Download Automatic Dose Rate and Image Quality Control Logic

Beth Israel Deaconess Medical center
Harvard Medical School
Automatic Dose Rate and Image
Quality Control Logic of
Interventional Fluoroscopy
Systems
Pei-Jan Paul Lin, Ph.D., FACR, FAAPM
Department of Radiology
Beth Israel Deaconess Medical Center
and
Harvard Medical School
Boston, MA 02215
Presented at the Joint Meeting of
Southwest Chapter-American Association of Physicists in Medicine, and
Federacion Mexicana de Organizaciones de Fisica Medica
Features Designed to Reduce Patient
Exposure (Air Kerma)
(1) Last Image Hold (LIH),
(2) Fluoroscopic image loop; 10 sec loop,
(3) Pulsed Fluoroscopy; 30, 15, 7.5 f/s
(4) Interleaved Pulsed Fluoroscopy;
15 f/s pulse rate displayed at 30 f/s,
(5) Pre-collimation under LIH mode,
(6) Spectral Shaping Filters,
(7) Automatic Dose Rate & Image Quality
Control Logic
© PPLin, 8 February, 2007
Features Designed to Reduce Patient
Exposure (Air Kerma)
© PPLin, 8 February, 2007
Features Designed to Reduce Patient
Exposure (Air Kerma)
(2) Fluoroscopic image loop,
(1) Last Image Hold (LIH)
“Last Image Hold” feature permits
fluoroscopist to stop the radiation to the
patient with the last frame of fluoroscopic
image displayed on the monitor. This
permits the fluoroscopist to attend to
matters pertain to the catheterization and
consider the “next” move with the last
image displayed!
© PPLin, 8 February, 2007
Features Designed to Reduce Patient
Exposure (Air Kerma)
(3) Pulsed Fluoroscopy; 30, 15, 7.5 f/s
(a) Compared against the 30 f/s continuous
fluoroscopy, the 30 f/s pulsed fluoroscopy
generally has less motion unsharpness and
can be setup to reduce patient exposure.
(b) Pulse rates less than 30 f/s show
reduced patient exposure.
© PPLin, 8 February, 2007
Typically, this feature will “loop” the last 10
seconds (300 frames) of fluoroscopic
images.
This is a dynamic display which takes
place of the “Last Image Hold”.
© PPLin, 8 February, 2007
Features Designed to Reduce Patient
Exposure (Air Kerma)
(4) Interleaved Pulsed Fluoroscopy;15
f/s pulse rate displayed at 30 f/s,
(a) As the lower pulse rate of 15 f/s becomes the
prerequisite to reduced patient exposure, one
frame of 15 f/s image can be displayed twice
before advancing to the next image.
(b) Displaying each frame of the 15 f/s images
twice improves the continuity of motion. This is
similar to the 30 f/s cine images were projrcted
twice by the use of a shutter to achieve the 60 f/s
motion continuity.
© PPLin, 8 February, 2007
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Features Designed to Reduce Patient
Exposure (Air Kerma)
(5) Pre-collimation under LIH mode.
Under the LIH mode, collimator can be
adjusted to desired filed size without
having to activate fluoroscopy.
Features Designed to Reduce Patient
Exposure (Air Kerma)
(6) Spectral Shaping Filters
(a) Use of 0.1 mmCu ~ 0.3 mmCu in place
of the aluminum filter resulted in reduced
patient exposure in early version of
spectral shaping filter application in
cardiovascular imaging systems.
(b) Cu filters ranging 0.1 mm to 0.9 mm are
being employed for cardiovascular
angiographic equipment.
© PPLin, 8 February, 2007
Features Designed to Reduce Patient
Exposure (Air Kerma)
(7) Automatic Image Quality & Exposure
Control Logic
© PPLin, 8 February, 2007
Features Designed to Reduce Patient
Exposure (Air Kerma)
(7) Automatic Image Quality & Exposure
Control Logic
(a) A sophisticated software programming
is required to respond to a change in the
copper filter thickness.
