First things First...Learn and understand iodine contrast BOLUS TRACKING Techniques before learning CT Angiography technique

smart prep protocol on a GE CT scanner

"Can you do angiography?"...That's an irritating question right?.."Of course I can because I am a medical radiographer and that's  what I do". 

But I get the "heat". A lot of us have been made to believe that CT angiography studies are very complex studies and it creates a lot of tension around the procedure especially for younger medical radiographers who are just getting introduced to the practice or older ones who have not been working with a CT scanning equipment.

abdominal angiography 

cerebral angiography showing circle of willis

Angiographic images are beautiful...Ain't they??

Now let's pour some ice on the heat. Let's just chill and get this things straight. I have been thinking. What is it about Angiography studies? What the heck?? The only angiography study that should seem complex is the coronary (cardiac) angiography. So whatever we are going to discus here doesn't really apply to cardiac/coronary angiography which unarguably involves a much more complicated technique including slowing the heart rate down and synchronizing ECG readings with the scan. I have done a coronary angiography only once in my practice and that was as an intern and I was simply observing.

The big deal in Angiography really is the approach we take especially while learning the procedure. So there are lots of things to learn all at once. Patient preparation, positioning, contrast administration, volume reconstructions and so one and so forth. How about we take the most essential of the things to learn and learn and understand it first before even trying to learn CT angiography techniques.

Ever heard of BOLUS TRACKING??? If you haven't then I am wondering how you even bothered to learn CT angiography. Now what is bolus tracking and why the h*ll do I have to learn about it? Let me break this down very well for you so you will understand it. First the real essence of angiography is to inject iodine contrast into the arteries and take scans while the iodine contrast is still within the arteries. How do you monitor the contrast flow to know when the contrast has fully entered the arteries? You use the technique called bolus tracking. The essence of this bolus tracking is that you visualize the contrast as it fills an artery selected as the region of interest (ROI). No vex!!! My technical grammar can be confusing. But what I mean is with bolus tracking, you see the contrast as it fills up the artery and then decide when the contrast has optimally filled the arteries and then you start the scan .

Some of us that have used GE CT machines are very familiar with SMART PREP. The very first thing you should learn even before you learn how to position a patient for angiographic studies is bolus tracking technique that is peculiar to your CT machine. If you are using  a GE CT scanner, it is called Smart prep, CARE bolus for Siemens; SUREStart for Toshiba; Predict scan in Hitachi and Philips refused to name it anything but bolus tracking. So whatever it is called in your machine, the function is same. Bolus tracking technique is used to monitor the flow of iodine contrast into the arteries to be sure that when you take the scan, there will be maximum enhancement of the arteries. By maximum enhancement, I mean that the arteries are very visible.

I am going to try to explain how bolus tracking works in a GE CT machine. It's called smart prep. I already mentioned what Siemens and others call theirs. The user interface might be different for different  CT brands makers but the technicalities are relatively the same. It's your responsibility to learn how it works in your own machine. I am sure that your colleagues who know it better will gladly teach you. It's a technique that you learn on the job.

