Blood oxygen level-dependent measurement of acute intra-renal ischemia

Technology insight: Clinical role of magnetic resonance angiography in the diagnosis and management of renal artery stenosis

Noninvasive modalities, including duplex ultrasonography, renal scintigraphy, CT angiography and magnetic resonance angiography (MRA), may usefully contribute to diagnosis and treatment planning in patients with suspected renal artery stenosis. Important technical developments have increased the accuracy and feasibility of MRA for the detection of renal artery stenosis. A number of different MRA techniques can be applied to the study of renal arteries, but contrast-enhanced MRA represents the most valuable approach; several studies corroborate the high diagnostic accuracy of this technique, especially for the detection of atherosclerotic renal artery stenosis. A combined MRA protocol, which might include angiographic information provided by contrast-enhanced technique in addition to renal flow information derived from phase-contrast imaging, could help in classifying patients appropriately. Limitations of renal MRA include low accuracy in the evaluation of renal fibromuscular dysplasia and in the assessment of patients who undergo stenting of the renal arteries. This review describes the MRA techniques applied to the study of renal artery stenosis, including the technical features of current approaches and forthcoming developments. An overview of the clinical role of MRA, in conjunction with the other diagnostic modalities, in the identification and management of patients with renal artery stenosis, is also presented.

Is hypertension a tissue perfusion disorder? Implications for renal and myocardial perfusion

Structural alterations in the microcirculation form a major link between hypertension and target organ damage. More than 60% of the overall peripheral resistance of the circulatory system arises at the level of the microcirculation. The primary function of the microcirculation is to supply oxygen and nutrients to tissues. In hypertension, remodelling of the microvascular vessels occurs, leading to an early, functional then anatomical reduction in the number of arterioles or capillaries in a given vascular bed. Such changes have been seen in the structure and density of the microvasculature of different target organs such as the myocardium and the kidneys. In hypertension, capillary rarefaction induces an increase in blood pressure, a relative decrease in tissue perfusion and an increased cardiovascular risk. Recent in-vivo non-invasive techniques for exploring the human microcirculation have allowed the detection of myocardial and renal microvascular impairment in hypertensive patients. In comparative therapeutic studies, antihypertensive drugs have been shown to have different capacities for preventing or reversing changes to the microvasculature of affected organs.

Functional MRI of the kidney: tools for translational studies of pathophysiology of renal disease

Magnetic resonance imaging (MRI) provides exquisite anatomic detail of various organs and is capable of providing additional functional information. This combination allows for comprehensive diagnostic evaluation of pathologies such as ischemic renal disease. Noninvasive MRI techniques could facilitate translation of many studies performed in controlled animal models using technologies that are invasive to humans. Such a translation is being recognized as essential because many proposed interventions and drugs that prove efficacious in animal models fail to do so in humans. In this article, we review the state-of-the-art functional MRI technique as applied to the kidneys.

Angiotensin II decreases the renal MRI blood oxygenation level-dependent signal

Acute experimental reduction of renal blood flow decreases the renal blood oxygenation level-dependent (BOLD) MRI signal in animals. Angiotensin II also reduces renal blood flow, but the ability of BOLD MRI to dynamically detect this response has not yet been investigated in humans. Six healthy male volunteers underwent an individual dose-finding study to identify the intravenous doses of angiotensin II, norepinephrine, and sodium nitroprusside necessary to induce a 15-mm Hg peak mean arterial blood pressure change. MRI studies followed within 3 weeks, when angiotensin II (8.8+/-1.4 ng/kg), norepinephrine (52+/-12 ng/kg), and sodium nitroprusside (2.0+/-0.3 microg/kg) were given twice in an unblocked, randomized sequence while imaging experiments were performed on a 1.5-T Siemens Sonata. A multiecho echo-planar imaging sequence was used to acquire T2* maps with a temporal resolution of 1 respiratory cycle. Averaged over a renal cortex dominated region of interest, angiotensin II caused a shortening of T2* between 6% and 10%. Sodium nitroprusside and norepinephrine, although of equal potency concerning blood pressure responses, did not alter the renal BOLD signal. The renal BOLD response to angiotensin II appeared with short onset latency (as early as 10 seconds after peripheral intravenous angiotensin II bolus administration) suggesting that this response is a consequence of altered perfusion rather than increased renal oxygen consumption. The methods described here are suitable to assess renal responsiveness to angiotensin II and may, thus, be of great value in human hypertension research.

