MR ANGIOGRAM PROTOCOLS

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. 

T1 weighted spin echo (axial - coronal - sagittal) T2 weighted fat-sat spin echo or IR (axial - coronal - sagittal) Gradient-echo axial (TOF) w/o saturation slab Pre- & post-contrast MR Angiography (multiple phases) (gradient echo sequence) Post-contrast fat-sat T1 weighted spin echo (axial - coronal - sagittal) MRI PROTOCOL for MR ANGIOGRAM Normal Sequences Localizer sequence  Transverse unenhanced and contrast-enhanced T1-weighted  Fast low-angle shot (FLASH)  Gradient-echo fat-saturated two-dimensional (2D) acquisitions Transverse T2-weighted 2D fast spin-echo acquisition  2D transverse and coronal short tau inversion recovery (STIR) acquisition coronal contrast-enhanced time-resolved high-spatial-resolution 3D MR angiography.

TECHNIQUES

 Numerous techniques are currently available to perform peripheral MRA. These include all 2d TOF, multiinjection 3d Gad with or without 2d TOF, and stepping table 3d Gad MRA with or

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.

 MANUAL STEP-TABLE METHOD

 A similar approach may be adapted for gadolinium-enhanced 3D GRE MR angiography. We devised a method to rapidly move the patient between 3D-GRE acquisitions without having to scout or to execute time-consuming pre-scan adjustments. After acquiring a  Gadoliniumenhanced 3D GRE MR angiogram of the abdominal aorta, this method allowed us to

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.

 AUTOMATED STEP-TABLE METHOD

 Most MR system vendors have recently introduced new software programs which

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