• larger data storage demands

• potential dead pixels or defects over time; replacement is expensive

• greater sensitivity to electronic noise; heat and environmental stability important

why is grid selection important in flat panel image acquisition 

grids reduce scatter, improving image contrast. the grid ratio and frequency must match system design; improper grid use leads to artifacts or loss of image quality 

what is automatic data recogintion (ADR) in digital radiography

ADR is the system’s ability to detect metadata (like exam type, body part) automatically from the image or user input; helps to select correct imaging plate/LUT, reduce user error

what are “dead pixels” in flat-panel detectors, and how are they managed

pixels on the detector that no longer respond (always on or always off). they’re managed through calibration maps and software interpolation; parts of the detector can be masked; replacement if too many

how does heat and electronics influence noise in FPD systems 

thermal noise in the electronics can introduce random variations; sensors and amplifiers produce noise. cooling, stable power supplies, and proper shielding help mitigate 

describe how FPD detectors are tested for unifiormity

uniform exposure tests: image a flat, homogeneous phantom or uniform x-ray field and examine for variations (shading, drop off edges, defective row/columns). correction or calibration required if nonuniform

in what clinical situations would a direct conversion FPD be preferred over an indirect one, and why

where high spatial resolution is critical (mammography, bone imaging for trauma) or when detecting fine detail; direct reduces light scatter so sharper image. also when low dose is required with high image fidelity

how might an indirect dectetor’s thicker scintillator both help and harm image quality 

helps by absorbing more x-ray photons ——> greater sensitivity (lower dose); harms by increased light spread, which reduces spatial resolution/blurs edges 

consider a portable x-ray unit used in ICU with a FPD. what device maintenance or QC steps would be especially important

frequent uniformity checks, inspection for physical damage, checking for dead pixels, battery power stability, ensuring the detector is clean and undamaged; calibration after shocks since portability increase risks of mechanical stress

how does FPD technology influence patient dose compared to CR and film 

because of higher DQE and no need for plate scanning losses, FPD can achieve similar image quality at lower doses than CR or film in many cases; but practices must monitor to avoid dose creep 

when selecting pixel size for a FPD, what factors must be balanced

need for spatial resolution vs. noise (smaller pixels have more noise), cost of detector, size of image file and storage, and clinical need

how does the concept of speed class apply in FPD 

speed class is analogous to exposure index and detector sensitivity- rate at which detector produces adequate signal for image; faster detectors allow lower exposure but may sacrifice some resolution or increase noise 

what role dose upward calibration or fine tuning of LUTs play in FPD

ensures that image contrast and brightness are optimized for the body part; proper LUTs help maintain consistent image appearance across different exposures, body habits, and exam types