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Figure 3 Cine-phase-contrast flow curves for different degrees of RAS. A semiquantitative grading scheme is applied on the basis of distinct changes in the waveform pattern. This scheme was shown to the readers as a guideline for the grading of hemodynamic changes. Note that the absolute scaling is the same for all four flow curves. (A) Normal flow profiles reveal a characteristic ESP and a midsystolic maximum. (B) Low-grade stenoses typically reveal only a partial loss of the ESP (solid arrow). (C) Moderate stenoses demonstrate an almost complete loss of the ESP and a decrease of the midsystolic maximum (open arrow). (D) High-grade stenoses have a featureless flattened flow profile. Abbreviations: RAS, renal artery stenosis; ESP, early systolic peak. Source: From Ref. 26.

Figure 4 (A) A 75-year-old female patient with high-grade left renal artery stenosis and scarring of the kidney demonstrated on MR angiogram. (B) In the phase-contrast MR-flow measurement, a substantially altered flow profile of the affected side with loss of the systolic velocity components becomes visible. Abbreviation: MR, magnetic resonance. Source: Courtesy of Dr. Stefan O. Schoenberg and Dr. Henrik Michaely, Department of Clinical Radiology, Ludwig-Maximilians-University, Munich, Germany.

time (msec)

Figure 4 (A) A 75-year-old female patient with high-grade left renal artery stenosis and scarring of the kidney demonstrated on MR angiogram. (B) In the phase-contrast MR-flow measurement, a substantially altered flow profile of the affected side with loss of the systolic velocity components becomes visible. Abbreviation: MR, magnetic resonance. Source: Courtesy of Dr. Stefan O. Schoenberg and Dr. Henrik Michaely, Department of Clinical Radiology, Ludwig-Maximilians-University, Munich, Germany.

Renal Perfusion with Contrast-Enhanced Dynamic Studies

The degree of perfusion depends on both the arterial flow rate and such local factors as regional blood volume and vasoreactivity. Theory of perfusion calculation as well as imaging methods depends on the type of contrast agent used. Diffusible Gd-chelates as well as two categories of agents without interstitial diffusion or GFR

(iron oxide particles and blood-pool Gd-chelates: macromolecular or albumin bound) have been proposed for renal perfusion (27,28).

First-Pass Dynamic Studies Using Intravascular Agents with a T2* Effect. Because absolute quantification of regional perfusion is not straightforward with iron oxide particles and requires several signal processing steps together with several assumptions, most studies have been conducted using either qualitative or semiquantitative indices; maximal signal decrease (MSD), time to MSD (TMSD), or wash-in and wash-out slopes can be measured for comparison from right to left kidney, from cortex to medulla, or from one territory to another.

Because these agents are considered having a unicompartmental distribution within the kidney, absolute regional RBF can be calculated (in mL/min/g) according to Stewart and Hamilton's central volume theorem:

rRBF = rRBV x MTT, where rRBF is the regional RBF, rRBV is the regional renal blood volume, and MTT is the mean transit time. Calculation of rRBV (in mL/g of renal tissue) corresponds to the area under the fitted first-pass concentration-time curve, denoted as C(t), normalized to AIF denoted as Ca(t), and to the mean renal density (q = 1.04 g/mL), and taking into account the difference (kh) in arterial and capillary hematocrits (29):

The real MTT is difficult to measure because it would require a better understanding of the range of microvasculature structure within the tissues. However, it has been shown that the first moment of the renal-fitted concentration-time curve is a reasonable estimation of the relative MTT (30).

Application of this quantitative technique to a series of patients with RAS have been reported recently (31). A decreased rRBF was noted only for severe stenosis (Fig. 5) or in kidneys suffering from chronic damage related to other renal diseases, illustrating the complementary role of morphological information provided by MRA and hemodynamic data.

First-Pass Dynamic Studies Using Diffusible Gd-chelates. Renal perfusion can also be evaluated with standard Gd-chelates that are freely diffusible within the interstitial space and excreted exclusively by GFR. First-pass studies with these agents allow measurement of both the renal perfusion and the GFR during the same acquisition protocol. Renal perfusion is calculated from the first-pass renal curve following an instantaneous bolus of the contrast agent, which requires a high temporal resolution.

The most widely used perfusion model is derived from Peters's model, developed for nuclear medicine with 99mTc-diethylene-triamine-pentacetate (DTPA) as radiopharmaceutical (32). Because 99mTc and these Gd-chelates have similar phar-macokinetical properties, the obtained dynamic uptake curves by MRI is comparable with that obtained with a gamma camera. The simple kinetic description of microspheres can be applied to the initial wash-in of the renal MR signal-time curve (33). Introducing the arterial changes of R1 allows [A(R1)art] calculation of renal perfusion per unit of volume and can be extracted from the mathematical expression:

RBF/vol = max sloperenal/max A(R1)

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