Receptors & Image acquisition

digital image capture (3 Cs)

Capture element, coupling element, collection element

Capture element (1/3 C’s)

absorbs exit radiation

Coupling element (2/3 C’s)

transfers signal to collection element

collection element (3/3 C’s)

collects and temporarily stores signal

Computed Radiography (CR)

two separate steps in image acquisition: image capture and readout

Direct radiography

combines image capture and image readout into one device

CR system components

imagine plate (IP) in a cassette, reader, workstation

The imaging Plate: Basic construction

support, protective & phosphor layers

The imaging plate: phosphor layer

barium fluorohalide doped in europium

CR Reader

drive mechanism, optical system, photodetector (PMT), ADC

CR Image production step (1/4)

Remnant radiation interacts with Image plate

CR image production step (2/4)

latent image stored in electron traps

CR image production step (3/4)

energy released in the reader in the form of light

CR image production step (4/4)

computer processing makes the latent (invisible) image visible (manifest)

direct radiography uses

flat panel detectors (FPD)

DR system component (1/3)

solid-state IRs using a large active-matrix array (AMA)

DR system components (2/3)

combines image capture & readout

DR system components (3/3)

“self-scanning”

Flat panel detector (FPD) Design: basic layers

X-ray converter, TFT array, substrate (glass)

Flat panel detector (FPD) Design: detector elements (DEL)

array is divided into detector elements (microns)

decreased DEL size = increased _____

spatial resolution

3 Parts of the DEL

detection surface, capacitor, TFT

DEL design

solid-state diode

sensing area = signal collection (sensitive to light or x-rays)

dead space = no signal collection

fill factor

% of DEL dedicated to sensing area

greater fill factor increases sensitivity

DEL: efficiency is measured as

DQE

From patient → IR → Monitor: Voxel

a volume element (or volume of tissue) within the patient

From patient → IR → Monitor: Dexel

a detector element (DEL) in the image receptor

From patient → IR → Monitor: Pixel

a picture element in the final image

information from different voxels (in patient) is collected by the _____

DEL’s (of IR) and computer-processed to become the pixels (of final images)

The TFT requires a ____

small electrical charge before exposure (hardwired, tether, connection, rechargeable battery)

Generator and ____ must communicate to synchronize image acquisition

FPD

Pre-exposure: ____ elements are charged to prepare to receive signal

TFT

during exposure: signal ____ and ____

conversion, collection

post-exposure: signal storage, ______, and prep for next exposure

readout

FPD Categories

indirect & direct conversion

All indirect detectors

use a scintillator, use a coupling element, two-step process

3 types of indirect detectors

TFT, CCD, or CMOS

scintillators

produce light when stimulated with radiation

2 types of scintillators

unstructured/turbid, structured

scintillator: turbid

gadolinium oxysulfide

scintillator: structured

cesium iodide

indirect conversion image production: scintillator

converts exit radiation into visible light

indirect conversion image production: photodetector/photodiode

converts light to electrical charges

indirect conversion image production: electrical signals are

digitized by the ADC

Indirect conversion

Scintillation material (gadolinium oxysulfide, cesium iodide)

“contact layer”

Amorphous silicon TFT array

glass substrate

indirect TFT image production step (1/5)

exit radiation causes scintillator to fluoresce

indirect TFT image production step (2/5)

TFT detectors convert light into electrical charges

indirect TFT image production step (3/5)

electrical charges stored in the capacitor

indirect TFT image production (4/5)

TFT acts like a switch to release stored signals

indirect TFT image production step (5/5)

signal converted by ADC (indirect)

Charge-coupled-device (CCD) components

very light sensitive & wide dynamic range

uses Csl scintillator

tiling used to make larger IR

CCD image production steps (1/3)

exit radiation causes scintillator to fluoresce

CCD image production steps (2/3)

optical lens or fiber optics channel light to CCD

CCD image production steps (3/3)

CCDs convert light into electrical charges

CMOS components

scintillator (crystalline silicon matrix)

Amplifier

photodiode

storage capacitor

ADC

CMOS image production: (1/5)

exit radiation causes scintillator to fluoresce

CMOS image production: (2/5)

Optical lens or fiber optics channel

light to CMOS

CMOS image production: (3/5)

CMOS converts light into electrical

charges

CMOS image production: (4/5)

Electrical charges stored in a

capacitor

CMOS image production: (5/5)

signal digitized in each CMOS DEL

Direct TFT components

one step/single stage conversion process

does not need a coupling element

amorphous selenium TFT array

Direct TFT image production: (1/5)

amorphous selenium converts radiation into electrical charges

Direct TFT image production: (2/5)

signal stored in a capacitor (DR)

Direct TFT image production: (3/5)

electrical signals are sent to the ADC for digitization

Direct TFT image production: (4/5)

TFT acts like a switch to release stored signals

Direct TFT image production: (5/5)

signals converted by ADC

converting the signal

signal sent to ADC

digitized information arranged to form matrix

each pixel has a brightness level

ALWAYS signal loss

ADC & quantization: pixel

numeric value assigned to each sampling point

ADC & quantization: shades of gray

pixel bit depth - contrast resolution

Sampling the analog signal: X-axis of graph

sampling points per unit time (frequency)

Sampling the analog signal: Y-axis of graph

sampling pitch

Aliasing

too few samples will create and aliased image

preventing aliasing: nyquist frequency

ensures accurate representation

sampling must be at least 2x the highest spatial frequency

DR panel care

manufacturer-approved disinfectants

avoid drops, excessive pressure, improper handling

protective cover to protect panel from contamination

damaged panels often returned for detailed failure analysis

quality control: physicists & vendors

equipment testing, calibration, & maintenance

quality control: radiographers

QC tasks

visual checks (artifacts) & routine cleaning

quality control: performance monitoring

identify quality changes that may indicate malfunction

Quality control key takeaway

Quality control is a shared responsibility

Regular QC by radiographers

testing by physicists and vendors, helps ensure optimal

image quality and system reliability