(c) Heavy copper filter preferentially
removed low energy photons and the mean
x-ray beam energy is, thus, increased.
(b) The automatic control logic may be
designed to various imaging parameters
including the focal spot size, kVp, mA,
pulse width, etc.
(d) For the same applied tube potential this
would require a higher “tube current” to
produce an acceptable image quality. --Thus, a “high power” x-ray tube is required.
© PPLin, 8 February, 2007
Automatic Dose Rate & Image
Quality Control (ADRIQ) Logic
• Fluoroscopic exposure parameters vary as
functions of “Patient Thickness”,
• Focal spot selection (switching) CAN be
programmed into the ADRIQ depending on the
Power Loading to the anode,
• Copper filters (mmCu) are introduced into the
primary x-ray beam in accordance to the
penetration sensed by the (flat panel) detector.
• Upon reaching the maximum allowable tube
loading condition, the ADRIQ works just like the
classical Automatic Brightness Control logic; isowatt loading.
© PPLin, 8 February, 2007
© PPLin, 8 February, 2007
Verification Testing of the Automatic Dose
Rate & Image Quality Control Logic
Flat Panel Detector
Ionization Chamber #2
PMMA Phantom
TID=38 cm
SID=105 cm
Ionization Chamber #1
Examination Tabletop
X-ray Tube Assembly
Geometrical Arrangement
© PPLin, 8 February, 2007
2
30 cm
Tube Potential (kVp)
100
1.0
0.8
80
0.6
0.4
0.2
Tube Current (mA)
60
Pulse Width (mSec)
Input Sensitivity
in front of the
Flat Panel Image
Detector
100
80
60
40
20
Patient Skin Dose
(Air Kerma)
16
14
12
10
Input Sensitivity (μ Gy/sec)
10 cm
Copper Filter (mm)
Tube Potential (kVp)
20 cm
Copper Filter (mmCu)
Entrance Exposure rate (mGy/min)
10 cm
120
20 cm
30 cm
0.9
0.8
0.7
0.6
100
10
1
8
0.1
2
4
6
8
10
12
Nominal Phantom Thickness (inches)
14
0
2
© PPLin, 8 February, 2007
Why Better Image Quality &
Lower Patient Dose?
• Image quality is “better” because of
consistently lower tube potential is
employed---higher image contrast!
• Radiation dose to the patients, especially,
small and average size patient, is
significantly reduced due to the use of
spectral shaping filters --- considerable
amount of low energy portion of spectrum
is removed before hitting the patient.
4
6
8
10
12
14
Nominal Phantom Thickness (inches)© PPLin, 8 February, 2007
3.0 mmAl HVL (Conventional)
6.5 mmAl HVL (with 0.2 mmCu Filter)
Relative X-ray Intensity
0
0
20
40
60
80
100
X-ray Tube Potential (kVp)
Schematic X-ray Spectra of (a) Conventional Filter; 3.0 mmAl
HVL and (b) Heavily Filtered 6.5 mmAl HVL; 0.2 mmCu Filter.
© PPLin, 8 February, 2007
Advances in Clinical
Applications
• Rotational Angiography (RA)
• CT-like Images From (RA)
• 3D Image Display
© PPLin, 8 February, 2007
Simplified Basic Principle of CT-like Image and 3D
Image Reconstruction with Rotational Angiography
Equipment
Flat Panel Im
One projection image is
obtained every 1.5o of
rotation resulting in 128
images in 5 seconds.
Each image has a matrix
size of 1024 X 1024.
Patient at
iso center
X-ray Tube
© PPLin, 8 February, 2007
age Detect
or
Through back projection
image reconstruction the
CT-like images can be
generated.
For a 512 X 512 CT-like
image, two pixel rows of
the projected image is
“binned” together for
processing.
© PPLin, 8 February, 2007
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Rotational Angiography
There are 128 projected
images. Each image is
composed of 1024 (lines)
slices.
