Magnetic resonance imaging enhanced by superparamagnetic iron oxide particles: Usefulness for distinguishing between focused ultrasound‐induced blood–brain barrier disruption and brain hemorrhage

Purpose

To investigate the usefulness of a fully flow‐compensated heavy T2*‐weighted imaging enhanced by superparamagnetic iron oxide (SPIO) particles for distinguishing between focused ultrasound‐induced disruption of blood–brain barrier (BBB) and brain hemorrhage.

Materials and Methods

Focused ultrasound (frequency: 1.5 MHz) was used to induce disruption of the BBB in 39 rats. Two T2*‐weighted images were obtained before and after SPIO administration. Preenhanced T2*‐weighted images were used to detect hemorrhage. Detection of BBB disruption was performed on SPIO‐enhanced images. Thirty‐four rats were sacrificed after magnetic resonance (MR) scanning for histological confirmation of brain lesions. Theremaining five animals were followed up for 35 days. Prussian blue staining was performed on histological sections to detect SPIO particles in the brain.

Results

After SPIO injection the areas of BBB disruption in rat brain were significantly enlarged. The area of mismatch between the T2*‐weighted images indicated a safe region where BBB opening occurred without hemorrhagic complications. In the longitudinal study, removal of SPIO occurred at a faster rate in hemorrhagic areas, albeit being closer to that occurring in the liver. The presence of SPIO was confirmed by Prussian blue staining in brain parenchyma and capillary endothelial cells in areas of BBB disruption.

Conclusion

T2*‐weighted images—either with and without SPIO enhancement—may differentiate focused ultrasound‐induced BBB disruption from brain hemorrhage. J. Magn. Reson. Imaging 2009;29:31–38. © 2008 Wiley‐Liss, Inc.     

Noninvasive MR imaging-guided focal opening of the blood-brain barrier in rabbits

PURPOSE: To determine if focused ultrasound beams can be used to locally open the blood-brain barrier without damage to surrounding brain tissue and if magnetic resonance (MR) imaging can be used to monitor this procedure. MATERIALS AND METHODS: The brains of 18 rabbits were sonicated (pulsed sonication) in four to six locations, with temporal peak acoustic power ranging from 0.2 to 11.5 W. Prior to each sonication, a bolus of ultrasonographic (US) contrast agent was injected into the ear vein of the rabbit. A series of fast or spoiled gradient-echo MR images were obtained during the sonications to monitor the temperature elevation and potential tissue changes. Contrast material-enhanced MR images obtained minutes after sonications and repeated 1-48 hours later were used to depict blood-brain barrier opening. Whole brain histologic evaluation was performed. RESULTS: Opening of the blood-brain barrier was confirmed with detection of MR imaging contrast agent at the targeted locations. The lowest power levels used produced blood-brain barrier opening without damage to the surrounding neurons. Contrast enhancement correlated with the focal signal intensity changes in the magnitude fast spoiled gradient-echo MR images. CONCLUSION: The blood-brain barrier can be consistently opened with focused ultrasound exposures in the presence of a US contrast agent. MR imaging signal intensity changes may be useful in the detection of blood-brain barrier opening during sonication.     

增强MRI评价可逆性血脑屏障开放的可靠性及价值

目的评价增强ＭＲＩ在监测可逆性血脑屏障（ＢＢＢ）开放的可靠性及价值。方法３６只家兔被随机分为３组：Ａ、Ｂ两组分别经颈内动脉灌注２０％甘露醇和生理盐水，Ｃ组经耳缘静脉灌注２０％甘露醇。灌注后，立即用０．５Ｔ超导型ＭＲ机进行兔脑冠状位扫描，随后经耳缘静脉注入ＧｄＤＴＰＡ，迅速行同层间隙扫描（增强后５、１０、１５、２０分钟，之后每隔１０分钟），持续２小时。以该层双侧对称的脑实质为兴趣区，测得不同时间兴趣区信号强度（ＳＩ），求其增强率（％）。各组１０只兔于扫描前经耳缘静脉注入２％Ｅｖｅｎｓ蓝（２ｍｌ／ｋｇ），２小时后处死；另有６只分别于灌注后２．５、３小时注入Ｅｖｅｎｓ蓝，１０分钟后处死，取脑观察有无脑实质蓝染，以此来推测ＢＢＢ开放持续时间。所有脑标本送病理检查，并与对应层的ＭＲＩ进行比较。结果Ａ组１０只皆出现灌注侧脑实质增强及蓝染，Ｂ、Ｃ组无一只出现明显强化及蓝染。ＢＢＢ开放持续时间小于３小时。未发现死亡及脑实质病理改变。结论增强ＭＲＩ是监测ＢＢＢ开放过程、指导临床个体化用药的一种理想方法。     

