MOD 4 - Digital Detectors

Digital Radiography vs. Digital Imaging

• radiography = process of x-ray beam digitization in the detector

• imaging = digital signals → electronic signals that are displayed on the monitor

Classification of Digital Imaging

• CR

• DR (Direct Radiography)

  • Flat Panel (Mammography) 

• IDR (Indirect)

  • Flat Panel (C-arm)

  • Non-Flat Panel (Siemen’s Detectors)

• Scanned Projection Radiography (CT)

Components of a Digital Detector

1. Capture: x-ray → light/electrical signal

2. Coupling: transmit the signal into the collection element

3. Collection: collects and converts the signal into electronic/analog signal

4. ADC

Which capture element structure increases spatial resolution

A columnar structure (CsI) allows for less light spread, therefore it provides better spatial resolution

Indirect non flat panel detectors are typically found within

Image Intensifier based fluoroscopy machines

CCC Elements of Indirect Non-flat Panel

1. Capture:CsI

2. Coupling: fiber optic cables

3. Collection: Charge Coupled Devices (CCD)

CCD advantage

highly sensitive to low light, responds to very dim and very bright lights therefore has a wide dynamic range

CCC Elements of Indirect Flat Panel

1. Capture: CsI or GaO

2. Coupling: contact layers

3. Collection: photoiodide a-Si + TFT or CCD

TFT’s function

controls when each DEL sends it's information; the info is stored in the top-thin layer of the TFT called the storage capacitator

CCC Elements of Direct Flat Panel Detectors

• Capture and Couple: a-Se which is the photoconductor layer

• Collection: TFT

Pixel Fill Factor

• where the capture element makes up 80% of the pixel space, and the rest 20% are the capacitators and TFT where it isn’t able to respond to scintillator created light

• MEANING, the SR increases with decreased pixel size ONLY if the TFT and Capacitator size are created smaller

Pixel Pitch

the distance between each central point of two adjacent pixels in a detector array

Indirect vs. Direct Digital Systems

Indirect Scintillation Layer Differences

General System Type Comparisons

• best resolution

• fastest and slowest readout speed

• best DQE

• most and least cost

• best resolution = direct flat

• fastest = indirect flat (a-Si TFT), slowest = direct flat

• best DQE = direct flat

• most cost = direct flat, least cost = indirect CCD

Principal of Digital Fluoro (DF)

increased acquisition speed and the ability to post-process digital images

Interrogation Rate

time required for the generator to activate the tube to the desired exposure = acquisition rate 

Extinction Time

time required to terminate the exposure

Duty Cycle

The fraction of time that the tube is activated

Types of DF Receptors 

• CCD

• Flat Panel DR receptors

CCD DF

coupled to the output phosphor either directly or with fiberoptics, the CCD linear response improves IQ compared to conventional fluoro

Advantages of FPD over II Fluoroscopy

• improved image accuracy

• consistent image quality across the whole image

• more compact and lighter

• High Detector Quantum Efficiency (DQE) → fast duty cycles

FPD C-arm

• allows for 3D multiplanar reconstructions to be performed during a surgery

• beneficial for surgeons to confirm alignment of bone fragments and fixation hardware

Magnification Mode in FPD Fluoroscopy

reduces the amount photon interactions for each detector element → increased image noise → increased mA

contrast and spatial resolution relationship

while CR is independent of SR, SR increases with increased CR

higher spatial frequency =

more line pairs within one mm

Modulation Transfer Function

• detector's ability to accurately detect and display objects true to life

• MTF = 1, if system can perfectly transfer object characteristics to the image that we view, BUT impossible and all MTF < 1

the MTF as the objects imaged get smaller

MTF rate decreases when object becomes smaller then the pixel/detector element size

Exposure Latitude

• the ability to produce images at different exposure values all with good/same quality

• higher dynamic range = wider exposure latitude

Detective Quantum Efficiency (DQE)

• the detector absorption efficiency for a wide range of photon energies, therefore high DQE would mean less dose required to create images

• DQE 1 = 100% of photons detected

increased kV = ?DQE

decreased DQE

which detector material has the highest DQE

a-Se used in Direct DR

Digital Image Lag

previous image residues shown when the capacitator fails to reset to zero