Renal Functional Contrast-Enhanced Magnetic Resonance Imaging

Probing renal blood volume with magnetic resonance imaging

Damage to the kidney substantially reduces life expectancy. Renal tissue hypoperfusion and hypoxia are key elements in the pathophysiology of acute kidney injury and its progression to chronic kidney disease. In vivo assessment of renal haemodynamics and tissue oxygenation remains a challenge. Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) is sensitive to changes in the effective transversal relaxation time (T₂ *) in vivo, and is non-invasive and indicative of renal tissue oxygenation. However, the renal T₂ * to tissue pO₂ relationship is not governed exclusively by renal blood oxygenation, but is affected by physiological confounders with alterations in renal blood volume fraction (BVf) being of particular relevance. To decipher this interference probing renal BVf is essential for the pursuit of renal MR oximetry. Superparamagnetic iron oxide nanoparticle (USPIO) preparations can be used as MRI visible blood pool markers for detailing alterations in BVf. This review promotes the opportunities of MRI-based assessment of renal BVf. Following an outline on the specifics of renal oxygenation and perfusion, changes in renal BVf upon interventions and their potential impact on renal T₂ * are discussed. We also describe the basic principles of renal BVf assessment using ferumoxytol-enhanced MRI in the equilibrium concentration regimen. We demonstrate that ferumoxytol does not alter control of renal haemodynamics and oxygenation. Preclinical applications of ferumoxytol enhanced renal MRI as well as considerations for its clinical implementation for examining renal BVf changes are provided alongside practical considerations. Finally, we explore the future directions of MRI-based assessment of renal BVf.

Geometrically confined ultrasmall gadolinium oxide nanoparticles boost the T(1) contrast ability

High-performance magnetic resonance imaging (MRI) contrast agents and novel contrast enhancement strategies are urgently needed for sensitive and accurate diagnosis. Here we report a strategy to construct a new T1 contrast agent based on the Solomon-Bloembergen-Morgan (SBM) theory. We loaded the ultrasmall gadolinium oxide nanoparticles into worm-like interior channels of mesoporous silica nanospheres (Gd2O3@MSN nanocomposites). This unique structure endows the nanocomposites with geometrical confinement, high molecular tumbling time, and a large coordinated number of water molecules, which results in a significant enhancement of the T1 contrast with longitudinal proton relaxivity (r1) as high as 45.08 mM(-1) s(-1). Such a high r1 value of Gd2O3@MSN, compared to those of ultrasmall Gd2O3 nanoparticles and gadolinium-based clinical contrast agents, is mainly attributed to the strong geometrical confinement effect. This strategy provides new guidance for developing various high-performance T1 contrast agents for sensitive imaging and disease diagnosis.

Contrast agents: magnetic resonance

Even though the intrinsic magnetic resonance imaging (MRI) contrast is much more flexible than in other clinical imaging techniques, the diagnosis of several pathologies requires the involvement of contrast agents (CAs) that can enhance the difference between normal and diseased tissues by modifying their intrinsic parameters. MR CAs are indirect agents because they do not become visible by themselves as opposed to other imaging modalities. The signal enhancement produced by MRI CAs (i.e., the efficiency of the CAs) depends on their longitudinal (r1) and transverse (r2) relaxivity (expressed in s(-1) mmol(-1) 1), which is defined as the increase of the nuclear relaxation rate (the reciprocal of the relaxation time) of water protons produced by 1 mmol per liter of CA. Paramagnetic CAs (most of them complexes of gadolinium) are frequently used in clinics as extracellular, hepatobiliary or blood pool agents. Low molecular weight paramagnetic CAs have similar effects on R1 and R2, but the predominant effect at low doses is that of T1 shortening (and R1 enhancement). Thus, organs taking up such agents will become bright in a T1-weighted MRI sequence; these CAs are thus called positive contrast media. The CAs known as negative agents influence signal intensity mainly by shortening T2* and T2, which produces the darkening of the contrast-enhanced tissue. These CAs are generally composed of superparamagnetic nanoparticles, consisting of iron oxides (magnetite, Fe3O4, maghemite, gammaFe2O3, or other ferrites). Iron oxide nanoparticles are taken up by the monocyte-macrophage system, which explains their potential application as MRI markers of inflammatory and degenerative disorders. Most of the contemporary MRI CAs approved for clinical applications are non-specific for a particular pathology and report exclusively on the anatomy and the physiological status of various organs. A new generation of MRI CAs is progressively emerging in the current context of molecular imaging, agents that are designed to detect with a high specificity the cellular and molecular hallmarks of various pathologies.

