Magnetic resonance angiography in the evaluation of renal arteries: imaging findings

Abstracts

OBJECTIVE: To describe indications, main findings and diagnosis of magnetic resonance angiographies of renal arteries. MATERIALS AND METHODS: A retrospective study including 56 imaging studies covering a total of 111 renal arteries, performed during the period between December 6, 2001 and March 11, 2004. The angiographies were performed in a 1.5 T scanner, in compliance with the Department protocol. RESULTS: As regards sex, it was found that 55.4% (n = 31) patients were male and 44.6% (n = 25) were female. The youngest patient was 12 years old and the oldest 88 years old. From a total of 25 different clinical indications, systemic arterial hypertension was the principal one with 26.7% (n = 15), followed by abdominal and/or lumbar pain with 12.5% (n = 7), abdominal aortic aneurysm with 10.7% (n = 6), renal artery stenosis with 8.9% (n = 5), and others. Among these 56 studies, 43 (76.7%) had different types of findings and 13 (23.2%) were normal. The majority of findings were related to vascular diameter and amongst them, parietal irregularities, aneurysms and stenosis were the most frequent. Parietal irregularity was the most frequent alteration in the right renal artery, with 17.87% (n = 10), and stenosis was the most frequent alteration in the left renal artery, with 25.45% (n = 14). CONCLUSION: Magnetic resonance angiography has shown to be an excellent non-invasive method for evaluation of renal arteries, because of its sensitivity and multiplanar capacity for demonstrating vascular structures.

Magnetic resonance imaging; Magnetic resonance angiography; Renal arteries

OBJETIVO: Descrever as indicações, os principais diagnósticos e os achados de imagem nas angiografias por ressonância magnética das artérias renais. MATERIAIS E MÉTODOS: Estudo retrospectivo, no período de 6/12/2001 a 11/3/2004, num total de 56 exames, totalizando 111 artérias renais estudadas. Os exames foram realizados em um equipamento de 1,5 tesla, segundo o protocolo do Serviço. RESULTADOS: Foi demonstrado que 55,4% (n = 31) pacientes eram masculinos e 44,6% (n = 25), femininos. O paciente mais novo tinha 12 anos e o mais velho, 88 anos. De um total de 25 diferentes indicações, a hipertensão arterial sistêmica com 26,7% (n = 15) foi a principal, seguida de dor abdominal e/ou lombar com 12,5% (n = 7), aneurisma da aorta abdominal com 10,7% (n = 6), estenose da artéria renal com 8,9% (n = 5), e outros. No que se refere às imagens, 43 (76,7%) exames tiveram algum tipo de alteração e 13 (23,2%) foram normais. Dentre os que tinham alterações, a maioria se deu no calibre, e dentre elas, as irregularidades parietais, aneurismas e estenoses foram as mais comuns. Na artéria renal direita as alterações mais comuns foram as irregularidades parietais com 17,87% (n = 10), e na artéria renal esquerda foram as estenoses com 25,45% (n = 14). CONCLUSÃO: A angio-RM mostrou-se excelente método no estudo das artérias renais, devido à sua sensibilidade e capacidade multiplanar para avaliar as estruturas vasculares.

Ressonância magnética; Angiografia por ressonância magnética; Artérias renais

ORIGINAL ARTICLE

Magnetic resonance angiography in the evaluation of renal arteries: imaging findings^*

Marcelo Souto Nacif^I; Alair Augusto Sarmet Moreira Damas dos Santos^II; Edson Marchiori^III

^IProfessor of Radiology at Faculdade de Medicina de Teresópolis (CCBM-FESO) and for Course of Specialization in Radiology at Instituto de Pós-Graduação Médica Carlos Chagas (VOT-Imagem), Master Degree in Radiology at Universidade Federal do Rio de Janeiro

^IITitular Professor for Course of Specialization in Radiology at Instituto de Pós-Graduação Médica Carlos Chagas (VOT-Imagem), Adjunct Professor and Chief for Service of Radiology at Hospital Universitário Antônio Pedro, Universidade Federal Fluminense

^IIITitular Professor and Chief for Department of Radiology at Universidade Federal Fluminense, Adjunct Coordinator for Course of Post-Graduation in Radiology at Universidade Federal do Rio de Janeiro

^{Mailing Address}

ABSTRACT

OBJECTIVE: To describe indications, main findings and diagnosis of magnetic resonance angiographies of renal arteries.

