Modeling of Look‐Locker estimates of the magnetic resonance imaging estimate of longitudinal relaxation rate in tissue after contrast administration

This paper models the behavior of the longitudinal relaxation rate of the protons of tissue water R₁ (R₁ = 1/T₁), measured in a Look‐Locker experiment at 7 Tesla after administration of a paramagnetic contrast agent (CA). It solves the Bloch‐McConnell equations for the longitudinal magnetization of the protons of water in a three‐site two‐exchange (3S2X) model with boundary conditions appropriate to repeated sampling of magnetization. The extent to which equilibrium intercompartmental water exchange kinetics affect monoexponential estimates of R₁ after administration of a CA in dynamic contrast enhanced experiment is described. The relation between R₁ and tissue CA concentration was calculated for CA restricted to the intravascular, or to the intravascular and extracellular compartments, by varying model parameters to mimic experimental data acquired in a rat model of cerebral tumor. The model described a nearly linear relationship between R₁ and tissue concentration of CA, but demonstrated that the apparent longitudinal relaxivity of CA depends upon tissue type. The practical consequence of this finding is that the extended Patlak plot linearizes the ΔR₁ data in tissue with leaky microvessels, accurately determines the influx rate of the CA across these microvessels, but underestimates the volume of intravascular blood water. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Studies of Gd-DTPA relaxivity and proton exchange rates in tissue

The image intensity in many contrast agent perfusion studies is designed to be a function of bulk tissue T₁, which is, in turn, a function of the compartmental (vascular, interstitial, and cellular) T₁s, and the rate of proton exchange between the compartments. The goal of this study was to characterize the compartmental tissue Gd-DTPA relaxivities and to determine the proton exchange rate between the compartments. Expressing [Gd-DTPA] as mmol/liter tissue water, the relaxivities at 8.45 T and room temperature were: saline, 3.87 ± 0.06 (mM. s)^?1 (mean ± SE; n = 29); plasma, 3.98 ± 0.05 (mM·s)^?1 (n = 6); and control cartilage (primarily an interstitium), 4.08 f 0.08 (mM·s)^?1 (n = 17), none of which are significantly different. The relexivity of cartilage did not change with compression, trypsinization, or equilibration in plasma, suggesting relaxivity is not influenced by interstitial solid matrix density, charge, or the presence of plasma proteins. T₁ relaxation studies on isolated perfused hearts demonstrated that the cellular-interstitial water exchange rate is between 8 and 27 Hz, while the interstitial-vascular water exchange rate is less than 7 Hz. Thus, for Gd-DTPA concentrations, which would be used clinicallly, the T₁ relaxation rate behavior of intact hearts can be modeled as being in the fast exchange regime for cellular-interstitial exchange but slow exchange for interstitial-vascular exchange. A measured relaxivity of 3.82 ± 0.05 (mM·s)_?1 (n = 8) for whole blood (red blood cells and plasma) and 4.16 ± 0.02 (mM·s)^?1 (n = 3) for frog heart tissue (cells and interstitium) (with T₁ and Gd-DTPA concentration defined from the total tissue water volume) supports the conclusion of fast cellular-extracellular exchange. Knowledge of the Gd-DTPA relaxivity and maintaining Gd-DTPA concentration in the range so as to maintain fast cellular-interstitial exchange allows for calculation of bulk Gd-DTPA concentration from bulk tissue T₁ within a calculable error due to slow vascular exchange.     