Evaluation of Intra-Renal Oxygenation by BOLD MRI

This is a review of blood oxygenation level-dependent (BOLD) MRI as applied to the kidney. It has been shown that BOLD MRI measurements reflect changes in renal oxygenation, especially in the medulla. Renal medulla functions in a hypoxic milieu and is extremely sensitive to further decrease in blood flow or increase in oxygen consumption. Availability of a non-invasive technique such as BOLD MRI should allow for better understanding of the factors involved in the maintenance of renal oxygenation status, not only in animal models, but also in humans.

Update of renal imaging

Significant technical improvements have allowed the use of radiological techniques to play a growing role in the imaging of renal diseases. Noninvasive ultrasound methods (ie, sonography and Doppler) are now positioned as first-line methods for the evaluation of renovascular diseases. Multidetector computed tomography is able to provide high spatial resolution images of the kidneys and renal arterial vessels. Magnetic resonance imaging, which provides higher signal-to-noise ratio and higher spatial and/or temporal resolution, can display both morphological information about renal parenchyma and vessels and functional data, including perfusion, filtration, diffusion, or oxygenation. In renovascular diseases, these techniques have the potential to drive new strategies, including Doppler sonography as a first-line method, followed by computed tomography angiography or magnetic resonance angiography, depending mainly on renal function. Imaging of parenchymal renal diseases is developing toward more quantitative (volumetric and functional measurements) and more specific (through in vivo cell targeting) acquisitions for obtaining the adequate information on tissue characteristics relevant either for diagnosis or for prognosis or treatment follow-up.

Functional renal imaging: nonvascular renal disease

Functional renal imaging-a fast-growing field of MR-imaging-applies different sequence types to gather information about the kidneys other than morphology and angiography. This update article presents the current status of different functional imaging approaches and presents current and potential clinical applications. Apart from conventional in-phase and opposed-phase imaging, which already yields information about the tissue composition, BOLD (blood-oxygenation level dependent) sequences, DWI (diffusion-weighted imaging) sequences, perfusion measurements, and dedicated contrast agents are used.

Assessment of acute renal transplant rejection with blood oxygen level-dependent MR imaging: initial experience

PURPOSE

To prospectively assess the oxygenation state of renal transplants and determine the feasibility of using blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging to differentiate between acute tubular necrosis (ATN), acute rejection, and normal function.

MATERIALS AND METHODS

This HIPAA-compliant study had institutional human subjects review committee approval, and written informed consent was obtained from all patients. BOLD MR imaging was performed in 20 patients (age range, 21-70 years) who had recently received renal transplants. Six patients had clinically normal functioning transplants, eight had biopsy-proved rejection, and six had biopsy-proved ATN. R2* (1/sec) measurements were obtained in the medulla and cortex of transplanted kidneys. R2* is a measure of the rate of signal loss in a specific region and is related to the amount of deoxyhemoglobin present. Statistical analysis was performed by using a two-sample t test. Threshold R2* values were identified to discriminate between transplanted kidneys with ATN, those with acute rejection, and those with normal function.