You will need to have prepared the patient, positioned the patient connected the IV line on the patient to the auto-injector, obtained a scout image and possibly taken plain scans before you activate the smart prep (bolus tracking technique). Please let me repeat again; don't go looking for smart prep in a Siemens CT scanner. It is called  CARE bolus. So moving on, once you activate the smart prep protocol, you are able to bring out the localizer. The localizer is more like a line that shows you where the actual slice/ cut will take place. Depending on the angiographic study you are doing,  you can place the localizer on the region of the arch of aorta for cerebral, carotid, upper limb(peripheral) and pulmonary angiography  and if it's renal, abdominal aortic and lower limb (peripheral) angiography, the localizer is best placed around the region of the descending abdominal aorta. Once you are ok with the localizer placement, you accept and then initiate the scan. A single slice is acquired. You place your Region of Interest (ROI). The ROI is usually the best suited arterial vessel to use in monitoring the flow of contrast. when you eventually start the scan, this is where you will see contrast fill and on maximum enhancement, the scan is taken. Please be very careful with placing your ROI especially when you  are using dynamic transmission or auto-scan ( which means that the system detects the maximum enhancement by itself and initiates the scans by itself). On dynamic transmission or auto-scan, the system recognizes when the set Hounsfield Unit/ HU value ( contrast enhancement)for the ROI has been reached and then sets off the scan automatically. So if you place your ROI on the wrong vessel or outside the arterial vessel, the system will either not initiate the scan or it will delay the initiation and you will make a mess of the procedure. Personally I deactivate dynamic transmission so that I am able to initiate the scan myself ( manually). If I am working with a Toshiba CT scanner, I use manual  scan instead of auto-scan. It's the same thing I am trying to achieve. So once you have placed your ROI corrected and of course checked that the auto-injector and IV line and connectors  are also set with the appropriate contrast volume and flow rate, you start the scan (monitor phase) together with the injection. As the CT contrast injection goes on, the machine scans the ROI (only) and you see the contrast filling. On optimal enhancement, the scan phase is initiated either manually  by you (in manual scan) or automatically by the machine (in auto-scan or dynamic transmission). Scan phase is where the machine scans the entire length of the patient that you have selected in your planning.

Please  it is important to note that before you activate bolus tracking technique, you must have planned your localizer for the contrast phase of the study.

 Bolus tracking is not only used for angiography. It is used in liver studies (tri and dual phase  studies) and any other studies where you need to monitor the flow of iodine contrast real time

if you understand how bolus tracking technique works, you won't have problems with understanding how to perform an angiographic study.

please go back to my earlier post on Angiography and read more about the techniques. if you have any issues, please send me an email on cseiroegbu@yahoo.com or call me on 08034635555

TRI-PHASE LIVER STUDY: A big "No big deal"

TRI PHASE LIVER: What you need to know.

Tri-phase liver study sounds very complicated and technical..right?

Well, no doubt but the truth is that it’s just another CT scan procedure and there is really nothing much to it…but that if you really understand the technicalities behind it.

To understand tri-phase liver study, you will need to

i.                     Understand the anatomy of hepatic blood supply. I hope you remember that by hepatic I mean “liver”.

ii.                   Understand the concepts of hypervascular and hypovascular liver lesions. Hypervascular liver lesions hepatic arterial blood supply while hypovascular lesions are almost not supplied by hepatic arteries.

iii.                  You will also need to understand the dynamics of blood flow because this will help you know at which times after administration of contrast that you can acquire the different phases.

Liver blood supply

Before I confuse you, let me start by explaining the blood supply to the liver. I will assume you know that the liver has 2 blood supplies and one venous drainage. The hepatic portal vein carries blood to the liver from the GIT, spleen and associated organs. 70 % blood supply to the liver is from the hepatic portal vein. The hepatic artery brings the remaining 30% blood supply to the liver from the abdominal aorta. The hepatic vein empties blood from the liver into the inferior vena cava. You probably should go back and read up the anatomy of liver and its blood supply. But the vital point here is that the hepatic portal artery and the hepatic artery are the main focus in tri phase liver studies.

Liver lesions

Let me discuss a little about liver lesions/pathologies before I explain the technique of tri-phase liver study. I mentioned earlier that some liver lesions are hypervascular while some are hypovascular. I also explained that hypervascular means these tumors have more hepatic arterial blood supplies and hence we expect to see them enhanced during the hepatic arterial phase. (I will explain what a hepatic arterial phase is). All liver tumors get 100% of their blood supply from the hepatic artery¹.  Hepatocellular carcinoma, focal nodular hyperplasia, adenomas, hemangiomas are examples of hypervascular liver lesions.