Inversion recovery EPI of bolus transit in rat myocardium using intravascular and extravascular gadolinium-based MR contrast media: Dose effects on peak signal enhancement

Inversion recovery gradient recalled echo planar imaging (TI/TR/TE = 700/2000/10 ms) was used to dynamically monitor the first pass of an intravascular (GdDOTA-polylysine) and an extravascular (GdDTPA-BMA) contrast agent through normal rat myocardium. It was found that myocardial enhancement increased with dose of the intravascutar agent to a limiting value of ～50% of fully relaxed intensity, consistent with enhancement of 40% of myocardial water content during the first pass. Larger doses produced no further increase in peak response. On the other hand, the extravascular agent caused incrementally increased enhancement throughout the dose range examined to a final value of 68 ± 2% of fully relaxed intensity. The profile of dose dependence for both agents was inconsistent with monoexponential T₁ relaxation. It was concluded that (a) compartmentalization of myocardial water combined with restricted myocardial water diffusion limits the peak response during bolus transit; (b) extraction of the extravascular agent during transit elevates the peak response over that obtained from agent confined to the vascular volume; and (c) models that assume simple monoexponential r, relaxation to derive timedensity curves do not adequately describe the relationship between changes in signal intensity, R₁ and contrast concentration.     

Quantification of liver blood volume: comparison of ultra short ti inversion recovery echo planar imaging (ulstir-epi), with dynamic 3d-gradient recalled echo imaging

An ultra-short TI inversion recovery echo-planar imaging (ULSTIR-EPI) sequence was designed to reduce the influence of water exchange on fractional tissue blood volume (BV) estimation by measurement of T₁-changes induced by a gadolinium-based macromolecular contrast medium (MMCM). Fractional liver BV in rats, estimated by ULSTIR-EPI was compared for accuracy to a fast T₁-weighted three-dimensional gradient-echo (3D-SPGR, 3D-spoiled gradient recalled acquisition in a steady state) sequence using an in vitro inductively coupled plasma atomic emission spectroscopy (ICP-AES) assay for BV as a standard. Liver images for fractional BV estimation were acquired in eight rats using both ULSTIR-EPI and 3D-SPGR before and after (within 3 to 12 min) intravenous bolus administration of albumin-Gd-DTPA₃₀ (0.05 mmol Gd/kg). Whereas both MR techniques may be useful for fractional tissue BV estimation, ULSTIR-EPI offers certain advantages including greater accuracy, direct T₁ maps, and minimization of transendothelial proton exchange effects. 3D-SPGR imaging offers better spatial resolution, current availability on standard clinical MR systems, and acceptable accuracy.     

Insight into in vivo magnetization exchange in human white matter regions

Water exchange can play an important role in interpreting compartment‐specific magnetic resonance imaging data in brain. For example, an MR method of myelin measurement, known as myelin water fraction imaging, assumes that water exchange processes are slow compared with the measurement time scale. In this article, we examined whether water exchange processes have an effect on myelin water fraction values. A previously established four pool model of white matter was used to simulate the interactions between two aqueous compartments (myelin water and intra/extracellular water) and nonaqueous compartments (myelin and nonmyelin tissues). To extract the water exchange cross relaxation times, the Bloch equations were solved analytically. As the water exchange time scales are dependent on the spin‐lattice T₁ relaxation of each of these four pools and due to the current uncertainties regarding the T₁ associated with each pool, exchange cross relaxation times for three different T₁ scenarios were calculated. The corrections that need to be considered in order for myelin water fraction to be an accurate marker for myelin were found to be less than 15%. This work indicates that regional variations in white matter myelin water fraction values are most likely due to variations in myelin content rather than regional differences in exchange rates. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Tracer kinetic analysis of dynamic contrast‐enhanced MRI and CT bladder cancer data: A preliminary comparison to assess the magnitude of water exchange effects