MATERIALS AND METHODS: A retrospective study including 56 imaging studies covering a total of 111 renal arteries, performed during the period between December 6, 2001 and March 11, 2004. The angiographies were performed in a 1.5 T scanner, in compliance with the Department protocol.

RESULTS: As regards sex, it was found that 55.4% (n = 31) patients were male and 44.6% (n = 25) were female. The youngest patient was 12 years old and the oldest 88 years old. From a total of 25 different clinical indications, systemic arterial hypertension was the principal one with 26.7% (n = 15), followed by abdominal and/or lumbar pain with 12.5% (n = 7), abdominal aortic aneurysm with 10.7% (n = 6), renal artery stenosis with 8.9% (n = 5), and others. Among these 56 studies, 43 (76.7%) had different types of findings and 13 (23.2%) were normal. The majority of findings were related to vascular diameter and amongst them, parietal irregularities, aneurysms and stenosis were the most frequent. Parietal irregularity was the most frequent alteration in the right renal artery with 17.87% (n = 10) and stenosis, in the left renal artery, with 25.45% (n = 14).

CONCLUSION: Magnetic resonance angiography has shown to be an excellent non-invasive method for evaluation of renal arteries, because of its sensitivity and multiplanar capacity for demonstrating vascular structures.

Keywords: Magnetic resonance imaging; Magnetic resonance angiography; Renal arteries.

INTRODUCTION

The magnetic resonance (MR) angiography is a safe and sensitive method for evaluation of renal arteries, with the advantage of not requiring the use of potentially allergenic iodinated contrast agents, ionizing radiation, besides the absence of inherent risks of catheterization. Considering that, in the greatest majority of diseases affecting the renal vascularization, the lesions are localized in the main branches of the renal artery, the MR angiography is a valuable imaging method. Additionally, with the recent gains with the modern 3D technology with the use of venous contrast medium, MR angiography has become an excellent method, even in clinical centers with highly skilled professionals in the field of ultrasound (US) and computed tomography (CT)^(1–4).

Stenosis is the most significant lesion affecting renal arteries and causes parenchyma ischemia that is a difficult-to-control increase in the systemic arterial pressure. As time progresses, stenosis evolves to occlusion and permanent reduction in the renal function. The relevance of hypertension as a public health problem and the devastating consequences for the patient in cases of renal failure have directed several studies towards a safe, fast, non-invasive and less-expensive method than catheter-angiography for an early diagnosis of this disease^(2,5–8).

The present study objective was to describe MR angiography indications, main diagnoses and imaging findings in renal arteries.

MATERIALS AND METHODS

An observational, descriptive and retrospective study was developed at Service of Radiology of Hospital das Clínicas de Niterói, in the city of Niterói, RJ, Brazil. All the MR angiographies of renal arteries performed in the period between December 6, 2001 and March 11, 2004 were analyzed, in a total of 56 studies constituting our sample.

Images were obtained in compliance with the protocol to be further described, in a Siemens Magnetom Symphony 1.5 T model, with body coil, aiming at obtaining the following sequences:

1 – Localizing, breath-hold gradient echo, T1-weighted multiplanar sequence.

2 – Breath-hold gradient echo, T1-weighted 2D sequence for artery topography.

3 –Coronal gradient echo T1-weighted 3D sequence. This is a subtraction mask for MR angiography.

4 – Bolus test, axial or coronal gradient echo T1-weighted 2D low-resolution sequence, demonstrating one image per second.

5 – Gadolinium-enhanced MR angiography, coronal gradient echo T1-weighted 3D (TR: 4.5–5.3 ms; TE: 1.9–2.1 ms; FOV: 260–300; matrix: 256–512).

6 – Axial, fat-suppressed, gradient echo T1-weighted 3D.

7 – Arterial reconstruction (MIP and MPR). Additional (as necessary):

8 – Axial, fat-suppressed, breath-hold turbo spin echo T2-weighted sequence, also for anatomical relation.

9 – Left and right, sagittal, breath-hold T2-weighted (HASTE) for anatomical relations.

10 – Delayed sequence for venous and pyelographic study.

11 – Reconstructions: venous and terminal collecting ducts.

