IMAGING TECHNIQUES
Different imaging techniques currently available in patients with suspected renal artery stenosis have a highly diagnostic efficiency.
The main factors in the choice of technique is determined by the degree of impairment of renal function,
available technology,
the operators experience and the ability to work in a coordinated way with the clinician / nephrologist.
1.
US-Doppler Color:
Duplex Color Doppler Ultrasound technique is commonly used as first line,
and should be assessed:
✦ Grey scale: the absence of alterations in mode B does not exclude the presence of significant renal artery stenosis.
- Diminished renal size (less than 1.5 - 2cm with respect to contralateral)
- Parenchymal renal thinning (less than 1cm)
- Increased parenchymal renal echogenicity.
✦ Doppler: the findings that suggest stenosis greater than 60% are:
Direct signs: result the hemodynamic changes in the site of the stenosis.
Native kidneys:
- Peak systolic velocity higher than 200cm/sec at the stenosis with a turbulent post-stenotic flow (sensitivity 71-98% and 62-98% specificity) ( Fig. 3).
- Value > 2.7-3 inorder to the peak systolic velocities renal/aortic rate.
Transplant kidneys:
- Peak systolic velocity higher than 250 cm/sec at the stenosis with a turbulent post-stenotic flow.
- Value > 2.7 -3 inorder to the peak systolic velocities renal/external iliac artery rate.
Indirect signs: result the hemodynamic changes in the post stenosis renal vessels.
Study of the intraparenchymal arteries in the upper,
middle and lower third of the kidney to detect possible renal artery stenosis in a double or accessory artery (Fig. 4). Should be assessed:
- The presence of initial systolic peak.
The loss of the systolic peak has high sensitivity but low specificity.
- Acceleration time and presence of parvus tardus waveform.
An increase in the acceleration time (≥ 0.07) determines the appearance of a tardus parvus waveform (Fig. 4).
These findings should make suspect the existence of a significant proximal stenosis,
but their absence does not exclude their existence (13).
Information must be integrated: mode B,
direct signs,
indirect,
and clinical and laboratory data for diagnosis.
In case of diagnostic uncertainty,
must resort to other imaging techniques to clarify the diagnosis.
2. MDCT angiography:
Advantages:
- High diagnostic efficiency (sensitivity and specificity in the diagnosis of renal artery stenosis is 88-100% and 77-98%,
respectively).
- Provides vascular map for interventional treatment,
if required.
- Detailed information on the vascular walls and perivascular structures.
This information can be critical in defining the specific therapy,
mainly in the transplanted kidney (Fig. 5).
Disadvantages:
- Irradiation
- Risk of increased contrast nephropathy in patients with a glomerular filtration rate (GFR) <60 ml/min/1,73 m2.
This risk can be reduced greatly by a nephrology pattern protection.
- Patients allergic to iodine contrast (perform antiallergic pattern).
- Diagnostic efficiency diminished when there are dense parietal calcifications.
Acquisition technique:
- Injection pump,
infusion 4 ml/sec.
- Bolus "tracking".
ROI cursor on the aorta lumen,
at the level of the output of the renal arteries.
- Thin-sections acquisition (not greater than 15mm) and optimized parameters for angiographic studies.
Postprocessing technique:
- MPR: useful for quantitative analysis of the stenotic region,
allowing an assessment of the vascular lumen (Fig. 6).
- VR: overview and assessment of the vessel studied on 3D.
However it is not possible the precise stenosis measurement with this method because the appearance of the lumen varies depending on the technical parameters of the image (Fig. 7). The presence of calcification may hamper analysis (Fig. 6).
- MIP: overview of the vessel,
although the parietal calcification (Fig. 9) and the presence of stent also limit the evaluation.
- CPR: allow see the distribution of stenosis even in the presence of wall plaques.
The erroneous selection of the sample site can generate false information regarding the size of the stenosis.
3.
MR Angiography
The renal arterial system can be evaluated by angiography MRI with and without contrast administration.
3.a.
Contrast MR angiography: was until recently the technique of choice in patients with significant alteration of renal function and diagnostic uncertainty about the presence of renal artery stenosis.
The description of nephrogenic systemic fibrosis associated with gadolinium administration in patients with significant alteration of renal function or undergoing dialysis,
has changed the diagnostic algorithm.
Current guidelines:
- Do not use gadolinium in patients on dialysis or glomerular FG ≤ 30 mL per minute per1.73 m2,
except that the benefits are clearly greater than the potential risks.
✦The risk is not established in patients with a GF 30-60 mL/min.
The decision to administer gadolinium in this population requires consideration of risk factors,
availability of other alternative diagnoses and potential benefits.
Advantages:
- High diagnostic performance (similar to CT angiography).
