From image quality to spatial resolution, there are several factors that influence the price and performance of a computed tomography (CT) scanner. If you are in the market for a CT, one of the easiest ways to save money is to separate the specifications that you need from those that you can go without. So, where does slice count fit in this mix and what are its implications?
- Slice Thickness and Spatial Resolution
- Slice Count and “Coverage”
- Speed of Acquisition
- Human vs Veterinary Importance
Slice count is often the first specification mentioned in a discussion about CT, but many veterinarians do not understand its role in CT scanning. Slice count refers to the number of rows of detectors in the z-axis of a CT. You might see a particular CT referred to as an “8-slice CT scanner,” “16-slice CT scanner,” etc. In an 8-slice CT, there are eight slices of data captured for every rotation of the gantry.
Slice Thickness and Spatial Resolution
The primary effect of slice count is the amount of time it takes to complete a CT scan. Before I elaborate, it is first important to introduce another CT specification: “slice thickness.” This determines the spatial resolution of the scanner in the same way that pixel size determines the spatial resolution of a flat panel in digital radiography. Slice thicknesses of multi-slice detectors are all very close throughout the industry, ranging from 0.625mm to 1.5 mm.
“Coverage” and Calculating CT Scan Time
The number of times the gantry has to rotate determines the total scan time. The size of the “coverage area” per gantry rotation is determined with this formula:
Slice Thickness x Slice Count
So, a 64-slice scanner requires ¼ the number of revolutions to cover the same scan volume as a 16-slice scanner.
Let’s look at an example:
Given these specifications, a 16-slice scanner would require 31.25 seconds to scan the full anatomy.500 mm of anatomy ÷ 1 mm per slice ÷ 16 slices per rotation ÷ 1 rotation per second. However, a 64-slice scanner would only require 7.8 seconds to scan the full anatomy. 500 mm of anatomy ÷ 1 mm per slice ÷ 64 slices per rotation ÷ 1 rotation per second
As you can see, the slice count is directly proportional to the scan time:
Gantry Rotation Speed: 60 rpm (1 rotation per second)
Slice Thickness: 1 mm
Anatomy to Be Scanned: 50 cm (in the z-direction; 500 mm)
Given these specifications, a 16-slice scanner would require 31.25 seconds to scan the full anatomy. 500 mm of anatomy ÷ 1 mm per slice ÷ 16 slices per rotation ÷ 1 rotation per second. However, a 64-slice scanner would only require 7.8 seconds to scan the full anatomy. 500 mm of anatomy ÷ 1 mm per slice ÷ 64 slices per rotation ÷ 1 rotation per second. As you can see, the slice count is directly proportional to the scan time:
Human vs Veterinary Importance
In the veterinary world, though, the slice count doesn’t matter except in extreme cases because the size of the anatomy being scanned is relatively small (compared to a head-to-toe scan of a 6-foot tall human), so the typical scan time is short even for CTs with lower slice count.
Most animals are less than 1 m in length, and a nose-to-tail scan of an animal is rarely required. We’re usually dealing with a 30 cm skull, a 50 cm thorax, or in extreme cases, a 70 cm spine—but even in that case, it is often possible to isolate the pathology to one small area of the spine.
For a more specific example, let’s assume that we’re scanning a 60 cm abdomen using 8-slice and 16-slice CT scanners, both with 1 mm-thick slices and 1 rpm gantry rotation speed.
The 8-slice machine accomplishes the scan in 75 seconds (1.25 minutes) while the 16-slice machine only takes 37.5 seconds. That’s a factor of 2 difference (because the slice count is different by a factor of 2). But from a logistical standpoint, there is virtually no difference whatsoever. The duration of the scan for the 8-slice machine is still clinically feasible, and there is absolutely no difference in the image quality, spatial resolution, or motion artifact.
SOUND TOP TIP
As you do your diligence on CT scanners, walk through your hospital and measure the patients you would scan if your CT was delivered today. Determine the total length of your scan and do the simple calculations to determine how long the scan would take. Then compare with the length of an ultrasound scan or even a multi-site DR acquisition.
CT scan times in veterinary applications vary from seconds to less than two minutes. The deeper you dig into this concept, you’ll very quickly see that scan time (and therefore slice count) is a very insignificant part of your decisions, especially knowing that a factor of 2 in slice count (from 8- to 16-, for example, could cost you upwards of $25-40,000 in cost).