Transcript Spot Filming

Spot Filming
• Spot Film Cassettes
• Uses conventional radiographic
cassettes.
• Bypasses image intensifier for direct
exposure.
• Uses mA > 100 X that of photofluoro
camera.
Spot Filming
• Spot Film Cameras
• Image the output phosphor of II.
• Same mA as fluoroscopy (1-3 mA
typically).
• Exposes one frame/exposure.
• Uses 70, 90, or 100/105 mm film size.
• As you increase film size, increase image
quality and patient dose.
• Method of choice for spot filming compared
to spot film cassettes.
Spot Filming
• Framing (matching II output size to film
area)
Underframing
• Fluoro image < frame
• Should be avoided
Exact Framing
• Int. image = small
dimension of frame
• No part of image lost
• Only 58% of frame
used
P. 31
Overframing
Total Overframing
• Image circle > short
film dimension
• Circular image = diameter
of frame
• Part of image lost
• All of film used
• 39 % of image wasted
Cinefluorography
• The process of recording fluoro images
on movie (cine) film.
• Film sizes
– 16 and 35 mm
– The larger the film size, the greater the
resolution and greater the patient dose.
Cinefluorography
• Synchronization
• Camera shutter timed to the same
frequency
as the x-ray pulses.
• Shutter open only during x-ray pulses.
• Framing Frequency
• Number of exposed frames/second.
• Divisions or multiples of 60 Hz line rate.
• Examples: 7.5, 15, 30, 90, 120
frames/second.
Cinefluorography, cont’d
• Framing Frequency
• Patient Exposure
• Directly proportional to framing frequency.
P. 30
Cinefluorography, cont’d
• Framing Frequency
• Patient Exposure Conversions
• 2000 mR/frame X 1 mR/1000 mR = 2
mR/frame
• 2 mR/frame X 30 frames/second X 60
seconds/minute = 3,600 mR/minute
• 3,600 mR/minute X 1 R/1000 mR = 3.6
R/minute
P. 30
Cinefluorography, cont’d
• F-number
• Number indicating the ratio of the focal
length of the camera lens to the
diameter of the lens.
• F-number = Focal Length of
Lens/Diameter of Lens
• The smaller the f-number = more light
into the camera and less patient
exposure.
P. 30
Focal length of lens
F-num =
50 mm / 20 mm = F 2.5
Iris diaphragm
The smaller the f-number - the larger the
aperture opening.
The larger the f-number - the smaller the
aperture opening.
Spot Film and Cine Cameras
• Beam and image size match within 3 % of
SID
• A small aperture (large f-number) will
require greater patient exposure but result
in low noise image.
• Patient entrance exposures of 50 - 150
rads or more.
P. 110
Structural Fluoro Room
Shielding Provisions
• Barriers
• Primary
• A barrier to attenuate the useful beam to the
required degree.
• Secondary
• Used to attenuate stray (scattered and leakage)
radiation to the required degree.
• Useful Beam
• Radiation that passes through window, aperture, or
cone.
• Stray Radiation
• Leakage and secondary radiation. No useful
purpose.
p. 121
Primary/Secondary Barriers
Exception
would be in
R/F room
where all
walls would
be a primary
barrier.
(Source: Principles of
Imaging Science and
Protection. Thompson,
Hattaway, Hall, Dowd,
1994)
Structural Fluoro Room
Shielding Provisions
• Tube Leakage
• Maximum exposure @ 1 meter is 0.1 R/hour.
• Half-Value Layer (HVL)
• Thickness of material that reduces beam
exposure rate by half of original value.
• Tenth-Value Layer (TVL)
• Thickness of material that reduces beam
exposure rate to 1/10 of original value.
HVL Lead (mm)
125 kVp
0.3
150 kVp
100 kVp
0.25
80 kVp
0.2
0.15
60 kVp
0.1
0.05
40 kVp
0
1
2
3
4
5
6
Maximum
permissible =
0.1 R/hr. @ 5 mA
• Radiation Area
Radiation Areas
• Any area where whole body dose could be = or >
0.005 rem/1 hr. at 30 cm from source.
• High Radiation Area
• Any area where whole body dose could be = or >
0.1 rem/1 hr. at 30 cm from source.
• Controlled Area
• Any area where radiation safety rules enforced.
• Unrestricted Area
• An area in which access is neither limited nor
controlled.
Radiation Areas, cont’d.
• Restricted Area
• Any area where access is limited by the
licensee to protect individuals against
undue risks from radiation exposure.
Unrestricted/Controlled Areas
QCSemiannual
checks
• Video Tape
•Video Disc
•Electronic
Radiography
• Advantages
Instant Replay
No increase in patient exposure
• Disadvantages
Poor image quality
Fixed framing rate- 30 frames/sec.
• Advantages
Last image freeze “sticky fluoroscopy”
Instantaneous imaging
Short exposure times
95% dose reduction during fluoro
Variable framing rates – 1-30 frames/sec.
Multiple image storage
Random access of images
• Disadvantages
None significant
Low to High Resolution Values
20
10
0
40
30
50
60
70
80
90
Optical mirror
Spot film cass.
105mm spot film
70mm spot film
35mm cine
16mm cine
Digital Photospot
VHS
Image Resolution by Type
100
Low to High Dose
30
20
10
0
VHS/Video
Disc
Non-Fluoro
Film/Screen
Digital
Photospot
Pulsed Fluoro
80
70
90
60
50
40
100
16/35mm Cine
Cassette
Spot Film
70/105mm
roll film
Patient Exposure by Modality