Spiral CT and Helical CT
|This photo simulates the path that the x-ray beam makes as spiral CT data acquisition of the abdomen is being made. The highlighted area is a man's stomach (man is lying on his back with his arms over his head).|
In all original CT scanners (1974 to 1987), the x-ray power was transferred to the x-ray tube using high voltage cables wrapped around an elaborate set of rotating drums and pulleys. The rotating frame (or gantry) would spin 360ï¿½ in one direction and make an image (or a slice), and then spin 360ï¿½ back in the other direction to make a second slice. In between each slice, the gantry would come to a complete stop and then reverse directions while the patient table would be moved forward by an increment equal to the slice thickness.
In the mid 1980's, an innovation called the power slip ring was developed so that the elaborate x-ray cable and drum system could be abandoned. The slip ring allows electric power to be transferred from a stationary power source onto the continuously rotating gantry. State of the art CT scanners with slip rings can now rotate continuously and do not have to slow down to start and stop. The innovation of the power slip ring has created a renaissance in CT called spiral or helical scanning.
These spiral CT scanners can now image entire anatomic regions like the lungs in a quick 20 to 30 second breath hold. Instead of acquiring a stack of individual slices which may be misaligned due to slight patient motion or breathing (and lung/abdomen motion) in between each slice acquisition, spiral CT acquires a volume of data with the patient anatomy all in one position. This volume data set can then be computer-reconstructed to provide three-dimensional pictures of complex blood vessels like the renal arteries or aorta. 3D CT images from volume data allow surgeons to visualize complex fractures, for example of facial trauma, in three dimensions and can help them plan reconstructive surgery.
MR, ultrasound and digital x-ray fluoroscopy have all made significant improvements in their ability to image the chest, lungs and abdomen. However, spiral CT has kept computed tomography as the primary digital technique for imaging the chest, lungs, abdomen and bones due to its ability to combine fast data acquisition and high resolution in the same study. CT is also unique in that it can provide detailed information of nearly every organ in the upper abdomen and pelvis in one quick examination.
Advanced 3D CT Images and "Virtual Reality" Images
Spiral CT allows the acquisition of CT data that is perfectly suited to three-dimensional reconstruction. A wide range of software techniques and advanced computer systems are being developed that enable creation of amazing 3D "virtual reality" images.
|Virtual reality 3-D image of the lungs. The bronchial trees are colored in green and the heart, aorta and vertebrae are colored in red||.|
In addition to creating fantastic images of internal anatomy, these new 3D reconstruction techniques enable a number non-invasive "virtual endoscopy" procedures to be performed. Endoscopy involves the use of an endoscope to see inside organs of the body such as the colon or bronchi. Virtual endoscopy allows physicians to see the inside of these same structures, without the use of an invasive endoscope. Some virtual endoscopy procedures can be performed with CT that could not be acquired with conventional endoscopy, such as the image below of a wire stent (wire support) inside the abdominal aorta.
|This collage shows an abdominal aortic stent (metal wire support): outer view (left), inner view (lower right) and original axial CT image (upper right)|
New "Multi-slice" Spiral CT Scanners
New "multi-slice" spiral CT scanners are now being developed that can collect up to four slices of data during spiral CT mode and some rotate at speeds up to 120 rpm (rotations per minute). These systems can collect up to eight times as much data versus previous state-of-the-art spiral CT systems that rotate at 60 rpm and only collect one slice of data at a time. Multi-slice CT scanning will allow non-invasive imaging and diagnosis of wider range of conditions in less time and with greater patient comfort.
The combination of multi-slice CT and new 3D reconstruction promises to allow physicians to see even more than ever before. Multi-slice CT systems are at "the cutting edge" in terms of speed, patient comfort, and resolution. CT exams are now quicker and more patient friendly than ever before. As CT scan times have gotten faster, more anatomy can be scanned in less time. Faster scanning helps to eliminate artifacts from patient motion such as breathing or peristalsis.
The latest multi-slice CT systems can collect up to 4 slices of data in 250 ms to 350 ms and reconstruct a 512 x 512-matrix image from millions of data points in less than a second. An entire chest (forty 8 mm slices) can be scanned in five to ten seconds using the most advanced multi-slice CT system. An CT angiography images of the "peripheral runs-off" (leg vessels from the pelvis to the toes) are now being acquired for the first time with multi-slice CT. Finally, multi-slice CT with very short scan times is opening the door for CT to become more important in the management of heart disease and stroke.
Updated: September 13, 2007