Chest: Exam Technique and Protocols
MDCT requires a careful analysis and modification of many of our single detector CT protocols. The specific protocols used including mAs, kVp etc. are contained in the protocol section but several basic principles can be summarized as follows:
- there is no disadvantage to using MDCT over single detector spiral CT (SDCT) or non-spiral CT scanning.
- for a similar scan volume and protocol MDCT is at least 4 times faster than SDCT.
- less IV contrast can typically be used with MDCT.
- because of how we can vary slice thickness from a single set of detectors, and because of how raw scan data is stored and coupled with the speed of image reconstruction (>2 slices per second) it is now easy to create high resolution images of the lung (HRCT) and detailed soft tissue imaging from a single data set. Similar advantages can be noted when acquiring both bone and soft tissue images from a single dataset.
- because of the speed of data acquisition the potential pitfall now is of scanning too early during an IV injection rather than too late.
- volume datasets generated with 1 mm detectors are optimal for 3D imaging.
Another advantage of the faster scan times with MDCT is the ability to use a lower volume of contrast for most routine chest studies. Loubeyre et al. recently found that 80 ml of 350 mg/ml of iodinated contrast injected at 2 ml/sec provides good opacification of central vascular structures. One potential problem with the faster acquisition is artifact related to the contrast bolus in the axilla or SVC. Several articles have suggested using a diluter contrast while a recent article suggests the use of a double power injector setup to inject both iodinated contrast and saline. The authors also found that they could achieve a "20% reduction of contrast material volume to 60 ml with a similar degree of enhancement. In addition, perivenous artifacts in the superior vena cava are significantly reduced."
We have found that one solution is to scan the patient from the diaphragm cranially to the lung apex. This allows the contrast bolus to be injected with little or no artifacts off the dense contrast in the axillary vessels or SVC.
"Injection of contrast material followed by a saline solution bolus using a double power injector when performing thoracic helical CT allows a 20% reduction of contrast material volume to 60 ml with a similar degree of enhancement. In addition, perivenous artifacts in the superior vena cava are significantly reduced."Reduction of Contrast Material Dose and Artifacts by a Saline Flush Using a Double Power Injector in Helical CT of the Thorax
Haage P et al.
AJR 2000; 174:1049-1053
Specific scan protocols will vary on the clinical problem as well as to whether 3D rendering is needed. One of the advantages of MDCT is of course that the speed of data acquisition limits the length of breathhold required to successfully complete a study. This is especially true in pediatric patients where a lack of need for sedation was seen in many patients.
"During contrast enhanced helical CT examination for general thoracic evaluations, good opacification of central vascular structures is obtained with a low volume of high iodine concentration nonionic contrast medium."Using Thoracic Helical CT to Assess Iodine Concentration in a Small Volume of Nonionic Contrast Medium During Vascular Opacification: A Prospective Study
Loubeyre P et al.
AJR 2000; 174:783-787
MDCT also has several practical advantages as to how image data is reconstructed and then either filmed or sent to a PACS workstation. In the past with single detector CT, the user made a pre-study decision as to the slice thickness used. Although the interscan spacing could always be selected following data acquisition, and the reconstruction algorithm (i.e. soft tissue algorithm, high resolution/ bone algorithm) similarly selected retrospectively the slice thickness could not be adjusted. If a 5 mm slice collimation was chosen the CT slice of that study would always be 5 mm. With MDCT the flexibility is increased. Although one must choose either a 1.0 mm, 2.5 mm or 5 mm detector one can select a number of different slice thicknesses for each volume acquisition. For example, when the 1.0 mm detectors are used you can reconstruct 1.0 mm high resolution scans of the lung parenchyma as well as 5 mm thick scans at the soft tissue algorithm of the mediastinum and chest wall. With over 2 images per second reconstruction time selecting two different algorithms has little if any effect on patient throughput.
We are then able to film images of the lung parenchyma with high resolution and images of the mediastinum with ideal soft tissue resolution. Similarly images can be reconstructed at bone windows or other window settings depending on the clinical case and problem. This flexibility of MDCT is obviously one of its key advantages.
VolumeZoom: Slice Widths Available
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Detector: 4 x 5 mm
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