In vivo MR quantification of superparamagnetic iron oxide nanoparticle leakage during low-frequency-ultrasound-induced blood-brain barrier opening in swine

Purpose:

To verify that low‐frequency planar ultrasound can be used to disrupt the BBB in large animals, and the usefulness of MRI to quantitatively monitor the delivery of superparamagnetic iron oxide (SPIO) nanoparticles into the disrupted regions.

Materials and Methods:

Two groups of swine subjected to craniotomy were sonicated with burst lengths of 30 or 100 ms, and one group of experiment was also performed to confirm the ability of 28‐kHz sonication to open the BBB transcranially. SPIO nanoparticles were administered to the animals after BBB disruption. Procedures were monitored by MRI; SPIO concentrations were estimated by relaxivity mapping.

Results:

Sonication for 30 ms created shallow disruptions near the probe tip; 100‐ms sonications after craniotomy can create larger and more penetrating openings, increasing SPIO leakage ～3.6‐fold than 30‐ms sonications. However, this was accompanied by off‐target effects possibly caused by ultrasonic wave reflection. SPIO concentrations estimated from transverse relaxation rate maps correlated well with direct measurements of SPIO concentration by optical emission spectrometry. We have also shown that transcranial low‐frequency 28‐kHz sonication can induce secure BBB opening from longitudinal MR image follow up to 7 days.

Conclusion:

This study provides valuable information regarding the use low‐frequency ultrasound for BBB disruption and suggest that SPIO nanoparticles has the potential to serve as a thernostic agent in MRI‐guided ultrasound‐enhanced brain drug delivery. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.     

Iron-oxide labeling of hematogenous macrophages in a model of experimental autoimmune encephalomyelitis and the contribution to signal loss in fast imaging employing steady state acquisition (FIESTA) images

PURPOSE: To determine the contribution of blood-derived macrophages to the signal loss observed in MR images of inflammatory lesions in experimental autoimmune encephalomyelitis (EAE). MATERIALS AND METHODS: A relapsing-remitting form of EAE was induced in transgenic mice that express enhanced green fluorescent protein (EGFP) specifically in hematopoietic cells of the myelomonocytic lineage. Animals were injected with Feridex, a superparamagnetic iron oxide (SPIO) nanoparticle, 24 hours prior to in vivo MRI. MRI was performed using a 1.5T whole-body scanner; a high-performance, custom-built gradient coil insert; and a 3D steady-state free precession (SSFP) imaging pulse sequence. Comparisons were made between MR images and corresponding anti-GFP and Perl's Prussian blue (PPB)-stained brain sections. RESULTS: MR images revealed the presence of discrete regions of signal loss throughout the brains of EAE animals that were administered Feridex. Histological staining showed that regions of signal loss on MR images corresponded anatomically with regions of PPB- and GFP-positive cells. CONCLUSION: This experiment provides the first direct evidence that macrophages of hematogenous origin are labeled with SPIO after intravenous administration of Feridex, and contribute to the regions of signal loss detected in MR images of EAE brain.     

犬颅脑枪弹伤后血脑屏障的改变及意义

目的　探讨犬脑枪弹伤后血脑屏障 (BBB)通透性改变及意义。方法　杂种犬 12只 ,随机分成正常对照、脑贯通伤 (PCI)15min、1h、6h组 (n=3) ,采用德国小口径步枪 (子弹型号 5 5 6mm ,弹头质量 2 5 7g)按上述分组要求致伤。硝酸镧心内灌注法固定犬脑 ,透射电镜下观察BBB改变。结果　犬脑枪弹伤后 15min、1h组均出现BBB改变 ,紧密连接开放 ,血管基膜外以至组织间隙内存在硝酸镧颗粒 ,后者更明显 ;至 6h组BBB改变愈加严重 ,可见大量硝酸镧颗粒经BBB进入组织间隙 ,与 15min、1h组比较差异有统计学意义 (P <0 0 5 )。结论　BBB的开放是造成颅脑火器伤后早期脑水肿的原因之一。     

Applicability and limitations of MR tracking of neural stem cells with asymmetric cell division and rapid turnover: the case of the shiverer dysmyelinated mouse brain.