Contrast-Enhanced Magnetic Resonance Angiography

PURPOSE

To assess the gadolinium-based macromolecular intravascular contrast agent P792 for magnetic resonance angiography (MRA) at magnetic field strengths of 3.0 T, in comparison to 1.5 T, in rabbits.

MATERIALS AND METHODS

Eleven female New Zealand rabbits of the same age served as the animal model. Dose relationship testing was performed with 2 doses (13 and 25 micromol/kg; n = 4 per group) of P792 as compared with a single dose (100 micromol/kg; n = 3) of gadoterate meglumine (Gd-DOTA). All animals underwent contrast-enhanced MRA of the abdominal aorta and its branches on 2 occasions separated by 72 hours. The particular doses were administered in random order. Contrast-enhanced MRA was performed on 3.0 and 1.5 T whole-body MR systems, using a fast 3D spoiled gradient recalled echo sequence. Data acquisition was performed before and up to 10 minutes after administration of intravenous contrast material. Image quality was judged on a 4-point-Likert scale. Signal-to-noise and contrast-to-noise measurements were performed; statistical differences (P < 0.05) between the groups were determined.

RESULTS

P792 and Gd-DOTA yielded high-quality MR angiograms in rabbits in all cases. Although image quality within the first 3 minutes after contrast material administration was equal for both agents, P792 at a dose of 25 micromol/kg was considered superior to Gd-DOTA at the later time points. Signal-to-noise and contrast-to-noise values of the higher dose of P792 were statistically significantly higher than those of Gd-DOTA in the post-bolus phase.

CONCLUSIONS

P792 seems to be well suited for high-quality early phase and equilibrium phase MRA in rabbits at a field strength of 3.0 T, on the basis of this initial evaluation in an animal model.

Current issues and perspectives in small rodent magnetic resonance imaging using clinical MRI scanners

Small rodents such as mice and rats are frequently used in animal experiments for several reasons. In the past, animal experiments were frequently associated with invasive methods and groups of animals had to be killed to perform longitudinal studies. Today's modern imaging techniques such as magnetic resonance imaging (MRI) allow non-invasive longitudinal monitoring of multiple parameters. Although only a few institutions have access to dedicated small animal MR scanners, most institutions carrying out animal experiments have access to clinical MR scanners. Technological advances and the increasing field strength of clinical scanners make MRI a broadly available and viable technique in preclinical in vivo research. This review provides an overview of current concepts, limitations, and recent studies dealing with small animal imaging using clinical MR scanners.

Intraindividual Comparison of MR-Renal Perfusion Imaging at 1.5 T and 3.0 T

PURPOSE

The purpose of this study was to intraindividually compare fast gradient-echo semiquantitative renal perfusion measurements at 1.5 Tesla (T) and 3.0 Tesla.

MATERIALS AND METHODS

Fifteen healthy male volunteers underwent renal perfusion measurements at 1.5 T and 3.0 T after the bolus injection of 7 mL of Gd-BOPTA. At both field strengths a Saturation-Recovery-fast gradient echo sequence (SR-TurboFLASH) with a temporal resolution of 4 (1.5 T) and 5 (3.0 T) simultaneously acquired slices per second was used. At 3.0 T, a parallel-imaging factor 2 was applied. For postprocessing, semiquantitative perfusion parameters including mean transit time (MTT), time to peak (TTP), and maximal signal intensity (SMax) were determined. The signal-to-noise ratios (SNR) of kidneys and aorta were determined precontrast and after enhancement. The image quality was rated by 2 radiologists. After Bonferroni correction paired t-tests were performed for statistical analysis.

RESULTS

All measurements were successfully performed. At 3.0 T, a significant 63% increase in the baseline SNR (P = 0.00005) of the kidneys was found, the peak SNR was also increased though not statistically significant. Because of the higher SNR, the SMax was also significantly (P = 0.005) increased from 406 A.U. to 522 A.U., whereas MTT and TTP were not significantly changed. The image quality was rated very good to good for the 3.0 T images but only good to moderate at 1.5 T.

