MRI Protocol for Vascular Anomalies
MRI and MR angiography appear to be valuable for the assessment of vascular malformations of the extremities. The protocol for imaging such malformations should combine dynamic contrast-enhanced 3D gradient-echo MRI with STIR sequences. However, digital subtraction angiography and venography are still required for definitive treatment decisions.
MRI PROTOCOL for MR ANGIOGRAM Normal Sequences
|
TECHNIQUES
without 2d TOF. Peripheral vascular
coils have recently been introduced, which we have found to be very
helpful.When trying to determine the approach that is best, local institutional
factors come into play; is a PV coil available? Do the MR system have
stepping-table software etc. One “low-tech“ approach that we have used
successfully is to perform two or three separate gadolinium enhanced MR
angiograms during a single examination.Using this technique, a
gadoliniumenhanced 3D GRE MR angiogram is performed in the usual manner,
although a relatively low dose of gadolinium is delivered. The patient is then
moved 40-50 cm, and after scouting and prescribing another sequence, a second
gadolinium bolus is infused and another 3D GRE
MR angiogram is obtained. Together the two angiograms cover as many as 90 cm
of anatomy. A third station can also be performed in the same manner as
long as a total of 0.3 mmol/kg of gadolinium is not
exceeded. Use of the subtraction
technique is useful for these studies. Another useful method involves the use
of a stepping table. A dynamic step-table technique has been used routinely
with conventional and digital subtraction angiography to increase the amount of
anatomy imaged per contrast injection. With this method, the patient or x-ray
tube is incrementally translated so that
a bolus of contrast material is imaged as
it flows through the arterial system.
rapidly move the patient and
initiate another 3D-GRE acquisition centered on the thighs
within 5-10 seconds. With this
technique, we have been able to rapidly acquire two
contiguous contrast -enhanced
3D-GRE MR angiograms following a single gadoliniumDTPA infusion. A reference
mark is placed on the side of the gantry using a piece of tape, so that the
table can be manually repositioned back to this point when necessary. The
patient and
table can now be moved back and
forth between stations freely. To move the patient
between “stations“ the table is
manually disconnected from the mechanical drive
apparatus and both the patient and
table is withdrawn 40-45 cm. This is only possible
on some MR scanners. Check with the
manufacturer representative to see if
this is
possible on your scanner. Contrast is infused for a longer period of
time than for other MRA exams. Many methods are in use, we use a biphasic
injection of 1cc/sec for 20 sec followed by 0.5 cc/sec for 40 sec, all followed
by 20cc saline flush at 1 cc/sec. This yields a 60 sec
gadolinium “bolus“, which keeps the
arterial signal high for the duration of the stepping
table acquisition.
perform the stepping table exam
automatically. This has made it significantly easier to perform the study and
allows for more accurate table positioning. A single prolonged contrast
infusion is used as described above. One drawback of the technique is the
relatively lower resolution as compared to other MRA techniques. Since the
three (or four) stations need to be acquired before there is substantial venous
return, the acquisition time of each station must be reduced, necessitating
reduction in image matrix and therefore resolution. The use of peripheral vascular coils and newer
pulse sequences, such as the ellipitical-centric
phase ordered 3d sequence will
hopefully aid in increasing resolution
No comments:
Post a Comment