LacZ-transfected C17.2 neural stem cells (NSCs) were labeled with the superparamagnetic iron oxide formulation Feridex prior to ICV injection in shi/shi neonates. Feridex labeling did not alter cell differentiation in vitro and in vivo. Initially, MR images obtained at 11.7T correlated closely to NSC distribution as assessed with anti-dextran and anti-beta-galactosidase double-fluorescent immunostaining. However, at 6 days postgrafting there was already a pronounced mismatch between the hypointense MR signal and the histologically determined cell distribution, with a surprisingly sharp cutoff rather than a gradual decrease of signal. Positive in vivo BrdU labeling of NSCs showed that significant cell replication occurred post-transplantation, causing rapid dilution of Feridex particles between mother and daughter cells toward undetectable levels. Neural differentiation experiments demonstrated asymmetric cell division, explaining the observed sharp cutoff. At later time points (2 weeks), the mismatch further increased by the presence of non-cell-associated Feridex particles resulting from active excretion or cell death. These results are a first demonstration of the inability of MRI to track rapidly dividing and self-renewing, asymmetrically dividing SCs. Therefore, MR cell tracking should only be applied for nonproliferating cells or short-term monitoring of highly-proliferative cells, with mitotic symmetry or asymmetry being important for determining its applicability.     

Magnetically labeled cells can be detected by MR imaging

To determine the feasibility of MR imaging of magnetically labeled cells, different cell lines were labeled with monocrystalline iron oxide (MION) particles. Phantoms containing MION labeled cells were then assembled and imaged by MR at 1.5 T using T1-weighted and T2-weighted pulse sequences. MION uptake ranged from 8.5 × 10⁴ to 2.9 × 10⁵ particles/cell for tumor cells (9L and LX1, respectively) to 1.5 × 10⁶ to 4.8 × 10⁸ particles/cell for “professional phagocytes” (J774 and peritoneal macrophages, respectively). On the T1-weighted images, cell-internalized MION appeared hyperintense relative to agar and similar to MION in aqueous solution. On T2-weighted images, signal intensity varied according to concentration of MION within cells. Cell-internalized MION caused similar MR signal changes of cells as did free MION; however, at a dose that was an order of magnitude lower, depending on the pulse sequence used. The detectability of MION within cells was approximately 2 ng Fe, which corresponded to 10⁵ tumor cells/well or 5 × 10³ macrophages/well. We conclude that a variety of cells can be efficiently labeled with MION by simple incubation. Intracellular labeling may be used for MR imaging of in vivo cell tracking.     

Contrast enhanced MRI. Evaluation of a canine model of osmotic blood-brain barrier disruption

An osmotic model of blood-brain barrier (BBB) disruption was studied by magnetic resonance (MR) imaging (0.5 T) in 17 canines. The animals were killed after imaging and the lesions confirmed on gross pathology by the presence of Evans blue dye. No accompanying cerebral edema was demonstrated on histologic examination. The disrupted BBB could be identified in only one of five control animals on unenhanced MRI, despite the use of calculated T1 and T2 images. In a second group of five animals, the area of abnormal vascular permeability was consistently demonstrated after IV injection of 0.25 mmol/kg Gd DTPA. The time course of enhancement was evaluated in four additional animals. The brain tissue concentration of the gadolinium ion responsible for the observed enhancement was determined by ion coupled plasma analysis in the last three canines. In a study of pulse techniques, spin echo sequences with both short TRs and TEs (ie, SE 500/30) and inversion recovery techniques proved to be most efficacious for the detection of contrast enhancement. However, contrast could be demonstrated on more T2 weighted sequences.     

Comparison of two superparamagnetic viral-sized iron oxide particles ferumoxides and ferumoxtran-10 with a gadolinium chelate in imaging intracranial tumors