CONCLUSION

Renal perfusion measurements at 3.0 T are feasible and directly benefit from the inherently higher SNR at 3.0 T. The higher SNR also translates into an increased SMax, whereas MTT and TTP are independent of the field strength.

Improved Evaluation of Myocardial Perfusion and Viability With the Magnetic Resonance Blood Pool Contrast Agent P792 in a Nonreperfused Porcine Infarction Model

OBJECTIVES

To investigate whether a magnetic resonance (MR) blood pool contrast agent enables both evaluation of myocardial perfusion and viability in nonreperfused infarction in pigs.

MATERIALS AND METHODS

An optimized MR protocol using the blood pool contrast agent P792 (0.026 mmol/kg, twice the clinical dose, Guerbet, France) was investigated to evaluate nonreperfused myocardial infarction in an animal model. P792 was compared with the extracellular contrast agent Gd-DOTA (0.1 mmol/kg). The MRI findings were compared with histomorphometry performed with microspheres to evaluate perfusion and triphenyltetrazolium chloride (TTC) to evaluate viability. Contrast-enhanced MR imaging of the heart was performed on a 1.5-Tesla scanner 2 days after instrumentation in 6 minipigs. A saturation recovery steady-state free precession sequence was used for perfusion imaging and an inversion recovery fast low-angle shot sequence for evaluation of myocardial viability.

RESULTS

P792 tended to depict areas of reduced perfusion more accurately than Gd-DOTA (17.2% +/- 11.1% versus 13.7% +/- 8.0%) in comparison to the gold standard of histomorphometry with microspheres (18.2% +/- 9.8%). Moreover, P792, but not Gd-DOTA, depicted ischemic areas for 30 minutes after intravenous injection. The change in myocardial signal intensity during first pass was not significantly different after P792 compared with Gd-DOTA (140.3% +/- 64.4% versus 123.3% +/- 22.5%, P = 0.56). P792 was highly accurate in depicting infarcted areas (11.1% +/- 7.1%) compared with Gd-DOTA (12.1% +/- 8.2%, r = 0.98, P < 0.001) and histomorphometry with TTC (12.2% +/- 8.0%, r = 0.99, P < 0.001).

CONCLUSIONS

Unlike Gd-DOTA, the blood pool contrast agent P792 allows evaluation of myocardial perfusion for a period of 30 minutes and shows good agreement with histomorphometry. P792 must be examined in further studies to evaluate its potential in evaluating early myocardial lesions and reperfusion. In addition, P792 also allows for evaluation of myocardial viability.

(Paediatric) magnetic resonance urography: just fancy images or a new important diagnostic tool?

PURPOSE OF REVIEW

Magnetic resonance urography has become an established imaging tool in uroradiology. Its potential to assess anatomy and function makes it an ideal tool for evaluation of urinary tract malformations, renal cysts, genito-urinary tract tumours, infections and renal transplants. This review tries to highlight the potential of magnetic resonance urography in the light of new advances, particularly focusing on paediatric applications.

RECENT FINDINGS

Technical innovations such as diaphragmatic tracking, parallel or propeller imaging, faster gradients and higher field strength improve applicability in infants and children. Dynamic studies enable assessment of renal functional parameters such as split renal function, glomerular filtration rate or urinary drainage. Recent advances in magnetic resonance spectroscopy, diffusion imaging and perfusion imaging and new contrast agents promise to widen the potential of magnetic resonance urography as a functional imaging tool, not only in paediatrics but also for other magnetic resonance applications in the genito-urinary tract, such as prostate imaging or in the staging of ovarian and endometrial cancer.

SUMMARY

Besides ultrasound being used as the initial imaging method, particularly in children (and as computed tomography in adults), magnetic resonance urography can be envisioned as the major imaging modality for almost all (paediatric) uroradiological queries, consequently creating a growing demand for available equipment and procedural expertise.

Advances in Magnetic Resonance (2006)

Advances in the field of magnetic resonance (MR) as it pertains to clinical diagnostic radiology are examined in this review on the basis of publications in Investigative Radiology over the past 2 years (2005-2006). The articles published during that timeframe are discussed, organizationally wise, by anatomic region with an additional focus on studies involving MR contrast media.