The purpose of this study was to determine the impact of water exchange on tracer kinetic parameter estimates derived from T₁‐weighted dynamic contrast‐enhanced (DCE)‐MRI data using a direct quantitative comparison with DCE‐CT. Data were acquired from 12 patients with bladder cancer who underwent DCE‐CT followed by DCE‐MRI within a week. A two‐compartment tracer kinetic model was fitted to the CT data, and two versions of the same model with modifications to account for the fast exchange and no exchange limits of water exchange were fitted to the MR data. The two‐compartment tracer kinetic model provided estimates of the fractional plasma volume (v_p), the extravascular extracellular space fraction (v_e), plasma perfusion (F_p), and the microvascular permeability surface area product. Our findings suggest that DCE‐CT is an appropriate reference for DCE‐MRI in bladder cancers as the only significant difference found between CT and MR parameter estimates were the no exchange limit estimates of v_p (P = 0.002). These results suggest that although water exchange between the intracellular and extravascular‐extracellular space has a negligible effect on DCE‐MRI, vascular–extravascular‐extracellular space water exchange may be more important. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Magnetization Transfer and T2 Relaxation Components in Tissue

T₂ relaxation makes an important contribution to tissue contrast in magnetic resonance (MR) imaging. Many tissues are known to exhibit multicomponent T₂ relaxation that suggests some compartmental segregation of mobile protons on a T₂ timescale. Magnetization transfer (MT) is another relaxation mechanism that can be used to produce tissue contrast in MR imaging. The MT process depends strongly on water-macromolecular interactions. To investigate the relationship between multicomponent T₂ relaxation and the MT process, multiecho T₂ measurements have been combined with MT measurements for freshly excised samples of cardiac muscle, striated muscle, and white matter. For muscle, short T₂ components show greater MT than long T₂ components, consistent with the belief that they represent distinct water environments. For white matter, quantitative MT measurements were identical for the two major T₂ components, apparently because of exchange between the T₂ compartments on a timescale characteristic of the MT experiment. Implications for accurate modeling of MT in tissue and the use of MT for MR image contrast are discussed.     

T2 exchange agents: A new class of paramagnetic MRI contrast agent that shortens water T2 by chemical exchange rather than relaxation

Exchange of water molecules between the frequency‐shifted inner‐sphere of a paramagnetic lanthanide ion and aqueous solvent can shorten the T₂ of bulk water protons. The magnitude of the line‐broadening T₂ exchange (T_2exch) is determined by the lanthanide concentration, the chemical shift of the exchanging water molecule, and the rate of water exchange between the two pools. A large T_2exch contribution to the water linewidth was initially observed in experiments involving Eu³⁺‐based paramagnetic chemical exchange saturation transfer agents in vivo at 9.4 T. Further in vitro and in vivo experiments using six different Eu³⁺ complexes having water exchange rates ranging from zero (no exchange) to 5 × 10⁶ s^?1 (fast exchange) were performed. The results showed that the exchange relaxivity (r_2exch) is small for complexes having either very fast or very slow exchange, but reaches a well‐defined maximum for complexes with intermediate water exchange rates. These experimental results were verified by Bloch simulations for two site exchange. This new class of T_2exch agent could prove useful in the design of responsive MRI contrast agents for molecular imaging of biological processes. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

In vivo quantification of regional myocardial blood flow: validity of the fast-exchange approximation for intravascular T1 contrast agent and long inversion time.

In the present study we investigated the effects of water exchange between intra- and extravascular compartments on absolute quantification of regional myocardial blood flow (rMBF) using a saturation-recovery sequence with a rather long inversion time (TI, 176 ms) and a T1-shortening intravascular contrast agent (CMD-A2-Gd-DOTA). Data were acquired in normal and ischemically injured pigs, with radiolabeled microsphere flow measurements used as the gold standard. Five water exchange rates (fast, 6 Hz, 3 Hz, 1 Hz, and no exchange) were tested. The results demonstrate that the fast-exchange approximation may be appropriate for rMBF quantification using the described experimental setting. Relaxation rate change (DeltaR1) analysis improved the accuracy of the analysis of rMBF compared to the MR signal. In conclusion, the current protocol could provide sufficient accuracy for estimating rMBF assuming fast exchange and a linear relationship between signal and tissue concentration when quantification of precontrast T1 is not an option.     