Hypovascular lesions like metastases, cysts, abscesses etc are not enhanced in the hepatic arterial phase but will be enhanced in the portal venous phase

NECT                                  Non enhanced CT

35 sec                                  Hypervascular lesions (arterial phase)    

70 sec                                  Hypovascular lesions (portal venous or hepatic phase)   

600sec                                 Fibrotic lesions (delayed phase)

   

TECHNIQUE

Now let’s look at the technique of dual phase liver studies. Usually, every CT scanner has a default protocol for Tri-phase liver study. Preparation of patient is same for general abdomen CT scan procedure. Some departments prefer to give oral contrast to also assess the GIT because there might be significant diagnoses made. But where the focus is entirely on the liver, there is really no need to give oral contrast. The scan phases include the scout, the plain/ non-contrast enhanced phase (both same as abdominal CT scan) and the post contrast or contrast-enhanced phase (which is where the real tri-phase study is done).

In the contrast enhanced phase, we have

·         The arterial phase (or hepatic arterial phase). This is obtained 25-35sec from the time contrast injection starts.

·         The portal venous phase (or hepatic phase) which starts 60-70 sec  

·         The delayed phase after 5-6mins.

Let me tell you how I do my own tri-phase liver study. And since I learnt from very good CT specialists and have worked with high caliber radiologists and they give it an excellent rating, I think it’s a good technique and you should use it. I will give it in steps.

Step 1: Position the patient in the scanner and get a scout, plan the plain scan to cover from 5cm above the diaphragm to the lower margins of the pubis symphysis.

Please note that a liver study must be done using an automatic injector. So before you position the patient for scanning, the patient must have had a vein cannular set (preferably the cubital vein). Set up the automatic injector very well. It is very important you do because errors in injection of contrast are very difficult to manage and could lead to rescheduling the patient for another day. 80 – 100mls of non-ionic iodine contrast is recommended.

Step 2: After you have obtained the plain scan, the next scan is the post-contrast (or contrast enhanced phase). You are going to plan both hepatic arterial phase and portal venous phase. This is preset in most scanners. If it’s not, you will need to learn how to “add scan”. So you can scan both hepatic arterial and portal venous phases simultaneously. For the arterial phase, plan the slices from above the diaphragm to the iliac crest (this will allow for enlargement of the liver /hepatomegaly). The portal venous or hepatic phase is planned from above the diaphragm to the lower margin of the pubis symphysis. But where the focus is entirely on the liver and no GIT and pelvic studies are considered, the planning is same as for the arterial phase. Don’t forget that standard slice thickness and interval is 5mm/5mm. ensure other parameters in the protocol are accurate

Step 2: Next, set up the bolus tracking protocol on the scanner. Should I explain what a bolus tracking technique is? Well it will prolong this discussion. But you must know or remember that bolus tracking is used to monitor contrast enhancement before the scan is initiated. I will probably explain bolus tracking in our next discussion. Obtain the bolus tracking slice and set the region of interest (ROI) on the abdominal aorta*plenty grammar abi?* .You can either set automatic scanning or manual scanning; in the earlier case, the scanning is automatic once the preset HU value is attained during contrast injection. (It is important you set this accurately especially if you are using automatic transmission/scanning).In the later, you initiate the scanning manually when you see maximum enhancement. I use manual when I am not sure of where I have placed my ROI. Once the arterial phase scanning is done, 30 sec later, the portal venous / hepatic phase is automatically initiated (depending on the scanner)

Step 3: allow 5-6mins and do a delayed scan. This completes the tri-phase study and at this phase, fibrotic lesions like cholangiocarninomas and fibrotic metastases are seen.

All this grammar thus far is just to say that for tri-phase liver study, you carry out 3 scans at time intervals of

i.                     25-35sec for arterial phase

ii.                   60-70sec for portal venous/ hepatic phase

iii.                  5-6mins for delayed phase.

All the time is from the onset of injection of contrast and not after injection of contrast.