The C-arm frontal plane is employed for the raw data acquisition.
3D Imaging
Scan Parameters For CT 16 cm CTDI Phantom
Siemens File Name
CT-like
70 kVp/20 mA per frame (AEC)
5S-1KDR
Angle of Rotation
192
204.8
Angles Per Frame
1.5o
0.8o
Number of Frames
128
256
Peripheral Dose
(reference only @
12 O’clock)
Center Dose
(reference only)
Matrix Size
o
mR
452
947
mGy
3.84
8.05
mR
9
12
mGy
To save processing time,
two rows and two columns
of data may be fused
(binned) together to form a
512 X 512 matrix CT-like
image.
10S-1KDR
o
0.077
0.102
1024 X 1024
1024 X 1024
This is illustrated in the
next slide.
The original DR images are in matrix size of,
say, 1024 X 1024.
Clinical Images; Courtesy of Arra S. Reddy, M.D.
© PPLin, 8 February, 2007
© PPLin, 8 February, 2007
Pixel (1, 512) -------------------------------- Pixel (xxx, 512)
Digitized values are assigned to each pixel.
(in one dimension only is shown.)
In the case of Flat Panel Detector, the signal
is digitized in the internal structure of flat panel
assebly while the analog TV signal from the
image intensified system is put through a digitizer.
Clinical Images; Courtesy of Arra S. Reddy, M.D.
© PPLin, 8 February, 2007
© PPLin, 8 February, 2007
Acquisition of CT-like & 3D Images
with Rotational Angiography Unit
This is a simulated
image.
128 images of 512
X 512 matrix size
CT-like images are
reconstructed.
These images are
further processed
to generate 3D
images.
© PPLin, 8 February, 2007
© PPLin, 8 February, 2007
4
Aluminum
Ramps
#1
#2
Teflon
Acrylic
#4
#3
Polyethylene
Air
#5
Water
Left Side: Rotaional Angiography Equipment | Right Side: CT-scanner
Projection “Raw” Fluoroscopy Image;
Notice the divergent beam of CONE
shaped fluoroscopy beam causing
distortion. (negative image)
Scout View of CT scanner shows
minimal distortion, or almost
distortion less image.
(positive image)
Left Side: Rotational Angiography Equipment | Right Side: CT-scanner
Slice width ramps in the center of
image with four plastic pins for
linearity test are clearly shown.
Notice the similarity of the artifacts
next to the slice width aluminum
ramps.
© PPLin, 8 February, 2007
0.4
0.6
0.65
0.75
0.85
1.0
(mm)
5
1.5
0.4%
Left Side: Rotational Angiography Equipment | Right Side: CT-scanner
This is the high contrast resolution
section of CT phantom. The 0.75 mm
square holes are resolved.
© PPLin, 8 February, 2007
The resolution of this CT scanner
under Standard Reconstruction
algorithm resolves 0.65 mm square
holes, and better with “BONE”
reconstruction algorithm.
© PPLin, 8 February, 2007
0.62%
4
3
2
2.0%
Left Side: Rotational Angiography Equipment | Right Side: CT-scanner
While CT scanners are designed for resolving “low contrast objects”, the
angiography equipment is able to show the nominal 2% contrast group
under the CT scanner. Notice that the contrast level will not be the same
due to the photon energy differences and the partial volume effect.
© PPLin, 8 February, 2007
Left Side: Rotational Angiography Equipment | Right Side: CT-scanner
The water bath section of the phantom shows the relatively noisy image
on the left as opposed to “smooth” looking CT image on the right.
Clinical Images; Courtesy of Arra S. Reddy, M.D.
© PPLin, 8 February, 2007
© PPLin, 8 February, 2007
5
Gracias por acompanarnos.
Thank you for coming.
• E-mail address:
[email protected]
Clinical Images; Courtesy of Arra S. Reddy, M.D.
The 3D images are best appreciated when presented in “Motion”.
© PPLin, 8 February, 2007
© PPLin, 8 February, 2007
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