Chapter 11

Digital Radiography

Digital Radiography• Digital Radiology Department

– Intrinsically digital: CT, MRI– Already digitalized: Ultrasound, PET, – DR: last image modality to be totally digital

• Screen–film radiography– High image quality: low motivation for digitalization

• Digital Radiography– Requires lots of space for digital media storage

• 2k×2k×8bit=4MB, 4k×4k×12bit=32MB– High bandwidth network– Costly high luminance & high resolution monitor

• 2k×2.5k

Computed Radiography• Imaging plate: Photo Stimulable Phosphor(PSP)

– X-ray emit light promptly + trap in PSP screen– Read our later by exciting with laser– Collecting emitted signal by PMT and digitization

• Plate material– Bariumfluorohalide

• BaFBr(85%)• BaFI(15%)• Eu(Europium) doping

Photo Stimulable Phosphor• By X-rays: electrons from Eu+2 F+ F• By red laser: electrons from F Eu+2 Eu+3

– Emit blue green light

• Erase by very bright light: reuse of plate• Greater dynamic range: Reduce retake rate

Charge Coupled Devices• Like in modern digital camera• With Image intensifier• Small input image

– Dental, mammography biopsy– Direct coupling with screen– Fiber optic coupling for

minification• Lens for large field of view

– Difficult to collect enough light photon

• Secondary quantum sink probelm

Flat Panel Detectors• TFT technology like laptop

display• Indirect Flat Panel

Detectors– Screen + FP detectors

• Detector– Light sensitive area(AL)– Detector electronics(AD)– Fill factor=AL/AD

• Compromise btw spatial resolution & contrast resolution

Readout Process• Select element by

vertical and horizontal scan line

• Sequential scan whole elements

Direct Flat Panel Detector• Photoconductor layer + TFT array

– X-ray electrons not light photons– Electrode for electron collection– Uniform electric field

• virtually no blurring• permit thicker sensor

• Selenium(z=34)– Less effective than

Ga(Z=64) or Ce(Z=55)– Efficiency compensated by

thicker layer

Indirect FPD

Direct FPD

Digital Mammography

• Digital stereotactic biopsy– Fiber optic coupled mirror

• 5×5cm area

• Full field mammography– Mosaic of CCD systems– Slot scan system

• Low scattering• Higher heat loading

PSP and FPDPhoto Stimulable

PhosphorFlat Panel Detector

Detector Price Relatively low High

Reader Separate Combined

Multiple detectors

Multiple Cassette per Reader

Single detector per reader

Throughput Low due to cassette handling process

Higher throughput by direct reading

Image display After cassette processing

Right after exposure

Patient Dose ~ 2 time of screen film

~ same with screen film

Hard copy & Soft Copy• Hard copy

– display image on film

• Soft copy– display image on monitor

• DR images– Difficult to display softly

• Need HR monitors

– Make hard copy using laser film printer

Imaging Mode

Image Size

SPECT/ PET 128×128

MRI 256×256

CT 512×512

DSA 1024×1024

DR 3500×4300

Digital Image Processing• Calibration to correct pixel

gain and bias– Using blank and dark image

• Simple but efficient– Windowing

or leveling – Contrast

reversal

Image Convolution

• g(x)=I(x) h(x)

=∫I(x’)h(x-x’)dx’– g(x): output– I(x): input image– h(x): kernel

1/9 1/9 1/9

1/9 1/9 1/9

1/9 1/9 1/9

-1 -1 -1

-1 9 -1

-1 -1 -1

Kernel for averaging

Kernel for edge sharpening

Processed by Convolution

Original Averaged

Edge sharpened

Harmonized = Original

- Averaged

Analog vs. Digital• Screen-film system

– Higher spatial resolution• 20lps/mm

• Digital system– Lower in spatial resolution– Higher contrast resolution

• By higher DQE of detector• Outweigh slight loss in spatial resolution

– Ability to transmit, archive, readjust, produce identical copy

Chapter 12

Digital Radiography

Geometric Tomography• Synchronized movement of tube & detector• Blurring out neighboring tissues

– Exposure to entire thickness– Higher contrast with large angle– Repetition for each slice

• 10 slices =5mGy×10=50mGy• CT: 30mGy for each slice

Digital Tomosynthesis

• Digital version of GT.– Using flat panel

detector– Multiple slice with

single set of exposure– Dose-efficient

Digital Subtraction Angiography

• Temporal subtraction– Before and after contrast

injection• Mask image – Contrast

image

• Remove anatomical structure– Easy to enhance contrast– Overlay faint version of

mask image if needed• Estimate vessel stenosis

Dual Energy Subtraction• Different material

– Bone (Zavg=13), tissue (Zavg=7.6)

• Different attenuation for each energy– Low energy/high energy image– Calculate bon or tissue image– Single/Double exposure

Tissue

Bone

keV

Atte

nuat

ion

Dual Energy Subtraction

• Problems– Broad beam

spectra– Scattered

radiation– Quantum

noise• Useful for ling

nodule detection

Low Energy High Energy

Bone only Tissue only