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by Michael H. Friebe, PhD
From RT Image
We keep hearing about these new 8- and 16-slice computed tomography (CT) scanners (or are we at 32 slices already?) and assume these systems are better than the single-slice spiral CT scanners installed three or four years ago. But do multi-slice scanners really perform better?
The new scanners are between 16 and 32 times faster (8 slice/16 slice, 0.5 seconds per rotation) when compared to a one-second spiral CT and can cover a larger body area in a shorter period of time. This is important when scanning organs like the heart, areas like the abdomen or for angiographic studies.
Multi-slice CT (MSCT) provides thinner slices, which, in combination with their fast acquisition, offers the possibility of "isotropic" imaging. Isotropic imaging is the acquisition of cubical voxels such that the spatial resolution of an acquired volume is the same in all three directions (X, Y, Z — e.g., 1mm x 1mm x 1mm cube-shaped voxel). In an isotropic data set, the same spatial resolution is present in any view: axial, coronal, sagittal and oblique (for example, lung scanning with a pixel resolution of 0.75 mm and a coverage of 300 mm requires 400 slices with 0.75 mm slice thickness). With a breathhold of 15 seconds, a 16-slice system with 0.5 seconds per rotation can acquire 480 slices — a feat not possible with a single-slice CT. As a result, clinicians reformat and reconstruct the acquired data in coronal and sagittal images from the axial images without any data loss.
MSCT is not much different from single-slice CT (SSCT) with the exception of the detector row, a fast reconstruction computer and a few other small differences. The detectors are either spaced equally or unequally in rows of four, eight and 16. Combinations of detectors determine the slice thickness (e.g., 8mm x 2mm x 1.25mm = 8 slices with 2.5mm each).
With an increasing number of detector rows, the so-called cone beam problem starts to distort the image reconstruction in the Z-axis if it is not properly corrected through the use of software algorithms. The cone beam (triangle between the X-ray source and the width of the detector array width) results in thinner slices close to the X-ray source when compared to the detector width. This is not a huge problem for very thin slices and for 4- or 8-slice systems, but it starts to have a significant impact on the 16-slice system.
Does a MSCT provide better image quality or reduce the X-ray dose? Not really. SSCT, for most general applications, provides better image quality with a reduced X-ray dose, and may actually be the better choice.
However, advanced applications requiring speed and detail will only be possible with MSCT systems. So the question arises: Should we use SSCT for general applications and MSCT for advanced procedures, or do we perform all CT scans on an MSCT system? Most smaller hospitals or practices with only one CT scanner will likely go for the second option in the very near future.
In fact, one CT vendor no longer offers SSCT systems. While most lower-slice MSCT units can be upgraded to image more slices, the upgrade prices of the original equipment manufacturers will, in most cases, be prohibitive. The results being that these units will be replaced rather than upgraded.
What does that mean for the used equipment market? It's not good news for companies dealing with used SSCT systems, but very good news for potential customers of these scanners. The supply of advanced and relatively new units (three to four years) will go up and the prices will go down as the demand for these CT scanners go down as well. Prices for MSCT systems will also decline, driving the pre-owned SSCT prices down further.
Buyers of CT systems should carefully evaluate whether they really need an MSCT system at this moment or if they can live with an advanced SSCT system for another two or three years, especially since service costs are higher and most general applications are not reimbursed any differently for SSCT or MSCT. By that time, many MSCT units will be available on the pre-owned medical equipment market at significant price advantages.
Michael H. Friebe, PhD, a long-term IAMERS member and technical contributor, is the principal of Tomovation¨, Recklinghausen , Germany , a company providing expert consultation to end-users and manufacturers in the diagnostic radiology modalities, especially CT and MRI. A university-trained engineer with a doctorate in physics and an MBA, Friebe founded Neuromed AG in 1994, Europe 's largest provider of mobile imaging services and pre-owned MRI/CT systems
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