F. Floemer

Contrast agents for MRA: Future directions

The purpose of this review is to outline recent developments in contrast agents for magnetic resonance angiography (MRA) and to give some idea of what the future might hold. Up to now, non‐binding gadolinium (Gd) chelates have been the agents used for MRA. Modified paramagnetic Gd‐based agents with varying degrees of protein interaction have been developed, and these, together with new superparamagnetic compounds, are currently under clinical evaluation. It is likely that two different types of contrast agent will soon be available: extracellular agents for first‐pass MRA and intravascular agents mainly for steady‐state MRA. Several agents also exhibit certain tissue specific properties in addition to conventional extracellular properties. This will lead to more comprehensive imaging approaches.J. Magn. Reson. Imaging 10:314–316, 1999.

Separation of arteries and veins in 3D MR angiography using correlation analysis.

Multiphase contrast‐enhanced 3D MR angiography (MRA) data sets allow the separate visualization of the arterial and venous pulmonary vasculature. However, due to short arterial‐tovenous bolus transit times in the lung, the generation of pure venograms without arterial overlay is difficult. To suppress arterial signal in venograms, early arterial phase data are typically subtracted from peak venous phase images. In this study, a correlation algorithm is used to postprocess the multiphase 3D MRA data sets. The cross‐correlation between a measured arterial or venous reference function and the local signal‐time course is computed which highlights image locations with a similar signal‐time curve as the reference function and suppresses constant signal. Conventional maximum intensity projections (MIP) are generated from the arterial and venous correlation maps. In a study with five volunteers, an increase in SNR by a factor of 2.1 (1.8) of arterial (venous) correlation MIP images over subtraction MIP images was observed. Magn Reson Med 43:481–487, 2000.

Assessment of gadobenate dimeglumine for magnetic resonance angiography: Phase I studies

Knopp MV, Schoenberg SO, Rehm C, et al. Assessment of Gadobenate Dimeglumine for magnetic resonance angiography: Phase I studies. Invest Radiol 2002;37:706–715. Rationale and Objectives. To assess the vascular contrasting properties of a new MR contrast agent (gadobenate dimeglumine [Gd-BOPTA]), which presents higher relaxivity because of reversible, weak protein interaction, and, to compare these properties with a standard gadolinium agent. Materials and Methods. Two phase I trials compared intraindividually: (A) the vascular contrasting properties of Gd-BOPTA at three doses (0.0125, 0.05, and 0.2 mmol/kg body weight) and two flow rates (0.5 and 2.0 mL/s) in 10 volunteers; and (B) 0.1 mmol/kg body weight doses of Gd-BOPTA and Gd-DTPA at 2.0 mL/s using a modified magnetic resonance angiography (MRA) sequence with a temporal resolution of 1 s/f. Quantitative (ROI analysis) and fully blinded qualitative (reader review) assessment of images was performed. Results. A dose of 0.2 mmol/kg resulted in higher maximum intensities, longer median peak widths, and larger areas under the curve than did the lower doses (0.0125 mmol/kg and 0.05 mmol/kg). In the intraindividual comparison, Gd-BOPTA demonstrated significantly better vascular enhancement characteristics in terms of signal peak duration (p < 0.05), maximum signal intensity (p < 0.05), and area under the enhancement curve (p < 0.01). The multireader assessment for overall vascular contrast preferred Gd-BOPTA at p < 0.03. Conclusions. Gd-BOPTA was shown to exhibit preferential and different vascular enhancement properties as compared with Gd-DTPA for MRA.

High-resolution pulmonary arterio- and venography using multiple-bolus multiphase 3D-Gd-mRA.