Aiming at achieving the best quality possible, before the examination, the patient was given an explanation on the procedure, his/her understanding and cooperation being of paramount importance.

Images were acquired with the patient in dorsal decubitus, asked to breath-hold, when necessary, and to keep a static position, avoiding even the minimal movement during the images acquisition.

The data obtained were reconstructed into 2D multiplanar or 3D images by means of the maximum intensity projection (MIP) or multiplanar reconstruction (MPR) techniques.

MR angiography examination duration was about the same of abdominal MRI, approximately ten minutes.

The parameters adopted for the studies analysis were: diameter (normal, parietal irregularities, stenosis and aneurysms), trajectory (normal or tortuous) and flow (normal, reduced or absent). Aorta, right and left renal arteries were evaluated.

For analysis of stenosis rates, the following parameters were adopted: (A) < 50%, (B) > 50% and < 75%, (C) > 75% and < 100%, and (D) complete (100%). Also, stenoses were classified according to their location in the artery: proximal third, middle third and distal third.

Kidneys evaluation was based on contour (regular or irregular), volume (normal, increased or reduced) and nephrogram (homogeneous, absent or heterogeneous).

All the studies were reviewed by at least two radiologists of the Service.

Paramagnetic contrast media was utilized in all the examinations included in this study.

The present study research project was approved by the Committee for Ethics in Research of Hospital Universitário Antônio Pedro, Universidade Federal Fluminense Faculty of Medicine under the number 004/05.

RESULTS

Epidemiological considerations

From a total of 56 patients included in this study, 55.4% (n = 31) were male and 44.6% (n = 25), female. The age range between 40 and 49 years has prevailed with 12 patients, followed by the age range between 70 and 79 years. The youngest patients was 12 years old and the oldest, 88 years old (mean age 55.32 years).

From an array of 25 different clinical indications for the 56 MRI-angio studies, systemic arterial hypertension was the principal one with 26.7% (n = 15), followed by abdominal and/or lumbar pain with 12.5% (n = 7), abdominal aortic aneurysm with 10.7% (n = 6), and renal artery stenosis with 8.9% (n = 5).

Imaging considerations

Amongst the 56 studies, 43 (76.7%) presented different types of findings and 13 (23.2%) were normal.

AORTA

As regards diameter, 25 aortas (44.66%) were found with parietal irregularities, 20 (35.71%) were normal, ten (17.85%) aneurysms — 3.57% (n = 2) involving renal arteries and 14.26% (n = 8) not (Figure 1) —, and a case of stenosis (Figure 2).

The evaluation of the aorta trajectory demonstrated the existence of 67.85% (n = 38) normal courses and 32.15% (n = 18) tortuous.

The flow inside aorta was present in 100% (n = 56) of images.

RIGHT RENAL ARTERY

Reviewing the diameter of the 56 right renal arteries evaluated, we have found 67.85% (n = 38) with normal diameter, 17.87% (n = 10) with parietal irregularities (Figure 3), 14.28% (n = 8) with stenosis. No aneurysm was identified.

As regards the right renal artery trajectory, we could analyze 91.07% (n = 51) of cases, all of them normal. In 8.93% (n = 5) of cases, we could not do it due the presence of a complete stenosis.

The flow was normal in 85.71% (n = 48) of cases, reduced in 5.35% (n = 3), and absent in 8.94% (n = 5).

LEFT RENAL ARTERY

Fifty-five left renal arteries were reviewed, since one patient presented left renal agenesis. As regards diameter, 56.36% (n = 31) were normal, 25.45% (n = 14) had stenosis, 18.19% (n = 10) presented parietal irregularities. No aneurysm was identified.

As regards trajectory, 83.65% of left renal arteries (n = 46) were normal, 3.63% (n = 2) were tortuous and 12.72% (n = 7) could not be analyzed due the presence of complete stenosis obstructing the contrast mean passage, the signal of the flow not being detectable at MR angiography. In 74.54% (n = 41) of cases the flow was normal, in 12.73% (n = 7) reduced, and in 12.73% (n = 7) absent.

RENAL ARTERIES STENOSIS

From 111 arteries evaluated, 22 presented stenosis, 25.45% (n = 14) at left and 14.28% (n = 8) at right. As regards topography, 59.09% (n = 13) were located in renal arteries proximal third, 31.81% (n = 7) in the middle third and 9.10% (n = 2) in the distal third.