RESULTS

R2* values for the medulla were significantly lower in the acute rejection group (R2* = 15.8/sec +/- 1.5) than in normally functioning transplants (R2* = 23.9/sec +/- 3.2) and transplants with ATN (R2* = 21.3/sec +/- 1.9). The differences between the acute rejection and normal function groups (P = .001), as well as between the acute rejection and ATN groups (P < .001), were significant. Acute rejection could be differentiated from normal function and ATN in all cases by using a threshold R2* value of 18/sec. R2* values for the cortex were higher in ATN (R2* = 14.2/sec +/- 1.4) than for normally functioning transplants (R2* = 12.7/sec +/- 1.6) and transplants with rejection (R2* = 12.4/sec +/- 1.2). The difference in R2* values in the cortex between ATN and rejection was statistically significant (P = .034), although there was no threshold value that enabled differentiation of all cases of ATN from cases of normal function or acute rejection.

CONCLUSION

R2* measurements in the medullary regions of transplanted kidneys with acute rejection were significantly lower than those in normally functioning transplants or transplants with ATN. These results suggest that marked changes in intrarenal oxygenation occur during acute transplant rejection.

Validation of quantitative BOLD MRI measurements in kidney: application to unilateral ureteral obstruction

BACKGROUND

Blood oxygenation level dependent (BOLD) magnetic resonance imaging (MRI) provides a measure of deoxyhemoglobin content and therefore an indirect measure of the partial oxygen pressure (pO(2)). The main purpose of this study was to examine the relationship between the apparent relaxation rate (R2*) in the pig kidney by BOLD imaging and renal tissue pO(2) levels measured directly by oxygen-sensitive microelectrodes. Second, BOLD imaging was applied to kidneys in pigs subjected to acute unilateral ureteral obstruction (UUO) to examine whether this condition is associated with changes in intrarenal oxygenation.

METHODS

Oxygen-sensitive microelectrodes were inserted in the cortex and medulla of pig kidneys (N= 6). Different arterial and intrarenal levels of pO(2) were obtained by stepwise changing the oxygen-to-nitrogen ratio supplied by a respirator. Simultaneous BOLD MRI measurements using an R2*-sensitive Echo Planar Imaging (EPI) sequence were performed on the contralateral kidney. In another group of pigs (N= 3) BOLD imaging was performed following 24 hours of UUO.

RESULTS

When the inhaled oxygen fraction was 5% to 70%, R2* was linearly related to pO(2) levels (cortex DeltaR2*/DeltapO(2)=-1.2 ms(-1)kPa(-1), and medulla DeltaR2*/DeltapO2 =-1.7 ms(-1)kPa(-1)). Twenty-four hours of UUO was associated with an increased R2* in the cortex and a decreased R2* in medulla as compared with baseline, which remained augmented after the release of UUO, indicating that pO(2) levels were reduced in the cortex and increased in the medulla during and after release of obstruction.

CONCLUSION

BOLD MRI provides noninvasive estimates of regional renal oxygen content and our study demonstrates that this technique may provide a useful tool in UUO which is associated with altered renal oxygen consumption.

Detection of acute renal ischemia in swine using blood oxygen level-dependent magnetic resonance imaging

PURPOSE

To determine the feasibility and sensitivity of blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) to detect acute renal ischemia, using a swine model, and to present the causes of variability and assess techniques that minimize variability introduced during data analysis.

MATERIALS AND METHODS

BOLD MRI was performed in axial and coronal planes of the kidneys of five swine. Color R2* maps were calculated and mean R2* values and 95% confidence intervals (CIs) for the cortex and medulla were determined for baseline, renal artery occlusion and reperfusion conditions. Paired Student's t-tests were used to determine significance.

RESULTS

Mean R2* measurements increased from baseline during renal artery occlusion in the cortex (axial, 13.8-24.6 second(-1); coronal, 14.4-24.7 second(-1)) and medulla (axial, 19.3-32.2 second(-1); coronal, 20.1-30.7 second(-1)). These differences were significant for both the cortex (axial, P < 0.04; coronal, P < 0.005) and medulla (axial, P < 0.02; coronal, P < 0.0005). No significant change was observed in the contralateral kidney.

CONCLUSION

R2* values were significantly higher than baseline for medulla and cortex during renal artery occlusion. More variability exists in R2* measurements in the medulla than the cortex and in the axial than the coronal plane.