The purpose of this study was to demonstrate selective visualization of pulmonary arteries and veins with high spatial resolution for improved detection of vascular pathologies. Time‐resolved pulmonary three‐dimensional gadolinium‐enhanced magnetic resonance angiograms (3D‐Gd‐MRA) were acquired with an ultrashort 3D fast low‐angle shot (FLASH) sequence (TR/TE 2.3/0.9 msec) using asymmetric k‐space sampling in readout, phase‐encoding, and partition directions. In a single breath‐hold four consecutive acquisitions were performed with a scan time of 6.28 seconds for each phase. A spatial resolution of 1.9 × 1.4 × 2 mm3 could be achieved for a 12 cm thick 3D volume. To exploit the intrinsic enhancement kinetics of the pulmonary vascular system, the infusion of two gadolinium chelate boli was synchronized with the acquisition of the first and third data sets. This infusion strategy created two predominantly pulmonary arterial and venous data sets with the arterial and venous signal being maximally anti‐correlated in time. A further separation of arterial and venous signal could be achieved by means of a correlation analysis. Eight patients with suspected thrombi in the pulmonary veins were studied. The calculated correlation angiographies provided high‐quality pulmonary arterio‐ and venograms. Background signal was completely eliminated, allowing maximum intensity projection analysis of the full data set. In 7/8 patients no venous contamination of the arterial phase and vice versa was found. Patency of the pulmonary arteries and veins could be reliably assessed to the level of the segmental vessels. The combination of ultrafast multiphase 3D‐Gd‐MRA and dual‐bolus injection with a correlation analysis of vascular signal provides high‐resolution pulmonary arterio‐ and venograms. This imaging strategy initiates a new competitor to X‐ray angiography. J. Magn. Reson. Imaging 1999;10:339–346.

Combined assessment of obstructive sleep apnea syndrome with dynamic MRI and parallel EEG registration: initial results.

RATIONALE AND OBJECTIVES To detect obstructive pharyngeal changes in sleeping patients with obstructive sleep apnea syndrome by dynamic MRI and concurrent EEG monitoring during true apneic episodes. METHODS Five volunteers and eight patients with clinically diagnosed obstructive sleep apnea were polysomnographically monitored inside the scanner before, during, and after sleep discontinuation. After sleep interruption, the Mueller maneuver was performed to compare induced pharyngeal collapse with real collapse during sleep. RESULTS In all patients, on-line EEG registration was achieved in the static magnetic field. Sleep was proved in four of the eight patients who showed typical EEG findings. A complete pharyngeal collapse was shown in two of the four sleeping patients. The other patients predominately showed local epi- and oropharyngeal obstructions in the apneic state. Compared with the apneic episodes, the Mueller maneuver did not reveal the same extent of pharyngeal narrowing, nor a complete collapse. CONCLUSIONS Polysomnographically monitored MRI can directly assess findings induced by the pathophysiology of sleep apnea and shows promise for use in sleep investigation and therapy planning and monitoring.

Intraindividueller Vergleich dreier Kontrastmittel (Gd-DTPA, Gd-BOPTA und Gd-BT-DO3A) mit der multiphasischen MR-Angiographie

ZusammenfassungFragestellung. Mit der Verfügbarkeit neuartiger Kontrastmittel – proteininteragierend oder höher konzentriert – stellt sich die Frage, ob und wie sich diese von den Standardkontrastmitteln hinsichtlich der vaskulären Kontrastierung unterscheiden. Methodik. In einer intraindividuellen Vergleichsstudie des Abdomens (1,5 T-MRTSystem) wurden 5 Probanden mit einer multiphasischen MRA jeweils mit 3 verschiedenen MR-Kontrastmitteln (Gadopentetat-Dimeglumin 0,5 M;Gadobenat-Dimeglumin 0,5 M und Gadobutrol 1,0 M) untersucht. Dosis (0,15 mmol/kg Körpergewicht) und Flussrate (3,0 ml/s) waren für alle 3 Kontrastmittel identisch. Die Untersuchungen wurden geblindet qualitativ und quantitativ evaluiert. Ergebnisse. Das proteininteragierende Gadobenat-Dimeglumin erzielte im Vergleich mit den anderen Kontrastmitteln signifikant höhere Signalintensitäten in den arteriellen Gefäßen (p = 0,05). Es wurde aufgezeigt, dass zwischen Gadopentetat-Dimeglumin und Gadobutrol 1,0 M keine signifikante Differenz in der vaskulären Kontrastierungseigenschaften besteht. Schlussfolgerung. Gadobenat-Dimeglumin hat sich als vorteilhaftes Kontrastmittel für die multiphasische MRA herausgestellt. Höher konzentrierte Gd-Chelate führen unter standardisierter Bildgebung nicht automatisch zu verbesserten Kontrastierungseffekten.AbstractQuestion. The availability of new MR contrast agents having either a protein binding effect or higher concentration leads to the question if they differ from standard compounds in their vascular contrasting properties. Methods. By intraindividual comparison five volunteers were examined (1.5 T MR system) with a multi-phasic MRA of the abdomen each receiving three different contrast agents (gadopentetate dimeglumine 0.5 M; gadobenate dimeglumine 0.5 M, and gadobutrol 1.0 M).The dose (0.15 mmol/kg body weight) and flow rate (3.0 ml/s) were kept identical. All images were quantitatively and qualitatively evaluated by blinded assessment. Results. Significantly higher maximum signal intensity was found in the arteries for the protein interacting gadobenate dimeglumine (p = 0.05). No significant difference in vascular enhancement was demonstrated in the comparison of gadobutrol to gadopentetate dimeglumine. Conclusion. Gadobenate dimeglumine was shown to be a favorable contrast agent for multi-phasic MRA. A higher concentrated Gd-chelate does not automatically lead to improved vascular contrast if standard imaging protocols are used.