- No irradiation.
- The parietal calcifications limited unless the evaluation compared to CT angiography.
Limitations:
- Patients with altered GF
- Respect to CT presents:
- Major acquisition time and generates more motion artifacts.
- Worst temporal and spatial resolution,
less definition in vessels of smaller caliber.
- Insufficient information on the wall and adjacent structures.
- In case of technical problems or poor patient cooperation repeat is not recommended sequence for double dose of contrast.
Technique:
- Sequences that allow rapid volumetric acquisitions in short periods of time to be performed in 4-20 seconds of apnea,
depending of patient cooperation,
degree of temporal resolution and the degree of resolution spatial or anatomical definition we need.
- First,
sequence is performed without contrast (mask),
that will assist in the subtraction of subsequent sequences after bolus contrast.
- Injection pump,
infusion 4 ml / sec.
Dose of 0.1-0.2 mmol/kg.
- The sequences used are generally 3D gradient.
TR and TE as short as possible to obtain good saturation of tissues and seek more isometric voxel or cubic possible.
- Are using new 3D gradient echo sequences call Tricks.
This way we can get better image quality in the same time.
Postprocessing: can be used both MIP and VR images,
combined with the axial slices.
Keep in mind that this technique tends to overestimate the degree of stenosis by low spatial resolution compared to multidetector CT.
3.b.
Unenhanced MRI:
Advantages:
- Absence of administration of contrast agents,
which obviously prevents potential adverse effects.
- Good diagnostic performance in transplanted and own kidneys.
- May be repeated several times the sequence if exist respiratory artifacts.
In the past,
the most popular techniques of MR angiography without contrast were "time-of-flight" (TOF) and "phase-contrast imaging".
Sequences have long acquisition time and are susceptible to multiple artifacts.
In practice,
are not commonly used for evaluation of abdominal vascular.
Recently developed new techniques without contrast MR angiography.
Of these techniques,
"steady-state free precession" (SSFP) and "arterial spin labeling" (ASL) are most useful in the evaluation of renal arterial system.
Currently being developed new sequence in 4D phase contrast allow us to assess the direction of flow and their quantification (volume,
average speed,
peak and wall tension) in the different cardiac cycle phases.
- SSFP sequence (also known as Balanced FFE,
TrueFISP,
FIESTA,
and TrueSSFP) is a 3D gradient echo sequence associated with an inversion pulse with cardiac and respiratory synchronization.
Advantages: high signal / noise ratio and short acquisition time.
Limitations: high susceptibility /inhomogeneity artifact of magnetic field,
which in turn determines heterogeneity of tissue saturation and artifacts on the flow.
- ASL technique with SSFP (Time- slip,
NATIVE TrueFISP or IFIR) is summarized in Fig. 9 .
Used respiratory and cardiac synchronization,
a band of venous saturation below renal arteries and a pulse of BBTI to eliminate the signal of stationary tissue.
Advantages: high signal/noise with sufficient suppression of background images.
Limitations: higher acquisition time respect to the SSFP technique and potential overestimation of stenosis.
Currently there are new sequences of TIME SLIP IN 4D,
which get multiple sets of images on T-SLIP varying the value BBTI and subsequently processed into FILM.
Postprocessing:
Just as in MR angiography contrast,
the MIP and VR,
in combination with axial and coronal are useful for diagnostic (Fig. 10,
Fig. 11,
Fig. 12).
4. Captopril Renogram
The Captopril renogram is a nuclear medicine technique that provides functional information that can determine whether the cause of hypertension is renovascular on its origin.
The technique involves performing a renogram in basal conditions and other after administration of captopril.
The results are interpreted as probability "low",
"moderate" or "high" of renovascular HTA.
The Captopril renogram is indicated for patients who meet clinical criteria for high probability of renovascular HTA.
In these cases the specificity / sensitivity of the technique is 90% (28).
Limitations:
- Poor performance in patients with moderate / severe renal altered function.
- Long duration of the scan.
- Absence of anatomical information.
Currently,
this technique is used in limited circumstances.
DIAGNOSTIC ALGORITHM
In Fig. 13 and Fig. 14 show two different algorithms taking into account the degree of alteration of the FG.
Currently,
in patients on dialysis or with a GFR <30 mL / min and "non-diagnostic" Doppler we preferably take the risk of iodinated contrast nephropathy than the risk of a serious complication as nephrogenic systemic fibrosis.
In our experiences and those of other research groups,
the new techniques of non-contrast MR Angiography show promising results.
However,
given the recent development of these techniques,
we need more publications to confirm its results.
If so,
Non-contrast MR angiography would be a fundamental tool in the evaluation of patients with suspected renovascular hypertension,
especially in patients with significantly renal function alteration.