Axial Scanning
Step and shoot method
Earlier scanner operated exclusively in the step-and-shoot method. This method is also referred to as axial scanning, conventional scanning, serial scanning, or sequence scanning all imprecise term that tend to create confusion in the field. Key aspect of the step and shoot method are that the CT table moves to desired location within the gantry, collecting data. Early systems, which contained only a single row of detectors in the z axis, obtained data for one slice with each rotation. This method of scanning was used, in much the same fashion, by both 3rd and 4th generation scanners. A minor difference was that whereas 3rd generation systems typically used a 360 degree rotation for each acquisition, and 4th generation system most often used a 400 degree rotation. In all types of scanners using the method there is slight pause in scanning between data acquisitions, referred to as the interscan delay, as the table moves to the next location. In early scanners the time for a complete cycle like: table movement to correct position, gantry rotation for scan acquisition, table movement to next position, allowed only a single scan to be acquired each time the patient held her breath. Newer scanners shortened the cycle time dramatically, allowing axial scans to be clustered. This is the practice of grouping more than one scan in a single breath hold. The number of scans grouped together is dependent on the speed of the specific scanner used and on how long a patient can reasonably be expected to hold her breath. The number of scan per cluster and the breathing time between clusters are programmable features and can be adjusted as specific patient conditions dictate. Grouping scans in this fashion decreases examination time and reduces slice misregistration.
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| Axial slices lie parallel to one another. The slice beginning matches exactly the slice end, and perfect circles are formed that are perpendicular to the z axis. |
Advantage of Axial Scanning
In evaluation image quality using phantoms- ‘that do not breathe or move’, step and shoot method result in the highest image quality, superior to that of helical methods. This is because of their axial nature like for example, is slice are perpendicular, not slanted, and the fact that the patient table remains stationary during data acquisition.
Axial scans can be programmed to acquire data for contiguous slices, ‘in which one slice abuts the next’ or in a noncontiguous fashion, in which some areas of the patient are skipped between slices or slice data overlaps. Scanners can also be programmed to repeat scans at the same slice location like no table incrementation. Such protocols are often called cine or dynamic methods and are done when how a structure appears over time is of interest. Gapped images are taken a survey of an area is needed like; ‘representative slices are sufficient, and imaging of part of the region is not required’. Because some areas are not exposed, studies made up of gapped slices will reduce the radiation dose to the patient. Gapped slices are often used for high resolution chest studies and for the unenhanced portions of chest studies when aortic dissection is suspected. Axial protocols that use overlapping slices are rare because they increase the radiation dose to the patient but typically do not provide additional diagnostic information. For these reasons, all current scanners offer the option of axial scanning. Although helical scanning has replaces many axial protocols, there are still a number of procedures in which methods are preferred.
Disadvantage of Axial Scanning
If the axial scanning has some benefits on producing high quality images. Inversely, axial scanning has also disadvantage on providing CT scan images. The primary disadvantage to the axial method is that the cumulative effect of the pauses between each data acquisition adds to the total examination time. Although the cumulative delay may be less than 30 seconds, even this brief delay is often significant. This is particularly true when blood vessels, which remain contrast filled for very short periods, are of primary interest or when a patient’s breathing will result in motion artifacts on the image. In addition, compared with helical data, data acquired in the step-on-shoot method are more limited in how can be reconstructed.
Misregistration
The delay inherent in axial scan sequences decrease the likelihood that a patient will be able to hold his breath throughout the examination. When a scan sequence is longer than a single breath hold, scans must be briefly suspended to give the patient time to exhale, take in a new breath, and hold it once again. This can result in slice misregistration, which occurs when a patient breathes differently with each data acquisition.
To better visualize how misregistration can result in error, imagine this scenario: A shallow breath places a group of slice acquired at a specific level of anatomy. The patient is allowed to breathe and the table is moved to the next scan position. The next set of scan is taken but the patient takes a deep breath instead. This difference in breathing places the second group of scans in an incorrect anatomic position relative to the first set of slices. Valuable information may be missed because of this effect as seen on the image below.
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| Slice misregistration caused by patient breathing. These two slices are taken at contiguous table positions; slice (A) is the last slice in the first group of axial slices. The patient was allowed to breathe and then once again asked to hold his breath. Slice (B) was the first slice in the second group of scans. Slice (B) is just 5 mm more inferior, yet it appears dramatically more inferior. It is possible to miss lesions as large as 1 cm as a result of slice misregistration. |
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