Three‐dimensional spiral MR imaging: Application to renal multiphase contrast‐enhanced angiography

A fast MR pulse sequence with spiral in‐plane readout and conventional 3D partition encoding was developed for multiphase contrast‐enhanced magnetic resonance angiography (CE‐MRA) of the renal vasculature. Compared to a standard multiphase 3D CE‐MRA with FLASH readout, an isotropic in‐plane spatial resolution of 1.4 × 1.4 mm2 over 2.0 × 1.4 mm2 could be achieved with a temporal resolution of 6 sec. The theoretical gain of spatial resolution by using the spiral pulse sequence and the performance in the presence of turbulent flow was evaluated in phantom measurements. Multiphase 3D CE‐MRA of the renal arteries was performed in five healthy volunteers using both techniques. A deblurring technique was used to correct the spiral raw data. Thereby, the off‐resonance frequencies were determined by minimizing the imaginary part of the data in image space. The chosen correction algorithm was able to reduce image blurring substantially in all MRA phases. The image quality of the spiral CE‐MRA pulse sequence was comparable to that of the FLASH CE‐MRA with increased spatial resolution and a 25% reduced contrast‐to‐noise ratio. Additionally, artifacts specific to spiral MRI could be observed which had no impact on the assessment of the renal arteries. Magn Reson Med 48:290–296, 2002.

Non-invasive assessment of renal artery stenosis: current concepts and future directions in magnetic resonance angiography.

Magnetic resonance angiography is undergoing rapid development as a non-invasive technique to reliably assess renal artery stenosis. Invasive X-ray angiography is currently the gold standard imaging technique and gives excellent spatial resolution. However, its disadvantages include a necessary exposure to radiation and the need for iodinated contrast media. While magnetic resonance angiography does not have these disadvantages, its spatial resolution is inherently lower. On the other hand, magnetic resonance angiography enables true three-dimensional imaging. The use of rapid imaging techniques allows multiple image acquisition within one breath-hold thereby permitting the visualization of distinct vascular phases. The limitation in spatial resolution can be overcome readily by combining morphologic imaging with functional information on the hemodynamic relevance. This can be achieved by means of magnetic resonance phase contrast flow measurements. The use of such a combined approach enables the grading of vascular stenosis based on the combination of morphology and functional information. Magnetic resonance angiography is already able to demonstrate a clinical utility equivalent to that of invasive procedures, indicating that it is likely to become a premier method for the diagnosis and follow up of renovascular disease.