Improving MR quantification of regional blood volume with intravascular T1 contrast agents: Accuracy,precision, and water exchange

The goal of this work was to develop a comprehensive understanding of the relationship between vascular proton exchange rates and the accuracy and precision of tissue blood volume estimates using intravascular T₁ contrast agents. Using computer simulations, the effects of vascular proton exchange and experimental pulse sequence parameters on measurement accuracy were quantified. T₁ and signal measurements made in a rat model implanted with R3230 mammary adenocarcinoma tumors demonstrated that the theoretical findings are biologically relevant; data demonstrated that over-simplified exchange models may result in measures of tumor, muscle, and liver blood volume fractions that depend on experimental parameters such as the vascular contrast concentration. As a solution to the measurement of blood volume in tissues with exchange that is unknown, methods that minimize exchange rate dependence were examined. Simulations that estimated both the accuracy and precision of such methods indicated that both the inversion recovery and the transverse-spoiled gradient echo methods using a “noexchange” model provide the best trade-off between accuracy and precision.     

Effects of water exchange on the measurement of myocardial perfusion using paramagnetic contrast agents.

To investigate the effects of water exchange on quantification of perfusion, data were acquired in isolated hearts (n = 11) and used to develop a model of exchange. Myocardial T1 was measured 3 times/sec during step changes in concentration of intravascular (polylysine-gadolinium-diethylene-triamine-pentaacetic acid) and extracellular (gadoteridol) agents. For the intravascular agent, the change in 1/T1 (deltaR1) was lower than predicted by fast exchange (2.7+/-0.5 vs. 7.8 sec(-1), respectively), and suggested an intra-extravascular exchange rate of 3 Hz. For the extracellular agent, contrast kinetics were similar to those of similarly sized molecules (wash-in time constant 38+/-5 sec), and the data suggested fast interstitial-cellular exchange. Modeling showed that perfusion is underestimated for both agents if exchange is ignored, although the relationships of measured to actual perfusion were monotonic. We conclude that myocardial water exchange strongly affects first-pass enhancement but that ignoring the effects of exchange may still provide reasonable estimates of regional perfusion differences.     

Reperfused myocardial infarctions on T1- and susceptibility-enhanced MRI: Evidence for loss of compartmentalization of contrast media

The purpose of this study was to characterize the contrast caused by a susceptibility MRI contrast agents, on spin echo T₂-weighted imaging of reperfused myocardial infarction. Our interest in this model focused on the expected requirement that such agents be compartmentalized in the tissue to cause signal loss on spin echo images, a condition which may not be present in reperfused infarcted myocardium. Accordingly, nine rats were subjected to 2 h of left coronary artery occlusion followed by 3 ± 0.5 h of reperfusion prior to administration of contrast media. Three sets of MR images were acquired: (a) baseline axial images at the midventricle, both T₁-weighted (TR/TE = 300/20) and T₂-weighted (TR/TE = 1500/60); (b) T₁-weighted images after administering a T₁-enhancing agent, Gd-DTPA-BMA (0.2 mmol/kg), to document that contrast media is delivered to the reperfused infarction; and (c) T₂-weighted images after administering the susceptibility agent, Dy-DTPA-BMA (1.0 mmol/kg). Gadolinium-enhanced T₁ images depicted reperfused infarction as regions with greatly enhanced signal intensity compared with unin-farcted myocardium, indicating that contrast agent was delivered to the infarcted zone. Dysprosium-enhanced T₁ images depicted the injury as a region of persistent signal intensity relative to depletion of signal in normal myocardium, consistent with failure of the contrast agent to cause signal loss. Similar infarction sizes were observed for unenhanced T₂-weighted images (33 ± 5%), gadolinium-enhanced T₁ weighted images (36 ± 5%) and postmortem staining (30 ± 6%); strong correlations (r > 0.9) were noted in comparisons of these data. Dysprosium-enhanced images exhibited a smaller region of differential signal presumed to be infarction (20 ± 5%, P < 0.05) and weak correlations (r < 0.75) with the other measurements. We conclude that the smaller infarction depicted on dysprosium-enhanced images is a subregion of the true infarction in which myocardial necrosis is sufficiently advanced that the agent is homogeneously distributed throughout all tissue compartments, preventing T₂*-dependent phase loss on spin echo images.     

