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.