Quantitative Erfassung der renalen Funktion mit der Magnetresonanztomographie

ZusammenfassungFragestellung. Verschiedene Methoden der Magnetresonanztomographie zur quantitativen Erfassung der Nierenfunktion sollen aufgezeigt werden. Methodik. Es werden MR-Techniken zur kombinierten Beurteilung der renalen Morphologie, Hämodynamik und Funktion der Niere vorgestellt. Zur Anwendung kommen hierbei die multiphasische 3-D-Gadolinium-MR-Angiographie, die MR-Phasenkontrastflussmessung, die quantitative Perfusionsmessung mit intravasalen Kontrastmitteln sowie die MR-Renographie/MR-Urographie. Die Methoden werden exemplarisch am Beispiel renovaskulärer Erkrankungen demonstriert. Ergebnisse. Mit den vorgestellten Methoden lassen sich Nierenarterienstenosen akkurat detektieren sowieÄnderungen des renalen Blutflusses, der Nierenperfusion, der Filtrationsleistung sowie der renalen Ausscheidung erfassen. Auf der Basis quantitativer Parameter kann die hämodynamische und funktionelle Relevanz von Nierenarterienstenosen eingestuft werden. Renovaskuläre und renoparenchymatöse Erkrankungszustände lassen sich differenzieren. Schlussfolgerung. Funktionelle Verfahren in der Magnetresonanztomographie stellen bei der Funktionsuntersuchung der Niere eine bedeutende Alternative zu nuklearmedizinischen Techniken dar. Der prädiktive Wert dieser Methoden für die Abschätzung des Revaskularisationserfolgs ist zur Zeit noch Gegenstand laufender Untersuchungen.AbstractAim. To show the potential of various methods in magnetic resonance imaging for the evaluation of renal function. Material and Methods. A combined assessment of renal morphology, renal hemodynamics and function is proposed. Various techniques are explained, including multiphasic 3D gadolinium MR angiography, MR phase-contrast flow measurements, quantitative perfusion measurements with intravascular contrast agents, and MR renography and MR urography. The use of these techniques is demonstrated for renovascular diseases. Results. The combined use of these techniques allows renal artery stenosis to be accurately detected and evaluation of renal blood flow, perfusion, glomerular filtration rate, and renal excretion. Based on true quantitative parameters, the hemodynamic and functional significance of the stenosis can be assessed. Renovascular diseases can be differentiated from renoparenchymal disease. Conclusion. For the assessment of renal function, functional magnetic resonance imaging techniques are an important alternative to nuclear medicine. The predictive value regarding the effect of revascularization is currently under investigation.

MR-Bildgebung der Nieren

ZusammenfassungFragestellung: Darstellung neuer diagnostischer Möglichkeiten im Bereich der Niere mittels schneller Magnetresonanz (MR)- Bildgebung. Methodik: Vorgestellt wird ein kombiniertes morphologisches und funktionelles Untersuchungskonzept bestehend aus schneller morphologischer Bildgebung, multiphasischer 3D-Gadolinium-MR-Angiographie, MR-Urographie und MR-Flußmessung. In einer einzigen MR-Untersuchung sollen vaskuläre Erkrankungen erfaßt, eingestuft und auf ihre hämodynamische und funktionelle Signifikanz überprüft werden, renale Läsionen detektiert und differenziert sowie die Harnabflußwege beurteilt werden. Ergebnisse: Durch Integration der gewonnenen morphologischen und funktionellen Daten lassen sich Nierenarterienstenosen, benigne und maligne renale Tumoren, Harnabflußstörungen und kongenitale Fehlbildungen mit ähnlicher Genauigkeit wie in den konventionellen radiologischen Verfahren erfassen. Der Nachteil der geringeren räumlichen Auflösung kann dabei weitgehend durch den besseren Gewebekontrast und die zusätzliche, funktionelle Information kompensiert werden. Schlußfolgerung: Ein kombiniertes MR-Untersuchungskonzept stellt ein zuverlässiges, nicht-invasives und kosteneffektives Verfahren zur umfassenden diagnostischen Abklärung der Niere dar.SummaryAim: New diagnostic strategies for evaluation of the kidney by fast MR imaging techniques. Material and methods: A comprehensive morphologic and functional evaluation of the kidney is proposed using fast MR imaging of renal morphology, multiphase 3D gadolinium MR angiography, MR urography and MR flow measurements. A single MR examination is designed to grade renovascular disease and assess the hemodynamic and functional significance, detect and characterize renal lesions and evaluate the urinary tract. Results: The combined analysis of morphologic and functional data allows reliable assessment of renal artery stenosis, benign and malignant renal masses and diseases of the renal collecting system and ureters, as well as congenital abnormalities in good agreement to the results of conventional imaging modalities. The improved tissue contrast and additional functional information compensates for the disadvantage of a lower spatial resolution. Conclusion: Combined morphologic and functional MR examination represents a reliable, non-invasive and cost-effective alternative imaging modality for comprehensive diagnostic evaluation of renal disease.