I remember when I first started my internship as a fresh
graduate in Medical Radiography. My center had a 64 slice CT machine; one of
the 1st in Nigeria at that time and the only one in Lagos also at
that time. It was grossly over-exaggerated. It became news everywhere. While other centers
had dual slice, 4slice and 8slice, here was a GE 64 slice CT machine believed
to perform wonders. Then came the 128 slice Toshiba machine acquired by a PPP
in LUTH. News had it that it could “detect death in 5 seconds”. Very laughable.
64 slice GE optima CT scanner
Courtesy: internet
I am often confronted with people making arguments on number
of slices of CT scanners and the superiority of 128slice CT above 64 CT and how
the more the number of slices, the “better” the CT scanner. Most of these
arguments are based on very wrong assumptions and very poor understanding of
multi-slice CT technology. The objective of this write-up is not to uphold or
disprove any of the arguments, but to put relevant issues bordering on
multi-slice CT scanners in the right perspective. At the end, you will be the
judge of which CT scanner reigns superior.
You will need to understand
·
Why do we need a multi-slice CT technology?
·
How does a multi-slice CT technology work? What
really do the numbers represent? What does the 8 in an 8 slice CT stand for and
how is it different from a 64 slice CT?
Why do we need a
multi-slice CT technology?
The answer is very simple …To reduce the time of scan. So whatever advantages a multi-slice CT
has above single slice CT or one multi-slice CT has above another multi-slice
CT is in the amount of time required to complete a particular scan. I am not
going to make this complicated by going deeper into details. It is just this
simple: a dual slice (2 slice) CT scans faster than single slice (1 slice) CT
and a 128slice CT just scans faster than a 4slice CT. Don’t forget; the fundamental difference is speed.
The advantages of multi-slice CT scanners come from how well
they able to eliminate the issues caused by increased scan time. These issues
border around image unsharpness caused by patient’s motion (both voluntary and involuntary
movements) and the need to capture intravenous contrast as fast as the travel
through the arterial and venous circulations. Let’s put this in simple terms
again. A car with only gear 1 is obviously slower than a car with gear 5. So while
a car with gear 5 completes a race in 10 minutes, the car with only gear 1
might complete same race in a much longer time (say 30 minutes) but the bottom
line is that the 2 cars will complete the race. If you say the car with gear 5 is
better than the car with gear 1 because it is faster, you are not completely
right. What if the car with gear1 has a better chilling air conditioning system
and is fitted with a beats by Dre car sound system? So you have to define it
clearly…better because it is faster.
How does a
multi-slice CT technology work?
I would have loved to discuss slip ring technology first. It
will help us further understand the concept of spiral/ helical CT scanners. But
that might distract us and even confuse us further. So let’s stick to
understanding how a multi-slice CT technology works.
I will start by talking about what really we mean by
multi-slice CT. There are 2 ways of understanding this concept ( image acquisition
and number of detector arrays) but the both are like akara and bread…when you
take both together, they tastes better. So there is no understanding one and
leaving the other. You will only but have a shallow understanding.
·
A multi-slice CT scanner is a scanner that can
acquire multi-slice images in one complete (3600) rotation. Let me
explain further; depending on your level of understanding of Computed
Tomography, you must have known that to get one image( one slice), the gantry
of the CT containing the X-ray tube and the detector arranged directly opposite
each other must make a 3600 rotation round the part being imaged. Don’t
be confused. If you don’t understand, go back and read the lines until you do.
Tube –detector alignment
Courtesy : internet
Tube –detector alignment
Courtesy: internet
So what this means is that a dual slice CT
scanner acquires 2 images in one rotation and a 64 slice CT scanner is “able”
to acquire 64 images in one rotation. Notice that I have put able in quotes because it’s not every time
that a 64 slice produces 64 images per rotation. It depends on the scan and the
protocols selected. This applies to 4slice, 8slice, 16slice, 32slice, 64slice,
128slice, 256slice, 320slice etc scanners. Again, a 128slice CT scanner is able
to acquire 128 images in one rotation. Some schools of thoughts will tell you
that one rotation is completed in 1 second and hence state that 128 slice CT
scanner is able to give 128 images in 1 second. But it is safer to understand
this in terms of rotation as some rotations may go faster than 1second. In fact
disregard this one rotation in one second term. I mentioned it because someone
taught me that but it’s been disproved. The tube-detector system rotates very
fast.
To understand multi-slice CT scanners in
terms of number of detector arrays is to further appreciate the design of the
data acquisition system. I have talked about this in previous posts. But let’s
focus on just the detector panel which is where the x-ray passing through the organs
of the body are received and recorded. The detector arrays/rows can be arranged
parallel to each other in the detector panel. Now this is what happens in a multi detector
row CT scanner; the detector panel has more than one detector arrays/rows
corresponding to its number. So a 16 slice CT scanner has 16 detector arrays/
rows in its detector panel. Same goes for 4, 8, 32, 128slice CT scanner each
having 4, 8, 32, 128 detector rows.
·
single detector row versus multi detector row
Courtesy : internet pictures
To marry the image acquisition with number of detector rows;
depending on the number of images to be acquired to match the scan time made
possible by selected slice thickness and scan field, the detector rows can be
combined to produce more than one image per rotation.
In summary, I believe I have been able to demystify the concept
of numbers of slice in CT technology. A 64 slice CT is not necessarily better
than a 16 slice scanner. A 64 slice CT is made specifically to accommodate scans
requiring faster times like cardiac CT scans. So except if you have plan to do
cardiac CT scans in your facility, there is really no need to buy a 64 slice,
128slice etc CT scanner. An 8 slice CT scanner can give as good image quality
as a 64slice CT scanner. Besides, these high detector row Ct scanners are
really expensive. Most scanners have improved technologies to deal with
enhanced image contrast, image resolution, patient dose reduction etc. But this
has little or nothing to do with the number of slices the machine is capable of
acquiring per rotation. And by the way, GE healthcare just released a n 8 slice
CT scanner that can perform peripheral angiography and colonoscopy and also
fitted with very smart technologies that gives improved image qualities.
I encourage you to
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