From 2014 to 2017 in our department 150 patients underwent renal arteries MR evaluation with a 3T scanner using no contrast MRA sequences,
as a second line exam after suspicious findings at Duplex-US.
The standard protocol we use (fig.4) include an axial steady state free procession sequence,
beneficial to set the further sequences,
and coronal single shot fast spin echo sequence in order to visualize kidneys and to detect eventual collateral findings.
Then,
3D-IFIR and 3D-PC sequences are acquired both using respiratory gating.
IFIR sequence (fig.5) is based on a 3D SSFP pulse sequence data combined with chemical fat suppression,
acquired with ASSET parallel imaging,
useful in reducing acquisition time and banding artifacts.
First,
the volume of interest (VOI) is saturated with one or more inversion recovery pulses,
depending on the alignment of the evaluated vessels.
Normally,
an axial plane with an automatic IR band covering the entire VOI is acquired; another band is placed below the VOI in order to suppress venous flow.
3D IFIR advantages are:
- Intrinsic elevated SNR enhanced by the use of a 3T scanner,
with notable increase in spatial resolution and lengthening of T1 causing additional signal reduction of the stationary tissues.
- The complete suppression of background tissue and venous flow.
- A clear morphological evaluation of the renal arteries,
due to the bright blood signal in the final angiogram,
even with the identification of segmental arteries.
- The possibility of 3D reconstruction.
In peripheral arteries with reduced flow,
the inflow of tagged blood may reach the T1 of the surrounding tissues,
thus reducing the tagging effect and the contrast resolution,
that can be partially overcome by using multiple and thinner slabs,
though resulting in longer acquisition time [4,5]
3D phase contrast sequence (fig.6) is able to give semi-quantitative functional information about stenoses distinguishing between haemo-dynamically non-significant and significant stenoses.
PC generates an image by applying a bipolar velocity-encoding gradient during the pulse sequence twice,
in opposing directions,
which results in a null net phase change in stationary tissues while applying a phase change in flowing protons,
resulting in MR signal.
In case of a significant vessel narrowing,
the accelerated flow passing through the stenosis cause turbulence,
thus resulting in the final MR image as a focal black jet associated with a reduction/loss of MR signal,
due to dephasing phenomenon.
Mild RASs create only a blooming effect to the bright luminal signal.
Main limitation of 3D PC sequence is the small volume due to the long time of acquisition and the relative insensitivity to slow flow.
Advantages of these two sequences are the visualization of intra-parenchymal arteries with no enhancement of renal parenchyma better than CEMRA and the possibility to repeat them if the initial images are suboptimal.
[6,7]
Thanks to the high morphological potential of IFIR,
the entire renal artery may be analyzed,
evaluating proximal (post-ostial,
fig.7),
mid-level (fig.8),
distal (post-dicotomic,
fig.9) or sub-segmental arterial tracts (fig.10).
Based on these two sequences,
RASs were graded as:
- absence of stenosis (fig.11): no alteration on both MR sequences.
- non significant/mild (fig.12): reduction of arterial caliber on IFIR images with blooming on PC sequence,
stenosis than can be managed conservatively.
- significant (fig.13-14): reduction of arterial caliber at IFIR images with signal loss on PC (black jet),
which may need catheter-based treatment (stent/dilatation).
- critical/nearly occlusion (fig.15): PC overstimates the stenosis,
with complete signal loss of the artery.
IFIR is the only sequence that may reveal the stenosis and the patency of the distal vessel.
- complete ostial obstruction the renal artery (fig.16).