SWIFT‐CEST: A new MRI method to overcome T2 shortening caused by PARACEST contrast agents

The exchange of water molecules between the inner sphere of a paramagnetic chemical exchange saturation transfer (PARACEST) contrast agent and bulk water can shorten the bulk water T₂ through the T₂‐exchange (T_2ex) mechanism. The line‐broadening T_2ex effect is proportional to the agent concentration, the chemical shift of the exchanging water molecule, and is highly dependent on the water molecule exchange rate. A significant T_2ex contribution to the bulk water linewidth can make the regions of agent uptake appear dark when imaging with conventional sequences like gradient‐echo and fast spin‐echo. The minimum echo times for these sequences (1–10 ms) are not fast enough to capture signal from the regions of shortened T₂. This makes “Off” (saturation at ?Δω) minus “On” (saturation at +Δω) imaging of PARACEST agents difficult, because the regions of uptake are dark in both images. It is shown here that the loss of bulk water signal due to T_2ex can be reclaimed using the ultrashort echo times (<10 μs) achieved with the sweep imaging with Fourier transform pulse sequence. Modification of the sweep imaging with Fourier transform sequence for PARACEST imaging is first discussed, followed by parameter optimization using in vitro experiments. In vivo PARACEST studies comparing fast spin‐echo to sweep imaging with Fourier transform were performed using EuDOTA‐(gly) uptake in healthy mouse kidneys. The results show that the negative contrast caused by T_2ex can be overcome using the ultrashort echo time achieved with sweep imaging with Fourier transform, thereby enabling fast and sensitive in vivo PARACEST imaging. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Modeling 1H exchange: An estimate of the error introduced in MRI by assuming the fast exchange limit in bolus tracking

A simulation is presented which calculates the MRI signal expected from a model tissue for a given pulse sequence after a bolus injection of a contrast agent. The calculation assumes two physiologic compartments only, the intravascular and extravascular spaces. The determination of the concentration of contrast in each compartment as a function of time and position has been outlined in a previous publication (Moran and Prato, Magn Reson Med 2001;45:42–45). These contrast agent concentrations are used here to determine the NMR relaxation times as a function of time and position within the tissue. Knowledge of this simulated tissue ‘map’ of relaxation times as a function of time provides the information required to determine whether the proton exchange rate is fast or slow on the NMR timescale. Since with a bolus injection the concentration of contrast and hence the relaxation time may vary with position along the capillary, some segments of the capillary are allowed to be in fast exchange with the extravascular space, while others may be in slow exchange. Using this information, and parameters specific to a given tissue, the MRI signal for a given pulse sequence is constructed which correctly accounts for differences in proton exchange across the length of the capillary. It is shown that extravascular contrast agents show less signal dependence on water exchange, and thus may be more appropriate for quantitative imaging when using fast exchange assumptions. It is also shown that nondistributed compartment models can incorrectly estimate the water exchange that is occurring at the capillary level if exchange‐minimizing pulse sequences are not used. Magn Reson Med 51:816–827, 2004. © 2004 Wiley‐Liss, Inc.     

