Evaluation of CT performance in SPECT/CT

Technology primer Evaluation of CT performance in SPECT/CT The effect of computed tomography (CT) acquisition parameters on imaging Partha Ghosh, MD Siemens Healthineers Molecular Imaging Business Line siemens-healthineers.com/mi SIEMENS Healthineers Introduction The most compared parameter when evaluating CT system performance is the number of cross-sectional images, or “slices,” a scanner acquires during each rotation around the patient. In molecular imaging (MI) it is also commonplace to assess CTs based on the number of slices. However, when reviewing CT capabilities, slice count is not the only performance factor, nor is it the most critical. The basic function of a CT in SPECT/CT is for anatomical localization of the metabolic information, extracting addi- tional information from the patient’s specific anatomy, and attenuation correction (AC). When reviewing CT offerings, it is important to understand all of the factors impacting how the device performs. This includes looking beyond the number of slices and evaluating how CT acquisition settings such as dose, including kV and mAs, rotation speed, pitch and table speed, and total length and time of acquisition, directly affect scan speed and image quality. CT versus gamma camera imaging: similarities and differences CT systems and gamma cameras have some similar imaging approaches. In CT, a topogram or scout view is acquired, which is typically an anterior-posterior (AP) view of the anatomy. Similarly, a gamma camera generates a planar image that shows an overview of the patient’s anatomy. In CT, the tube rotates around the patient and creates a slice or image; a SPECT gamma camera, taking a bit more time, completes a rotation around a patient to create a tomographic image. With CT, the system can image while the table is in motion, which is not done on a gamma camera. While this guide does not detail the technical reasons behind this concept, it is important to note that this functionality is considered the standard of care for CT today and is referred to as spiral (or helical) CT. In spiral CT the table is constantly moving while the tube and detector rotate around the patient. This technique forces the user to make decisions to optimize acquisition, which varies widely from patient to patient. Spiral CT is the core reason slice number alone is not a true indicator of CT performance. Parameters impacting the acquisition speed and quality of resultant CT images As previously stated, multiple parameters influence CT speed and image quality. One key aspect is ‘pitch.’ Pitch is the relationship between the distance the table travels every rotation versus size of the collimated slice. Other primary considerations for spiral CT are noted below: 1. Rotation speed 2. Power reserve of the generator/tube 3. Speed of the table 4. Slice width (# of slices x slice width = total collimation) 5. Number of slices Pitch = table distance/revolution total collimation 2 CT performance | Effective Date: 14 Nov 2019 | HOOD05162003049694 Pitch values are fundamental in defining the time required to achieve desired image quality or image noise. Scan time may vary based on the clinical requirement, such as solely AC or diagnostic CT. Table 11 Slices 2 6 8 Collimated slice width 3 mm 3 mm 1.25 mm Total collimation 2 x 3 = 6 mm 6 x 3 = 18 mm 8 x 1.25 = 10 mm Rotation time 0.8 s 0.6 s 1 s Pitch2 1.8 1.8 1.35 Speed 13.5 mm/s 54 mm/s 13.5 mm/s Time to cover 1 meter 74 s 18.5 s 74 s Scan speed = total collimation x pitch rotation time Table 1 is an example of an AC scan on three CT systems with varying slice numbers. The reconstructed slice thickness of an AC scan is 3 mm to 5 mm. The speed of the scan is not always immediately recognizable. It is not important to memorize the formulas above, but only to understand the interdependency or relationship of many parameters to overall system performance and image quality. In this case, the number of slices for each scanner is factored with other CT parameters, thus resulting in acquisition speeds for each. (Note: slice width is kept constant for the duration of the scan.) Table 21 Slices 2 6 8 Collimated slice width 3 mm 3 mm 1.25 mm Total collimation 2 x 3 = 6 mm 6 x 3 = 18 mm 8 x 1.25 = 10 mm Rotation time 0.8 s 0.6 s 1 s Pitch2 1.5 1.5 1.35 Speed 11.3 mm/s 45 mm/s 13.5 mm/s Time to cover 1 meter 88.5 s 22.2 s 74.0 s If a diagnostic CT acquisition (a separate study done for interpretation of the CT scan) is performed, the acquisition protocol may change in order to present the CT images in the manner needed. See Table 2. Interesting to note is how the slight parameter adjustments made for the second acquisition affect acquisition speed. CT performance 3 These two