BACKGROUND AND PURPOSE: Ultrasmall superparamagnetic iron oxide particles result in shortening of T1 and T2 relaxation time constants and can be used as MR contrast agents. We tested four hypotheses by evaluating MR images of intracranial tumors after infusion of two iron oxide agents in comparison with a gadolinium chelate: 1) Ferumoxtran in contrast to ferumoxides can be used as an intravenous MR contrast agent in intracranial tumors; 2) ferumoxtran enhancement, albeit delayed, is similar to gadolinium enhancement; 3) ferumoxtran-enhanced MR images in contrast to gadolinium-enhanced MR images may be compared with histologic specimens showing the cellular location of iron oxide particles; 4) ferumoxtran can serve as a model for viral vector delivery. METHODS: In 20 patients, ferumoxides and ferumoxtran were intravenously administered at recommended clinical doses. MR imaging was performed 30 minutes and 4 hours after ferumoxides infusion (n = 3), whereas ferumoxtran-enhanced MR imaging (n = 17) was performed 6 and 24 hours after infusion in the first five patients and 24 hours after infusion in the remaining 12. MR sequences were spin-echo (SE) T1-weighted, fast SE T2- and proton density-weighted, gradient-recalled-echo T2*-weighted, and, in four cases, echo-planar T2-weighted sequences. Representative regions of interest were chosen on pre- and postcontrast images to compare each sequence and signal intensity. RESULTS: Despite some degree of gadolinium enhancement in all tumors, no significant T1 or T2 signal intensity changes were seen after ferumoxides administration at either examination time. Fifteen of 17 patients given ferumoxtrans had T1 and/or T2 shortening consistent with iron penetration into tumor. Histologic examination revealed minimal iron staining of the tumor with strong staining at the periphery of the tumors. CONCLUSION: 1) Ferumoxtran can be used as an intravenous MR contrast agent in intracranial tumors, mostly malignant tumors. 2) Enhancement with ferumoxtran is comparable to but more variable than that with the gadolinium chelate. 3) Histologic examination showed a distribution of ferumoxtran particles similar to that on MR images, but at histology the cellular uptake was primarily by parenchymal cells at the tumor margin. 4) Ferumoxtran may be used as a model for viral vector delivery in malignant brain tumors.     

A quantitative pressure and microbubble‐size dependence study of focused ultrasound‐induced blood‐brain barrier opening reversibility in vivo using MRI

Focused ultrasound in conjunction with the systemic administration of microbubbles has been shown to open the blood‐brain barrier (BBB) selectively, noninvasively and reversibly. In this study, we investigate the dependence of the BBB opening's reversibility on the peak‐rarefactional pressure (0.30–0.60 MPa) as well as the microbubble size (diameters of 1–2, 4–5, or 6–8 μm) in mice using contrast‐enhanced T₁‐weighted (CE‐T₁) MR images (9.4 T). Volumetric measurements of the diffusion of Gd‐DTPA‐BMA into the brain parenchyma were used for the quantification of the BBB‐opened region on the day of sonication and up to 5 days thereafter. The volume of opening was found to increase with both pressure and microbubble diameter. The duration required for closing was found to be proportional to the volume of opening on the day of opening, and ranged from 24 h, for the smaller microbubbles, to 5 days at high peak‐rarefactional pressures. Overall, larger bubbles did not show significant differences. Also, the extent of BBB opening decreased radially towards the focal region until the BBB's integrity was restored. In the cases where histological damage was detected, it was found to be highly correlated with hyperintensity on the precontrast T₁ images. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Uptake of dextran-coated monocrystalline iron oxides in tumor cells and macrophages

Although several dextran-coated iron oxide preparations are in preclinical and clinical use, little is known about the mechanism of uptake into cells. As these particles have been shown to accumulate in macrophages and tumor cells, we performed cellular uptake and inhibition studies with a prototypical monocrystalline iron oxide nanoparticle (MION). MION particles were labeled with fluorescein isothiocyanate or radioiodinated and purified by gel permeation chromatography. Two preparations of MION particles were used in cell experiments: nontreated MION and plasma-opsonized MION purified by gradient density purification. As determined by immunoblotting, opsonization resulted in C3, vitro-nectin, and fibronectin association with MION. Incubation of cells with fluorescent MION showed active uptake of particles in macrophages both before and after opsonization. In C6 tumor cells, however, intracellular MION was only detectable in dividing cells. Quantitatively, ¹²⁵I-labeled MION was internalized into cells with uptake values ranging from 17 ng (in 9L gliosarcoma) to 970 ng iron per million cells for peritoneal macrophages. Opsonization increased MION uptake into macrophages sixfold, whereas it increased the uptake in C6 tumor cells only twofold. Results from uptake inhibition assay suggest that cellular uptake of nonopsonized (dextran-coated) MION particles is mediated by fluid-phase endocytosis, whereas receptor-mediated endocytosis is presumably responsible for the uptake of opsonized (protein-coated) particles.     

