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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
Flat
panel
II
Field coverage / size advantage to flat panel
Image distortion advantage to flat panel
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: reduce structured noise
Left
Right
Shift images to select plane
Add to create tomogram
•
3 cm above detector
•
9 views, + to - 30°
•
1.4 x dose
Tomographic ramp
Niklason, L.T. et.al. Radiology 205:399-406