Contrast agents for functional and cellular MRI of the kidney

Low-molecular-weight gadolinium (Gd) chelates are glomerular tracers but their role in evaluation of renal function with magnetic resonance (MR) imaging is still marginal. Because of their small size, they diffuse freely into the interstitium and the relationship between measured signal intensity and concentration is complex. New categories of contrast agents, such as large Gd-chelates or iron oxide particules, with different pharmacokinetic and magnetic properties have been developed. These large molecules could be useful for both functional (quantification of perfusion, quantification of glomerular filtration rate, estimation of tubular function) and cellular imaging (intrarenal phagocytosis in inflammatory renal diseases). Continuous development of new contrast agents remains worthwhile to get the best adequacy between the physiological phenomenon of interest and the pharmacokinetic of the agent.

The cost of developing imaging agents for routine clinical use

The objective of this study was to estimate the financial cost of developing new imaging agents for clinical use and to discuss the effects of these costs on the future clinical imaging agent environment. Publicly available financial data from the annual reports of major companies developing and selling imaging agents were examined and the data used to develop cost estimates. These estimates were compared with the in-depth data and analyses available for the development costs of therapeutic drugs. The cost of developing a drug for diagnostic imaging to commercialization is in the 100 dollars to 200 million dollars range, whereas a blockbuster imaging drug has current sales of 200 dollars to 400 million dollars. Most of these blockbuster imaging agents have been on the market for some time. The majority provide morphologic images with general indications in a slowly changing section of the market. Future agents will most likely address smaller markets and be in the rapidly developing molecular imaging field. The costs are high and are a significant brake on the development of imaging agents for commercialization. If new imaging agents are to realize their commercial potential, ways must be found to make the financials more attractive. The prices per dose are currently low so they must either be greatly increased for new imaging agents, with a corresponding increase in the value of the information they provide, or the use of imaging agents must be widened and/or their development made less costly in time and money. Without addressing these issues, the commercialization of new imaging agents will continue to be slow and may get slower. This will impact the progress of imaging agents toward use as validated biomarkers.

Preclinical Safety Assessment of Vasovist (Gadofosveset trisodium), a New Magnetic Resonance Imaging Contrast Agent for Angiography

OBJECTIVES

Vasovist (EPIX Pharmaceuticals and Schering AG) is a newly developed blood pool contrast agent for magnetic resonance imaging with a high affinity for human albumin, making it an ideal tool for the detection of structural abnormalities such as stenosis and aneurysm. For the risk assessment of the single diagnostic use in patients, the toxicity of this compound was investigated.

MATERIALS AND METHODS

Studies of acute, repeated-dose, reproductive, and developmental toxicity as well as local tolerance, immunotoxicity, and mutagenic potential were performed.

RESULTS

Lethality was observed in rodents after single intravenous administration at doses of at least 2 orders of magnitude higher than the anticipated human dose of 0.03 mmol/kg. The no observed adverse effect level after repeated daily administration over the course of 4 weeks to monkeys exceeded the single diagnostic dose by a factor of 3.3. The main effect of repeated dosing in both rats and monkeys was vacuolation in kidney proximal tubules without concomitant effect on kidney function. Studies into reproduction toxicity have shown no evidence of effects on fertility or perinatal and postnatal development. Signs of embryo-fetal toxicity were observed in rabbits after repeated administration of high doses. No indications of immunotoxic and mutagenic effects were observed. In local tolerance testing, Vasovist was well tolerated after intravenous administration.

CONCLUSIONS

Vasovist was well tolerated with reasonable safety margins between the single diagnostic dose of 0.03 mmol/kg in humans and the doses resulting in adverse effects in animal studies.