As regards stenosis rate, in 50% (n = 11) of cases it was of 100%, in 27.27% (n = 6) > 50% and < 75%, in 18.18% (n = 4) > 75% and < 100%; in only 4.55% (n = 1) stenosis was > 50%. With the association of stenosis rate and topography, the most frequent finding was proximal third complete stenosis (D type) with 31.81% (n = 7), followed by middle third complete stenosis with 18.18% (n = 4).

RIGHT KIDNEY

From a total of 56 kidneys evaluated, 82.14% (n = 46) have shown regular contours and 14.86% (n = 10), irregular contours (Figure 4). As regards renal volume, 82.14% (n = 46) were normal, 12.51% (n = 7) were reduced and 5,35% (n = 3) were increased. Nephrograms showed 87.50% (n = 49) homogeneous, 10.72% (n = 6) heterogeneous and 1.78% (n = 1) absent.

LEFT KIDNEY

From the total of 55 kidneys evaluated, 81.81% (n = 45) have showed regular contours and 18.19% (n = 10) irregular contours. As regards renal volume, 87.27% (n = 48) were normal, 12.72% (n = 7) were increased and none was reduced. Nephrograms showed 85.45% (n = 47) homogeneous, 12.74% (n = 7) heterogeneous (Figure 5) and 1.81% (n = 1) absent.

OTHER FINDINGS

The review of the 56 studies has also evidenced 14.28% (n = 8) cases of accessory renal arteries, 7.15% (n = 4) extending towards the lower pole of the left kidney, 5.35% (n = 3) towards the lower pole of the right kidney (Figure 6) and 1.78% (n = 1) towards the upper pole of the right kidney.

As a result of other findings analysis, the following abnormalities were observed: thirteen single renal cysts — seven at right and six at left —, and a case each of mass on the right kidney upper pole, aortic dissection with low flow in the right renal artery (Figure 7), metal prosthesis in the right renal artery (Figure 8), right pelvic transplanted kidney (Figure 9), aorto-bi-iliac graft, flow artifact in the right renal artery middle third (Figure 10), solitary kidney at right (Figure 11) and right renal artery bifurcation at middle third (Figure 12).

DISCUSSION

Clinical and epidemiological considerations

From the total of 56 patients included in this study, 55.40% (n = 31) were male and 44.60% (n = 25), female. Systemic arterial hypertension was the main clinical indication for MR angiography and therefore the little percentage difference related to the predominant sex is due the fact that systemic arterial hypertension affects both men and women without any significant predilection^(9–12).

As regards the age range, there was a prevalence of the fifth decade, with 21.42% (n = 12) of the patients, and the eighth decade with 17.86% (n = 10). However, a homogeneous distribution was observed among the fourth, sixth, seventh and ninth decades.

Considering that more than half of patients affected by systemic arterial hypertension presented alterations related to parietal irregularities, and that such irregularities are precursors of arterial stenosis, one may observe that isolatedly it is a good clinical indication for MR angiography since it has accounted for the greatest number of altered results among the examinations included in this study.

Imaging considerations

From the 56 cases included in this study, 13 (23.2%) were considered normal and 43 (76.7%), altered. Amongst the altered studies, findings were the following: 22 renal artery stenosis (20.1%), 20 renal arteries parietal irregularities (18.3%), 11 reduced flows (10%) and five absent flows (4.5%). For all of these diagnoses the technique employed was gadolinium-enhanced GE 3D, compatible with the studies of Bongers et al.⁽⁷⁾, Fain et al.⁽⁸⁾, Mittal et al.⁽⁹⁾ and Shetty et al.⁽¹⁰⁾. The advantage of this technique is the possibility of acquiring high-resolution images of the main branches included in the renal arterial system, besides the feasibility of correlation with transverse images. On the other hand, distal branches may be affected by saturation due their smaller diameter and reduced flow, mimetizing a certain level of stenosis⁽¹¹⁾.

AORTA

The evaluation of aorta should be the first item to be analyzed, since the greatest part of indications occurred due arterial hypertension that may be secondary to a diffuse atherosclerotic disease, causing stenosis in the renal artery origin. In our study, we have found 44.66% (n = 25) of aortas presenting parietal irregularities secondary to atherosclerotic disease.

