128SLICE CT SCANNER...GROSSLY OVER-EXAGGERATED

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 1^st 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 source out books and other articles

Steps to learning Computed Tomography as an Intern Medical Radiographer

I understand that it can be very intimidating to face a CT scanner for the very first time. I was also intimidated…very intimidated. It’s easier if as a student you were introduced to practical CT procedures. But if you were unfortunate to meet a CT scanner for the first time as an intern, it’s pretty tasking to learn it under so much pressure.

I am going to share with us practical steps to learning CT scanning procedures. It could be that simple if you follow it sequentially than trying to learn everything at the same time. Scanning a patient using a computed tomography scanner requires the following

1.       Knowledge of patient surface anatomy. You will need this in patient positioning and centering.

2.       Knowledge of anatomical coordinates. You will be hearing lots of stuffs on axial, sagittal, coronal, cranio-caudal, caudo-cranial, prone, supine etc. It will help you a lot also in patient positioning and the actual scanning.

3.       Theoretical knowledge of the basic concepts of CT scans

4.       And every other relevant knowledge you can get.

CT scan procedures can be divided into these steps:

Step 1:

PATIENT’S PREPARATION: Some patients are given information a day to the scan. You ask the patient questions regarding what they ate and how best they adhered to instructions given earlier (if any). This is the right time to look at the request of the patient *again and review the clinical notes/ history. This is where the patient is given appropriate clothing for the procedure. You also explain the procedure to the patient and take vitals (blood pressure, temperature etc.). There is usually a departmental protocol for patient preparation. Patients requiring oral contrast are given instructions on how to drink it and the duration. For patients requiring IV injections, it’s better to cannulate the patient at before positioning. I have omitted patient registration and documentation pre and post scanning because it is mostly handled by medical records personnel or other designated persons.

What to learn:

·         How the patient request is reviewed; what important information to look at and how that information guides the scanning.

·         How the patient is prepared; what you explain to the patient about the scanning procedures, what the patient should eat and not eat.

·         What patient requires oral contrast and why and how do they take it.

Note: in most departments, it’s the job of the radiologist or medical officer to cannulate the patient and the radiology nurses take vital signs. It’s very important to understand the departmental protocol on patient preparations.  

Step 2:

PATIENT POSITIONING: Bad CT technique starts from bad patient positioning skills.

What to learn:

·         How the patient is supposed to lie; supine/ face up, prone/ face down, lateral/ on the sides, hands by the sides, hands above the head etc?

·         What buttons on the CT gantry moves the table up, down, in and out, angulates the gantry and other motions.

·         Where the centering for each examination is. How to properly use the beam lights/ coordinates in centering the patient. What buttons you need to press to centre a patient and so on.

Step 3

SCANNING: This is where the images are obtained. To do this, you will need to put in patient data, select the appropriate protocol, obtain the scout/topogram images, plan the locations of scan images and the region of interest, confirm and scan and repeat scan (for contrast phase if needed).

What to learn

·         How to put in patient data

·         How to select the appropriate protocol.

·         How the scout image(s) is obtained.

·         How you plan/ plot the locations of scan images and the region of interest

·         How to confirm and scan.

·         How to repeat scan for contrast phase.

NOTE: I have omitted how to adjust protocols and parameters. The simply reason is that the CT scanner has so many settings in it. From exposure factors through slice thickness/ interval, pitch and table increment to dose reconstructions, these figures are standardized for specific protocols. So brain protocol is already set for CT brain scans, Abdomen protocols for CT abdomen and so on. Most protocols also include thin slices/ volume image recons. It is not your business at this stage to worry about adjusting protocols and parameter. You will have enough time to learn that as you progress. However, the supervising radiographer will tell you what is necessary to adjust especially when you need bone windows and for other cases requiring any adjustments.

Step 4

REVIEWING THE IMAGES. This helps you to better appreciate the images and the quality of the work you have done. At this stage of learning, you are to learn how the anatomies as they appear on axial, coronal and sagittal images. You will be guided to know how the normal appears and how the abnormal are different. Text books on sectional anatomy will help you learn faster. But don’t sweat yourself out. You have enough time to learn while you practice.

Step 5

IMAGE REFORMATION: CT images are typically acquired in axial planes. You are going to need to learn how to reformat images into coronal and sagittal planes. These plans are very essential in comprehensive diagnosis by the radiologist. Some machines can produce reformatted plane images automatically. You will learn 3D/ Volume rendering reformations but this is when you have learnt how to scan properly.

1.       Printing and archiving images. This is the final step involving producing the images on film and storing the images in the departments PACS/ Picture Archiving and Communication System.

In summary

·         1. Learn how to prepare the patient

·        2.  Learn how to position the patient

·         3. Learn how to type in patient details

·         4. Learn how to obtain scout/ topogram image(s)

·         5. Learn how to plan/ plot locations for slices

·       6.   Learn how to confirm and scan

·         7. Learn how to repeat scan for contrast administration.

·         8. Learn how to reformat images into coronal and sagittal planes

·         9. Learn how to print and archive images.

 Like I said earlier, learning CT can become very challenging when you have to learn so many things at the same time when in fact CT scan is very easy...at least the basic ones. it is best to develop a timetable to guide you in learning. 1 and 2 can be learnt in a day or 2; 3,4,5, 6 and 7 can be learnt over 2 weeks to 1 month; 8 and 9 can be learnt over 1 week. so in all, within 1-2 months, you are good to go.

It is best to start learning CT scan with CT brain. After that, focus on CT chest and then CT abdomen.  Don't rush to learn CT angiography and the rest. You will end up being confused. Refer to my post on CT Brain: a preview of practice. It will help you.

Good luck