Mechanisms of blood-brain barrier breakdown

Summary The functional status of the blood-brain barrier (BBB) must be taken into account when designing and interpreting brain imaging techniques. The integrity of the BBB is affected in many diseases of the brain, with the potential involvement of a number of different but poorly understood cellular mechanisms. Factors known to disrupt the BBB experimentally include arachidonic acid and the eicosanoids, bradykinin, histamine and free radicals. These active compounds, released in pathological tissue, may alter cytosolic calcium levels and induce second messenger systems leading to an alteration in BBB permeability. Extravasation of plasma proteins may occurvia disrupted tight junctions, stimulation of fluid-phase vesicular transport or the formation of transcellular pores or channels.     

Contrast media-induced blood-brain barrier damage. Potentiation by hypertension

Large doses of an ionic contrast medium (CM) can disrupt the blood brain barrier (BBB) osmotically. Acute hypertension (HT) also is known to open the BBB. We tested the hypothesis that these two factors potentiate each other in a rat model. Adult male Wistar rats, anesthetized with pentobarbital, underwent tracheostomy. An external carotid artery catheter was placed so that it opened into the patent common carotid artery; arterial blood pressure was recorded continuously. Of three groups of animals, two (HT) groups received metaraminol to elevate and maintain blood pressure in the range of 165 to 190 mm Hg. The third (normotensive) group received an equivalent volume of saline. Five minutes after injection of Evan's blue, either sodium/meglumine diatrizoate or saline was infused into the carotid cannula (2 mL in 30 seconds). Twenty minutes later the cardiovascular system was flushed with saline, and the brain was removed, frozen, and sectioned for gross and histofluorescent microscopic examination of BBB opening. The carotid injection of CM at a concentration of 1000 mosm/kg water did not produce gross evidence of BBB opening in the normotensive group. Similarly, hypertension at levels below 190 mm Hg did not produce gross evidence of opening in the carotid saline group. However, the combination of carotid CM and HT produced significant BBB opening. These results suggest that the risk factor of acute HT potentiates CM-induced BBB opening.     

Iron particles enhance visualization of experimental gliomas with high-resolution sonography

BACKGROUND AND PURPOSE: Intraoperative MR imaging and sonography are used for navigation during neurosurgical procedures. The purpose of this experimental study was to evaluate the potential of high-resolution sonography using superparamagnetic iron oxide (SPIO) particles as a contrast medium to delineate brain tumors and to relate these findings with those of MR imaging. METHODS: C6 gliomas were implanted in 36 rats. Eleven days after tumor implantation, the animals underwent MR imaging with a 1.5-T MR imaging unit. Twelve animals received gadopentetate dimeglumine immediately before the MR examination, 12 animals were injected with SPIO particles 24 hours before MR imaging, and 12 animals received no contrast agent. Immediately after MR imaging, the animals were sacrificed and their brains were removed and placed in saline. Sonography was performed instantly after brain removal. Brains were embedded in paraffin, and sections were stained for iron with Perl's stain and for macrophages with ED-1 immunohistochemistry. RESULTS: At MR imaging, the tumors appeared hyperintense on T2-weighted and gadolinium-enhanced T1-weighted images. After application of SPIO particles, they became markedly hypointense on T2-weighted images and hypo- to hyperintense on T1-weighted images. On sonograms, gliomas were iso- to slightly hyperechoic to normal brain parenchyma on nonenhanced and on gadolinium-enhanced images. After application of SPIO particles, tumors became markedly hyperechoic and were distinctly demarcated from the surrounding brain tissue. CONCLUSION: SPIO particles improved the detection and demarcation of the experimental gliomas on sonograms, which may improve intraoperative neuronavigation with sonography.     

Intravascular contrast media and the blood-brain barrier. Testing the new nonionic agent ioxilan

The effects of diatrizoate, iohexol, and ioxilan on the blood-brain barrier (BBB) were investigated in normal and hypertensive rats. Anesthetized Wistar rats received 14C-inulin as an indicator for BBB disruption. Diatrizoate, iohexol, or ioxilan (350 and 175 mgI/mL) or normal saline was then injected into the carotid artery (2 mL in 30 seconds). Twenty minutes later, the cardiovascular system was flushed, the brain removed, and each hemisphere was digested. BBB disruption, expressed as counts/minute/mg protein, was compared for each hemisphere in each group by analysis of variance. BBB damage in the diatrizoate-350 group was significantly greater than that in all other groups. No significant BBB damage resulted from iohexol or ioxilan relative to normal saline.     