T1-relaxation kinetics of extracellular, intracellular and intravascular MR contrast agents in normal and acutely reperfused infarcted myocardium using echo-planar MR imaging

The objective of this study was to determine and compare if MR contrast agents distributed into various compartments can provide estimation of fractional distribution volume (FDV) in normal and infarcted myocardium using inversion recovery echo-planar MR imaging (IR EPI). Three different types of MR agents were investigated: (a) an extracellular agent, GdDTPA-BMA (0.1 mmol/kg); (b) an intravascular agent, GdDTPA-albumin (0.025 mmol/kg); and (c) an intracellular agent, manganese chloride (0.025 mmol/kg). The null point was determined from a series of IR EPI images in which TI was varied. Temporal changes in ΔR1 (ΔR1 = 1/T1_post-1/T1_pre) were measured during the initial 29–59 min after administration. Rats (n = 24) were subjected to 1-h coronary artery occlusion/reperfusion. Histochemical staining confirmed the presence and location of infarction. GdDTPA-BMA caused increase in ΔR1 of infarction < blood < < normal myocardium. ΔR1 ratios were 1.55 ± 0.08 for infarction and 0.33 ± 0.03 for normal myocardium, consistent with FDV of 0.82 ± 0.04 and 0.18 ± 0.01. The fractional distribution of this agent in normal myocardium approximated the extracellular space of myocardium. GdDTPA-albumin caused increase in ΔR1 of blood < < infarction < < normal myocardium. ΔR1 ratio in normal, but not infarcted, myocardium was constant at 0.10 ± 0.02 and approximated fractional blood volume. MnCl₂ caused equivalent increase in ΔR1 of normal and infarcted myocardium. ΔR1 of normal myocardium did not change overtime, whereas ΔR1 of blood rapidly decreased, leading to overestimation of FDV in normal and infarcted myocardium. In conclusion, extracellular, intravascular and intracellular MR contrast agents exhibited different T1-relaxation kinetics in both normal and infarcted myocardium. Constant ΔR1 ratio (myocardium/blood) after administration of MR contrast agent is a prerequisite for estimation of FDV of MR contrast agent in myocardium. Received: 22 December 1998; Revised: 7 April 1999; Accepted: 18 May 1999     

Magnetization exchange in capillaries by microcirculation affects diffusion-controlled spin-relaxation: A model which describes the effect of perfusion on relaxation enhancement by intravascular contrast agents

The effect of perfusion on relaxation time in tissue has only been considered for first-pass kinetics of NMR-signal after application of contrast agents. The importance of perfusion on relaxation has not yet been studied for steady state conditions, i.e., when the intravascular relaxation rate is constant in time. The aim of this study is to develop a model in which T, relaxation is derived as a function of perfusion and intracap-illary volume fraction (regional blood volume). Tissue is considered to be two-compartment system, which consists of intracapillary and extravascular space. Intracapillary relaxation differs from relaxation in the arterial system due to diffusion-exchange of magnetization from extravascular to intracapillary space. Perfusion tends to attenuate this difference and thus counteracts the effect on intracapillary relaxation. Relaxation in the extravascular space becomes a function of perfusion because extravascular and intracapillary magnetization are linked by diffusion. This dependence is presented in analytical form and a generic equation is derived. A T₁ experiment is considered in which all spins of tissue and blood are inverted at the beginning. Calculations are performed for the fast exchange model of tissue. Perfusion increases relaxation enhancement of intravascular contrast agents. This effect is considerable in highly perfused tissue like myocardium. The dependence of relaxation on perfusion implies an overestimation of the regional blood volume when the calculation of the latter is based on tissue models that neglect perfusion. The model presented here is applied to predict the effect of perfusion on T₁ imaging with FLASH-pulse sequences because this technique has been proven to be a powerful method to obtain T₁ maps within a short time interval. For the fast exchange model, two algorithms are suggested that determine perfusion and regional blood volume from T₁ imaging in the presence and absence of intravascular contrast agents.     

