CT fluoroscopy, or continuous imaging, depends on spiral/helical data acquisition methods, high-speed processing, and a fast image-processing algorithm for image reconstruction.
In conventional CT, the time lag between data acquisition and image reconstruction makes real-time display of images impossible.
CT fluoroscopy allows for the reconstruction and display of images in real-time with variable frame rates.
CT fluoroscopy combines the cross-sectional image targeting provided by CT with the real-time imaging, tracking and movement perception of fluoroscopy for interventional procedures.
It allows continuous update of images at a fixed position and is commonly used for CT-guided biopsies and fluid drainages.
The Fluoro Assisted Computed Tomography System, uses a unique flat-panel amorphous silicon digital detector coupled with an x-ray tube by a C-arm, which is a part of the CT gantry.
In 1993 Dr. K. Katada of the Fujita Health University, School of Health Sciences, in Japan initiated the idea for real-time imaging with use of CT scanner.
The first CT scanner capable of realtime imaging was introduced in North America in 1994.
In 1996 the U.S. FDA approved real-time CT fluoroscopy as a clinical tool for use in radiology.
The evolution of CT fluoroscopy has now made it possible to acquire dynamic CT images in real time.
precise needle placement
fluid collection aspirations
local drug injection
lumbar nerve root blocks
Improved accuracy in needle placement in CT fluoroscopy.
One of the problems with needle placement under CT fluoroscopic control with a single-slice CT scanner. The image shows that the needle has hit the target, which is simply not the case.
This problem is solved with multislice CT scanners that offer CT fluoroscopy because multiple images are obtained. It is apparent that the target has been hit.
limitations imposed by slice-by-slice CT scanning
One advance in CT technology that facilitates CT fluoroscopy is continuous scanning by using slip-ring technology. The cable wraparound typical of conventional slice-by-slice CT results in a delay that prevents real-time image reconstruction and display.
A, Reciprocating rotation. B, Fast continuous rotation.
The cable wraparound and unwinding is shown in Figure A.
This wraparound results from the fixed length of the high-voltage cable, which follows the x-ray tube as it rotates through 360 degrees around the patient.
The cable is unwound during the imaging of the next slice
In Figure B, the cable wrap around process is eliminated through the use of slip-ring technology, which allows the x-ray tube to rotate continuously as the patient moves continuously through the gantry. This is spiral/ helical CT scanning.
CT fluoroscopic principle
CT fluoroscopy is based on three advances in CT technology:
1.fast, continuous scanning made possible by spiral/helical scanning principles.
2.fast image reconstruction made possible by special hardware performing quick calculations and a new image reconstruction algorithm.
3.Continuous image display by use of cine mode at frame rates of two to eight images per second.
FAST CONTINOUS IMAGE
Fast continuous scanning was a major technologic development in CT, which resulted in spiral/helical scanning.
Spiral/helical scanning is made possible by slip-ring technology, which allows for continuous rotation of the x-ray tube.
Continuous rotation of the x-ray tube speeds up data collection and allows data to be collected for one rotation (360 degrees) per second.
When data are collected after one rotation (360 degrees), the first image is displayed on the monitor for viewing.
Subsequent images are displayed every time a data set has been collected for every 60-degree rotation.
The data set for every 60-degree rotation is used to refresh the previous image, which is discarded as new 60-degree data sets are processed.
This means that six images per second (360/60) can be displayed.
FAST IMAGE RECONSTRUCTION
In real-time CT fluoroscopy, fast image reconstruction is made possible by a set of hardware components dedicated to provide fast computations, together with a new image reconstruction algorithm.
The dedicated hardware components include a fast arithmetic unit, high speed memory, and a back-projection gate array.
All these components are housed in the image reconstruction unit.
In CT fluoroscopy, the interpolation algorithm is not used.
But in spiral helical CT scanning this algorithm is used to removes artifacts resulting from the simultaneous movement of the patient through the gantry while the x-ray tube rotates continuously during data acquisition.
In CT fluoroscopy motion artifacts are therefore present on the image and appear as streaks; however, these artifacts do not restrict visualization of relevant structures.
CONTINOUS IMAGE DISPLAY
Data are collected continuously, it is reconstructed by the fast reconstruction unit.
use of a small reconstruction matrix – 256 x 256
Images are subsequently displayed on a monitor in the cine mode (dynamic display) at frame rates that can vary from two to eight images per second.
overlapping structures can be removed, providing accurate spatial information
real-time display of images
consequent reduction in complications through finer needle control
reduced procedure time
increased operator confidence
video monitor will need to be displayed in the scanning room
an operator panel is required in the scanning room – with controls available for table movement, gantry lift, laser light control and fluoroscopic factors. Exposures will usually be activated using a footswitch
Generally performed with same KV but lower mA than conventional CT scanning.
Need for additional beam filtration to decrease patient radiation exposure.
Other technique parameters that must be considered in CT fluoroscopy are slice widths (collimator width), the FOV, and the maximum fluoroscopy time.
Typical tube current settings in CTF
10 mA paediatrics, 10-40 mA chest, 40-50 mA abdominal Staff
RADIATION DOSE IN FLUOROSCOPY
Radiation exposure to patient
the dose in CT fluoroscopy is also important because personnel are present in the room during the procedure.
The patient surface dosage may range between 2 and 10 mGy/sec, with exposure times lasting up to 200 seconds.
A number of factors influence the dose to the patient and operator in CT. One of the important factors is the length of the time of the exposure because dose is directly proportional to exposure time.
For safety reasons, departments should a monitor the patient dose based on departmental phantom studies and cease the procedure if the deterministic threshold is reached.
RADIATION EXPOSURE TO THE OPERATOR
The most significant area of radiation exposure to the radiologist is the hands. The radiologist’s hands will be close to the scan plane during image acquisition as they manipulate the needle under real-time imaging.
The solution to this problem was the development of needle holders, which are intended to keep the hands of the operator out of the x-ray beam.
This should be measured using a thermoluminescent dosimeter disc or ring. The absorbed dose to the hands in the direct beam is approximately 1.1 mGy per second.
Wearing a protective apron during the procedure will certainly reduce the effective dose to the operator.
Operators standing in the CT room during the procedure must wear protective lead aprons, thyroid shields, and lead glasses or goggles. This protective apparel must be at least 0.5 mm lead equivalent.
To reduce the dose to the patient is the use of a special x-ray filter.
use of lower-exposure CT fluoroscopic techniques (use lower mAs).
reduce section thickness (e.g. use 2 mm or 5 mm thickness, rather than 10 mm)
maximize distance from the primary beam and patient (inverse square law)
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