Evaluation of blood-brain barrier permeability changes in rhesus monkeys and man using 82Rb and positron emission tomography

Dynamic positron tomography of the brain with 82 Rb, obtained from a portable generator [82Sr (25 days) - 82 Rb (76 sec)], provides a means of studying blood-brain barrier (BBB) permeability in physiological and clinical investigations. The BBB in rhesus monkeys was opened unilaterally be intracarotid infusion of 3 M urea. This osmotic barrier opening allowed entry into the brain of intravenously administered rubidium chloride. The BBB opening was demonstrated noninvasively using 82Rb and positron emission tomography and corroborated by the accumulation of 86Rb in tissue samples. Positron emission tomography studies can be repeated every 5 min and indicate that dynamic tomography or static imaging can be used to study BBB permeability changes induced by a wide variety of noxious stimuli. Brain tumors in human subjects are readily detected because of the usual BBB permeability disruption in and around the tumors.     

Polyclonal human immunoglobulin G labeled with polymeric iron oxide: antibody MR imaging

Human polyclonal immunoglobulin (Ig) G was attached to a monocrystalline iron oxide nanocompound (MION), a small superparamagnetic probe developed for receptor and antibody magnetic resonance (MR) imaging. The resulting complex, MION-IgG, had a slightly negative surface charge, a molecular weight of 150-180 kDa, and 0.36 microgram of IgG attached per milligram of iron. After intravenous administration of MION-IgG to normal rats, most of the compound localized in liver, spleen, and bone marrow. In an animal model of myositis, MION-IgG caused reduced signal intensity (most apparent on T2-weighted spin-echo and gradient-echo images) at the site of inflammation. No change in signal intensity existed after an injection of unlabeled MION. Site-specific localization of MION-IgG was corroborated with scintigraphic imaging with indium-111 IgG and MION-In-111-IgG and was confirmed histologically with iron staining. These results indicate that antibody MR imaging is feasible in vivo. Target-specific and antibody MR imaging could be easily extended to other applications, including detection of cancer, infarction, and degenerative diseases.     

Blood-brain barrier injury following intracarotid injection of radiographic contrast media. In vivo quantification using magnetic resonance imaging and Gd-DTPA

Changes in signal intensity of the brain at magnetic resonance (MR) imaging before and after Gd-DTPA were used for in vivo quantification of injury to the blood-brain barrier (BBB). Immediately following intracarotid injection of 2 ml/kg of radiographic contrast medium (CM) 0.4 mmol/kg of Gd-DTPA was injected intravenously. MR imaging was performed with a 400/25 partial saturation pulse sequence. The maximum percentage changes (mean +/- SD) in signal intensity of the brain after CM and Gd-DTPA were 1.6 +/- 1.6% with saline, 3.2 +/- 2.0% with iotrolan, 4.3 +/- 1.7% with iohexol, 6.6 +/- 3.6% with ioxaglate and 8.2 +/- 3.6% with diatrizoate. Not only the osmolality but also the ionicity and chemotoxicity seemed to influence Gd-DTPA leakage. A subtle BBB injury had a stronger tendency to occur in the basal ganglia than in the cerebral cortex. MR enhancement is proposed as a sensitive method for in vivo quantification of the BBB injury caused by intracarotid CM injection.     

Novel double contrast MRI technique for intramyocardial detection of percutaneously transplanted autologous cells.

Bone marrow cells (BMC) labeled with iron particles can be injected into the heart and detected with MRI. Improvement in conspicuity of labelled cells would be advantageous. This study examined if double contrast with iron oxide and Gd-DTPA enhances cell MRI after transvascular transplantation in myocardial infarction. Ten pigs with week-old myocardial infarction had transvascular peri-infarct delivery of microspheres alone (Group I, n = 3) or mixed with iron-labeled BMCs (Group II, n = 7). Gradient-echo MRI before and 1 min after systemic Gd-DTPA administration produced regions of interest with hypoenhancement that were compared to contralateral regions for contrast-to-noise (CNR) and signal-to-noise (SNR) ratios. All hearts were harvested for gross and microscopic analysis. Areas of focal hypoenhancement corresponding to the BMCs were detected in the myocardium in Group II. Early after administration of Gd-DTPA CNR increased from 17.58 +/- 8.5 to 27.25 +/- 15.8 (P < 0.05) and SNR from 24.87 +/- 9.6 to 35.08 +/- 15.5 (P < 0.05). There was no hypoenhancement in Group I. Tissue examination confirmed presence of iron-containing cells and microspheres in corresponding segments of the heart. The distribution of microspheres was similar between the groups. Double contrast with cellular iron and Gd-DTPA in surrounding myocardium resulted in improved cell localization by MRI.