Detection of apoptotic cell death in vitro in the presence of Gd‐DTPA‐BMA

Due to variability in patient response to cancer therapy, there is a growing interest in monitoring patient progress during treatment. Apoptotic cell death is one early marker of tumor response to treatment. Using known extracellular concentrations of gadolinium diethylenetriamine pentaacetic acid bismethylamide (Gd‐DTPA‐BMA) to vary the exchange regime, T₁ and T₂ relaxation data for acute myeloid leukemia (AML) cell samples were obtained and then analyzed using a two‐pool model of relaxation with exchange. Leukemia cells treated with cisplatin to induce apoptosis exhibited a statistically significant (P < 0.05) decrease in intracellular longitudinal relaxation time, T_1I, from 1030 ms to 940 ms, a decrease (P < 0.001) in the intracellular water fraction, M_0I, from 0.86 to 0.68 and a statistically significant increase (P < 0.01) in transmembrane water exchange rate, k_IE, from 1.4 s^?1 to 6.8 s^?1. The changes in MR parameters correlated with quantitative histology, such as cellular cross‐sectional area and average nuclear area measurements. The results of this study emphasize the importance of accounting for water exchange in dynamic contrast‐enhanced MRI (DCE‐MRI) studies, particularly those that examine tumor response to therapies in which apoptotic changes occur. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Phase‐sensitive inversion recovery for myocardial T1 mapping with motion correction and parametric fitting

The assessment of myocardial fibrosis and extracellular volume requires accurate estimation of myocardial T₁s. While image acquisition using the modified Look‐Locker inversion recovery technique is clinically feasible for myocardial T₁ mapping, respiratory motion can limit its applicability. Moreover, the conventional T₁ fitting approach using the magnitude inversion recovery images can lead to less stable T₁ estimates and increased computational cost. In this article, we propose a novel T₁ mapping scheme that is based on phase‐sensitive image reconstruction and the restoration of polarity of the MR signal after inversion. The motion correction is achieved by registering the reconstructed images after background phase removal. The restored signal polarity of the inversion recovery signal helps the T₁ fitting resulting in improved quality of the T₁ map and reducing the computational cost. Quantitative validation on a data cohort of 45 patients proves the robustness of the proposed method against varying image contrast. Compared to the magnitude T₁ fitting, the proposed phase‐sensitive method leads to less fluctuation in T₁ estimates. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

A rapid 2D time-resolved variable-rate k-space sampling MR technique for passive catheter tracking during endovascular procedures

A new, fast, 2D MR imaging technique allowing passive catheter visualization adequate for use as a tool for guiding the movement of a catheter during endovascular procedures is described. This imaging technique samples low spatial frequencies more often than high spatial frequencies; it also uses both k-space view sharing and temporal interpolation. Unlike other techniques for passive visualization that exploit magnetic-susceptibility–induced artifacts, we have adopted a strategy that takes advantage of the T₁-shortening effect of paramagnetic contrast agents, such as Gd-DTPA and a projection dephaser. This not only permits visualization of the entire catheter length but also minimizes the risk of intravascular heating. Using this method, a temporal frame rate of up to eight images per second and a tip localization accuracy of ± 1mm (root mean square difference) can be achieved.     

Magnetic resonance perfusion imaging in ischemic heart disease.

This review explores the present status of contrast media available for myocardial perfusion studies, the magnetic resonance (MR) sequences adapted to multi-slice first-pass acquisitions, and the issue of myocardial perfusion quantification. To date, only low molecular weight paramagnetic gadolinium chelates have been used in clinical protocols for myocardial perfusion. With the availability of fast MR acquisition techniques to follow the first-pass distribution of the contrast agent in the myocardium, the bolus tracking technique represents the more widely used protocol in MR perfusion studies. On T1-weighted imaging, the ischemic zone appears with a delayed and lower signal enhancement compared with normally perfused myocardium. Visual analysis of the image series can be greatly improved by image post-processing to obtain relative myocardial perfusion maps. With an intravascular tracer, myocardial kinetics are in theory easier to analyze in terms of perfusion. In experimental studies, different intravascular or blood pool MR contrast agents have been tested to measure quantitative perfusion parameters. If a simple flow-limited kinetic model is developed with MR contrast agents, one important clinical application will be the evaluation of the functional consequence of coronary stenoses, ie, non-invasive evaluation of the coronary reserve.