Magnetic resonance imaging in acute and chronic kidney diseases: present status

Magnetic resonance imaging (MRI) of normal and diseased kidneys shows great promise because of the combined value of anatomical and functional information provided, as well as of specific contrast patterns that can be observed non-invasively. Multicontrast MRI is able to show infiltrative kidney disorders. Diffusion-weighted imaging can assess alterations in renal function and can suggest obstruction or inflammation when present. Due to the low nephrotoxicity, contrast-enhanced MR studies using serial dynamic enhancement with non-specific gadolinium chelates are able to provide information on glomerular filtration. Furthermore, contrast agents such as ultrasmall particles of iron oxide, specific of inflammation, should be used in the near future to detect active from quiescent involvement, both in native kidneys and renal allografts. Early results should indicate that these compounds might differentiate acute tubular necrosis from other acute nephropathies, as well as active proliferative nephropathies from chronic ones. Ongoing studies will obviously demonstrate the value of the combination of these various MRI sequences in the diagnosis of acute renal failure and chronic kidney disease.

Effect of Field Strengths on Magnetic Resonance Angiography

OBJECTIVES

We sought to compare the intravascular enhancement of an ultrasmall superparamagnetic iron oxide (USPIO) blood-pool contrast agent to gadopentetate dimeglumine for contrast-enhanced magnetic resonance angiography (CE-MRA) at field strengths of 1.5 and 3.0 T in rabbits.

MATERIALS AND METHODS

CE-MRA at 1.5 and 3.0 T was performed at several time points (50 seconds and 5, 10, 20, and 30 minutes) after the manual intravenous injection of 40 micromol Fe/kg body weight of an USPIO (SH U 555 C; Schering AG, Berlin, Germany) and 100 micromol/kg body weight gadopentetate dimeglumine (Magnevist; Schering AG, Berlin, Germany). MRA was performed with comparable acquisition parameters at both field strengths (Turbo-gradient sequence; 1.5 T: TR/TE/alpha: 5.5/1.7 milliseconds/40 degrees ; 3.0 T: TR/TE/alpha: 5.1/1.8 milliseconds/40 degrees ) on clinical imaging systems (both: Gyroscan Intera, Philips Medical Systems, Best, The Netherlands). At either field strength, 6 rabbits were studied with both contrast agents (n = 24 in total). Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated from signal intensity measurements in the abdominal aorta.

RESULTS

Compared with 1.5 T, the SNR and CNR of gadopentetate dimeglumine significantly increased at 3.0 T by a factor of 2.2 and 2.3, respectively (P <or= 0.01), measured 50 seconds after intravenous injection. SNR and CNR of SH U 555 C, measured 50 seconds after intravenous injection, did not change significantly with increasing field strength (P >or= 0.05). At both field strength and either time point, CNR and SNR of SH U 555 C were significantly higher compared with gadopentetate dimeglumine at 3.0 T (P <or= 0.01).

CONCLUSIONS

SNR and CNR of gadopentetate dimeglumine significantly increased with increasing field strength. No SNR or CNR gain was observed for SH U 555 C. However, blood-pool MRA with SH U 555 C is feasible at 3.0 T. Compared with gadopentetate dimeglumine, SNR and CNR of SH U 555 C were significantly higher from 5 to 30 minutes at both field strengths (P <or= 0.01).

Comparison of magnetic properties of MRI contrast media solutions at different magnetic field strengths

RATIONALE AND OBJECTIVES

To characterize and compare commercially available contrast media (CM) for magnetic resonance imaging (MRI) in terms of their relaxivity at magnetic field strengths ranging from 0.47 T to 4.7 T at physiological temperatures in water and in plasma. Relaxivities also were quantified in whole blood at 1.5 T.

METHODS

Relaxivities of MRI-CM were determined by nuclear magnetic resonance (NMR) spectroscopy at 0.47 T and MRI phantom measurements at 1.5 T, 3 T, and 4.7 T, respectively. Both longitudinal (T1) and transverse relaxation times (T2) were measured by appropriate spin-echo sequences. Nuclear magnetic resonance dispersion (NMRD) profiles were also determined for all agents in water and in plasma.

RESULTS

Significant dependencies of relaxivities on the field strength and solvents were quantified. Protein binding leads to both increased field strength and solvent dependencies and hence to significantly altered T1 relaxivity values at higher magnetic field strengths.

CONCLUSIONS

Awareness of the field strength and solvent associated with relaxivity data is crucial for the comparison and evaluation of relaxivity values. Data observed at 0.47 T can thus be misleading and should be replaced by relaxivities measured at 1.5 T and at 3 T in plasma at physiological temperature.