Rad Tech 265

More Digital Imaging

Digital Fluoroscopy

• Radiation dose– Patient dose for DF is significantly less than

conventional fluoro• At 7.5 pulses/second DF has a 75% drop in

exposure

• At 3 pulses/sec a 90% reduction in dose

• The lower dose is obviously advantageous for pediatric work.

Types of DF

• The most common methodology is to add a CCD to the video chain.

• A truly digital system can be either indirect or direct.– Pulse progressive fluoroscopy

• Uses a high frequency generator with regular mA values

Digital Fluoroscopy Room

Flat panel vs. Image Intensifier

Field coverage / size advantage to flat panelField coverage / size advantage to flat panel Image distortion advantage to flat panelImage distortion advantage to flat panel

FlatFlatpanelpanel

IIII

Flat vs. Fat Digital Flat Panel Conventional II

Dynamic Range Very Wide (5-10 times more

than conventional)

Narrow (TV camera limit)

Distortion No Distortion Distortion from curved input surface of II

Detector Size Weight and thickness much lower

Heavy, bulky detector

Image Area 41 cm x 41 cm square Round area is more than 20% smaller area for same diameter

Image Quality Good resolution, high DQE Good resolution, high DQE

Direct DF

• In direct capture or direct to digital systems, x-ray energy is not converted to light. Instead, it is captured by a thin film transistor matrix of a material such as amorphous selenium that changes it into electronic signals. No intensifying screen is required, and none of the energy is lost through scatter, as happens when x-ray energy is converted to light on its way to display of an image.

Indirect DF

• The indirect capture systems, including some flat panel displays that can be integrated into analog systems, are similar conceptually to the traditional film-screen technology. In one version, a cesium iodide scintillator captures the x-rays as they exit the patient and converts them to light. This light is turned into electronic signals by a matrix of amorphous silicon sensors. Each sensor corresponds to a single pixel of the image and is connected to a readout line. The signals from each cell in the matrix are read out in sequence row by row to obtain the image. High information transfer rates permit the display of moving images.

Indirect v. Direct

• Notice, that the comparison of direct v. indirect digital fluoroscopy is the same as direct v. indirect digital radiography.

• In the indirect systems, the photons are converted to a light image first before being digitized.

Direct v. Indirect

Direct v. Indirect Resolution

Digital v Conventional Film

Digital v. Analog

Acquisition steps

Image Manipulation with DF

• Pan/zoom, background noise reduction, adjustable contrast and brightness, edge enhancement, quantitative analysis of vessel diameter and stenosis severity, subtraction capabilities, roadmapping, and bolus chase are common.

Pixels (matrix)

Matrix size

32 x 32 128 x 128

Matrix size

128 x 128 512 x 512

Edge enhancement

Bits (gray scale)

Bits

4 bits 8 bits

Temporal averaging

Last image hold (lih)

DSA

DSA

DSA

Digital Tomosynthesis: Digital Tomosynthesis: reducereduce structured structured noisenoise

• 3 cm above detector

• 9 views, + to - 30°

• 1.4 x dose

Tomographic rampTomographic ramp

Shift images to select planeShift images to select planeAdd to create tomogramAdd to create tomogram

LeftLeft RightRight

Niklason, L.T. et.al. Radiology 205:399-406