Digital Fluroscopy

objectives

• describe parts of digital fluoro imaging and functions

• compare features of image-intensified units from digital fluoro units

• understand advantages of flat panel IR

Charged-Coupled Device

• light sensing

• creates electric charge when illuminated

  • Tiling: array of closely spaced CCD to create one large detector

1. capacitors store energy

2. energy passed to each neighboring capacitor

3. final capacitor transfers energy to ADC & amplifier

* Only 1 ADC and amplifier

Advantages

• increased DQE

• increased SNR

• Increased Spatial Resolution

• less radiation

• decrease patient does

Quantization & Sampling

• collects electronic signal and assigning values to pixel that correspond to x-ray exposure

• sampling collects exposure data from detector and releases electrons to ADC

• Quantization assignment of numerical value (binary code) to pixel

Flat Panel Detectors

• respond to rapid sequence to create dynamic range

  • 60 frames per sec

• rapid readout speeds

  • how quickly the matrix processes the image data

Backlighting System

• light-emitting diode array

• erases detector between frames

Scintillation Detectors

1. coverts x-ray energy to light energy

2. photodetectors absorb light and converts to electric charges

Indirect DR

Method 1: Phosphor + CCD

1. phosphor plate

   • scintillation phosphor

     • Gadolinium Oxysulfide - turbid (visible light spread)

     • Cesium Iodide - structured (less light spread)

   • emits light

     • fiber optic bundles or optical lenses

     • “couples” CsI plate and CCD

     • transmits light to CCD

   • x-rays → light

     • CCD

     • Light sensitive device

     • light → electronic analog signal

Method 2: Phosphor + TFT

1. Phosphor plate

   • scintillation phosphor

   • emits light

   • x-rays → light

2. Photodetector

   • a-Si

   • painted on DEL

3. TFT array

   • electronic components layered on glass substrate

   • configured into matrix of DELs or pixels

   • includes readout, charge, collector, & light-sensitive elements

Capture Area

• Sensitive or active area

• captures electrons

• largest area of DEL

Storage Area

• non sensitive areas

• part of DEL’s electronic components

• storage capacitor

• stires electrons after capture and before release

TFT Switch Gate

• non sensitive areas

• charge readout

• gate releases electrons to ADC

Direct Capture

1. photoconductor

   • a-Se

   • painted on DEL

2. TFT Array

   • electronic components layered on glass substrate

• x-rays are converted directly to electric charges

• no scintillation phosphor

• no light emission

• losing less information

  • always lose information when there is need to convert information to a different type of signal

Flat Panel Detectors Advantages

• more compact and durable

  • reduction in size

  • less weight

  • easier to manipulate

• rectangular FOV

  • eliminates distortion due to pincushion effect

    • no distortion

• faster acquisition speed

  • no image intensification

• operates in radiographic mode

  • 50 - 1200 Ma

• less dose

  • 50% lower

• no brightness loss

• increased contrast resolution

• increased DQE

• Wider Dynamic Range

• increased Spatial Resolutoin

  • 2.5 - 3.2 Lp/mm

• decreased electronic noise

Flat Panel Detector Disadvantages

• fill factor is about 80%

  • increases noise

• possible ghosting

• large data file for archiving

  • 8 MB for static images

  • 240 MB for dynamic images

*Fluoro is highest Tech dose

• high mA

• long exposure

Decreasing patient dose

1. automatic exposure rate control

   • auto adjusts technical factors to maintain radiation exposure

2. last image hold

   • last frame is held on monitor

   • lines appear on image

3. virtual collimation

   • adjusts collimation without exposing patient

4. electronic magnification

   • using smaller FOV

   • FOV is enlarged on display monitor

   • magnifies anatomic structures

   • spatial resolution remains the same

   • not increasing patient dose

5. pulsed fluoro and frame averaging

   • intermittent radiation exposure

     • lessens amount of time beam is on

   • operates in radiographic mode

     • 100 - 1200 mA (instead of 2-5 mA)

   • lowers patient dose

   • pulse rate

     • pulses per sec

     • capable of 1-30 pulses per sec

     • any under 30 pulses per sec reduces patient dose

   • Pulse width

     • length of each pulse ( how long each image is; Duty Cycle)

     • 6 milliseconds

     • - con: decreased spatial resolution pediatrics

     • up to 100 milliseconds for adults

   • Duty Cycle: time that x-ray tube is energized

   • Interrogation Time: time needed for x-ray tube to be switched on and reach selected levels of technique

   • Extinction Time: time needed for x-ray tube to be switched off

Interrogation & extinction times < 1 ms

• Frame Averaging

  • common method

  • lower patient dose

  • decrease image noise

  • CON: decreased spatial resolution