The adequate visualization of aorta in the 56 cases studied is in compliance with several studies^(10–14). The acquisition of images during breath-hold periods has allowed a significant minimization of respiratory artifacts compatible with findings observed in the studies by De Cobelli et al.⁽¹⁵⁾, Shetty et al.⁽¹⁰⁾, Bongers et al.⁽⁷⁾ and Fain et al.⁽⁸⁾.

RENAL ARTERIES AND STENOSES

The evaluation of the renal arteries trajectory, including their origin, was considered satisfactory in all of the cases, similarly to the studies by Holland et al.⁽¹⁶⁾, Postma et al.⁽¹⁷⁾ and Rieumont et al.⁽¹²⁾.

We have evaluated 111 renal arteries and found 19.81% (n = 22) of stenosis, 59.09% (n = 13) located in the proximal third and 31.81% (n = 7), in the middle third. This can be explained by the age ranges included in this study and the association with atherosclerotic disease. On the other hand, there were 9.1% (n = 2) of stenosis in the distal third related to the presence of fibromuscular dysplasia.

Because of the overrating of stenosis in the MR angiography, many authors ^{(7,9,13,17–19)} recommend the evaluation of functional and hemodynamic and functional alterations, instead of evaluating only the arterial stenosis level. This is important, since the most relevant factor in stenosis is the presence or absence of significant hemodynamic alterations, defining whether some renal revascularization process will be necessary or not^(9,17,18).

Notwithstanding, hemodynamically significant stenoses may be identified through the absence of signal on the stenosis site. Also, the functional consequences, including kidneys asymmetries, parenchymal impregnation difference, contrast media excretion and loss of cortical/medullary ratio, may be evaluated. For this reason, in our study, not only the renal arteries were evaluated, but also aorta and kidneys, aiming at trying to demonstrate these hemodynamic alterations.

KIDNEYS

Although the examination is specific for renal arteries, it is important to evaluate the renal function, a data that can be identified through indirect signs related to parenchymal enhancement and contrast media excretion^(13,19,20).

Some authors^{(10,12,13,19)} support the concept that the renal artery study is actually a dynamic study of parenchymal perfusion so the study has an arterial phase (arteries study), a nephographic phase (parenchyma study), a venous phase for the correct study of the venous vascularization and, finally, an excretory phase where the adequate contrast media elimination may be evaluated.

In our study, we have observed a good correlation between renal perfusion findings like heterogeneous or even absent nephrograms, and the presence of stenosis. In the right kidney, we have observed 10.72% (n = 6) of heterogeneous nephrograms and, in the left, 12.74% (n = 7). Correlating nephrograms with right renal arteries stenoses [14.2% (n = 8)], and left renal arteries stenoses [25.45% (n = 14)], we have observed that the higher the stenosis level is, independently from topography, the higher the probability of finding a perfusional alteration that could identify hemodynamically significant alterations.

FINAL CONSIDERATIONS

We may observe that MR angiography has become a dynamic method with differentiated diagnostic capacity when compared to other methods of examination. In our study, we have identified accessory renal arteries, a significant fact in the evaluation of renal vascularization, especially in cases of pre-transplantation evaluation. MR angiography also allows the identification of an array of other findings, which demonstrates the diagnostic capacity of this method.

The knowledge on artifacts and method limitations is of paramount importance for the study evaluation, since these factors may represent diagnostic pitfalls.

The digital subtraction angiography is considered as the best method for studying the renal vascular anatomy. However, innumerable studies are indicating an increase in MR angiography sensitivity and specificity, principally after the advent of new and faster techniques with higher resolutions, so we may say that this is a comprehensive method, considering that it is non-invasive, does not utilize ionizing radiations, does not utilize nephrotoxic contrast media and presents a non-significant complications rate when compared to digital angiography^(7–11,14).

REFERENCES

Received September 12, 2005.

Accepted after revision November 23, 2005.

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*

Study developed at Hospital de Clínicas de Niterói, Niterói, RJ, and Department of Radiology, Universidade Federal do Rio de Janeiro Faculty of Medicine/Hospital Universitário Clementino Fraga Filho, Rio de Janeiro, RJ, Brazil.

Publication Dates

• Publication in this collection
  26 Sept 2006
• Date of issue
  Aug 2006

History

• Accepted
  23 Nov 2005
• Received
  12 Sept 2005