The most common CT procedures that put everyone on edge are
unarguably CT Angiography procedures. But then, Angiographies as technical as
they may seem are really not a big deal. I am not going to talk about specific
steps to angiography procedures. But however, I am going to give insights into
some of the most basic things that every radiographer has to know before
attempting to do an angiography successfully.
Angiography also called Arteriography (some texts can use
angiogram or arteriogram; they all mean the same thing) is typically a study of
the arteries. Ordinarily, the arteries are radiolucent and this necessitates
that use of contrast media in angiography. Some of the most common
angiographies requested include: cerebral angiography (circle of Willis),
Carotid angiography, pulmonary angiography, renal angiography, abdominal aortic
angiography, peripheral angiography or extremity angiography. Coronal (cardiac)
angiography is obviously the most complicated and technical angiography and the
request are usually not common.
Please bear in mind that the context here is CT
angiographies which are studies of the arteries carried out using contrast
enhanced computed tomography scans. Conventional Angiographies are mostly
obsolete these days and you may never come across such a request in the
department again.
Basics to
understanding CT Angiography
For you to successfully attempt a CT angiography, you have to
have fundamental knowledge of the following
1.
What kind of CT scanner do you have and how does
it perform angiography?
I
have had discussions with Radiographers who tell me that their scanners cannot
perform Angiographies. Well the truth is that all multi-slice detector scanners
can perform Angiography scans but how well is what is to be argued. Once the
machine is able to acquire thin slice volume images, it has the capacity of
performing an angiography. But the issue of scan time is the reason why 16
slice CT scanners have been placed as the benchmark for CT angiography. The fundamental
of a good angiography study is the ability for the scanner to match very thin
slice image acquisition with very short scan time. Ideally, thin slices of
0.625mm -1.25mm are adequate for a good angiography study and will ensure
adequate volume images obtained within a single breathe hold and at the time of
arterial circulation with no venous circulation contamination. More details
will be discussed subsequently but the key point here is that any scanner that
allows 0.625-1.25mm slice thickness and a scan time of 5-25secs can perform a
good angiography study.
Usually most scanners have already pre-set
angiography protocols that has appropriate scan time, pitch, gantry rotation,
slice thickness, exposure factors etc. Angiographies are always *compulsorily
done with automatic injectors. You must also be familiar with how to operate
the auto-injector and most important how to synchronize it with the scanning.
(I will discuss that later).
2.
What kind of Angiography is being requested?
Like I pointed out earlier, there are
different types of CT angiographies depending on the part of the body under
review. So it is important that you have a good knowledge of the anatomy of arterial
circulation. When you have a request for cerebral CT angiography, you are
investigating arteries that make up the circle of willis and same goes for
other angiography studies.
3.
Why was the angiography study requested? What is
the doctor looking for?
You have an idea of what the clinical
condition of the patient is and why the doctors are requesting for the
angiography. If you don’t, you will not be properly guided in the procedure. You
must ensure that the referring doctor(s) attached a clinical note detailing the
conditions of the patient and the pathology under review. No clinical note…no CT
angiography.
The reasons for doing an angiography study can be generally
grouped as follows:
·
To find an aneurysm (a blood vessel that has become
enlarged and may be in danger of rupturing)
4.
What are the preparations given to patients
before the scan procedure?
In as much as CT angiography is presumed
safe for the patient, you must carefully prepare the patient before the scan. The
major concern is usually with the iodine contrast given to the patient. A small
percentage of patients react to iodine contrast and as such all patients must
be given the following instructions:
i.
Do not eat atleast 4-5 hours before the scan. The
reason for this is very simple. Patients can react to the iodine contrasts in
form of vomiting (among others). A patient who has eaten within 4-5 hours have
the tendency of vomiting food in the stomach and is likely to aspirate the food
particles into the airways. That will cause serious injury and in extreme cases
can lead to death.
ii.
Do a kidney function test for patients suspected
of impaired kidney functions. Usually a creatinine test is done and a normal
result is within the range of 0.7 to 1.3 mg/dL for men and 0.6 to 1.1 mg/dL for
women. Some departments make it a routine to do a creatinine for all
angiography patients; their reasons being precautionary.
iii.
For diabetic patients, metformin drug intake
should be withheld for 48hours after contrast administration. The reason is
that, metformin is excreted by the kidney and its accumulation is very toxic. The
kidney also has to deal with excreting the iodine contrast agent and so it’s
better to let the kidney excrete all the iodine contrast and we are sure that there
is no kidney failure before metformin is restarted. If you were told before to
withhold metformin intake 48 hours before and after contrast
administration, please disregard that information.
iv.
Patients are allowed to take clear fluid and
drugs not requiring food.
v.
It is important to take patients vitals before
and after the procedure. Iodine contrast reactions can include rise in
temperature, increased heart rate etc. Taking vitals will ensure adequate
patient monitoring.
5.
Can you perform post processing/ reconstructions?
Scanning a patient is important; but image
reformation is also as important because that where the images that make all
the differences are displayed. You must learn how to do 3D volume rendering. This
is usually best done in a work station; and trust me, that is where the real
work is done. Your expertise with work stations makes a lot of difference.
Spend time and learn it very well. Unfortunately, work station operations
cannot just be discussed on prints. They are real time practical and get better
with efforts in practice and learning.
 |
| this is what a post-processed 3D volume rendered cerebral CT angiography image looks like |
 MIP images for renal CT angiogram  |
| 3D volume rendered images for renal CT angiogram |
WE WILL REVIEW PRACTICAL APPROACHES TO CT
ANGIOGRAPHY STUDIES IN SUBSEQUENT DISCUSSIONS.