protocols also do not consider the power used to create the X-ray dose delivered to the patient, which has a significant influence on the image produced. As in SPECT where injected dose and time of acquisition define image quality (count statistics), CT, likewise, has a dose value (mA) and a rotation time component (t). The product of which is mA x t = mAs. mAs is the primary indicator of image quality, or more specifically, image noise. Figuratively, the number of counts and mAs are equivalent. Generally, more counts equal a better image and fewer counts equal a poorer image. Therefore, in CT higher mAs means less image noise or better image quality, at the expense of dose to the patient. For CT scans performed mainly for AC, the dose can be very low, as the computer knows how to segment the data from a noisy image. Diagnostic CT protocols, however, must have enough dose to produce an acceptable image quality based on noise and sharpness requirements. Diagnostic CT is a delicate balance of matching the clinical needs to the acquisition protocol. Thin slices (0.5 to 1.0 mm) are often used to obtain fine detail, with inner ear imaging for example, but these images tend to be noisy. Thick slices (3.0 to 5.0 mm) are used to achieve more subtle greys in the image, such as with brain or liver images. As such, weighing image quality versus dose is an ongoing consideration. As with SPECT, the ability to offer the necessary dose per image is a function of the generator and tube (or injected dose). Inadequate generator power will result in noisy images. Another way to add more dose is to slow down the rotation time so enough dose can be generated per rotation if the generator/tube is not strong enough to go faster, but this creates a blurry image due to motion. Rotation times can be compared to the shutter of a camera, where a faster rotation ‘freezes’ the image more so than a slower rotation. In short, it is important to keep in mind that the rotation speed must be matched with the appropriate power (mAs) in order to align with the clinical requirements. When considering a CT, the generator/tube must be large enough to deliver the dose necessary for the task at hand. An AC-only scan can suffice with poor image quality, noisy and slightly blurry. However, for examinations requiring diagnostic quality CT, appropriate dose needs to be delivered in order to obtain good image quality at fast rotation speeds. The CT generator and tube thus must be able to deliver adequate dose. 4 CT performance Conclusion Both CT image quality and acquisition speed are affected by the interdependencies of several specific components, beyond the number of CT slices. True quality is reliant on dose, time, system architecture, distance to detectors, etc. Evaluating a CT therefore needs consideration of multiple such factors rather than number of slices alone. CT image quality depends on the appropriate combination of a host of protocol settings such as rotation time, pitch, slice thickness, mAs, and total coverage in the Z direction. Different CT systems must use differing settings to ‘match’ a certain image quality in a defined amount of exam time. Therefore, CTs must be compared based on both speed and image quality at that particular speed. So, when selecting a CT system, the number of slices is one factor to be considered but should not be viewed as the most critical one. In SPECT/CT, AC-only scans are easily performed with a 2- slice CT. If more is required clinically, like diagnostic CT, system capabilities such as faster rotation times, higher pitch settings, appropriate power capability, and, yes, potentially more slices should be evaluated. There are different CT approaches to creating the best clinical protocols for a department. The key is to match the CT system to what is needed in the clinic, as per the protocols, the patient mix, and patient volume. CT performance 5 Trademarks and service marks used in this material 1 With the reconstructed slice width normalized to the are property of Siemens Healthcare GmbH. All other same. Based on typical customer utilization and factory company, brand, product and service names may recommended protocols. Competitive data on file. be trademarks or registered trademarks of their 2 Some CT vendors limit the selection of pitch values due respective holders. to reconstruction difficulties. Please contact your local Siemens sales representative for the most current information or contact one of the addresses listed below. Note: Original images always lose a certain amount of detail when reproduced. All photographs © 2019 Siemens Healthcare GmbH. All rights reserved. “Siemens Healthineers” is considered a brand name. 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