Edwin Heijman

Magnetic resonance imaging of high intensity focused ultrasound mediated drug delivery from temperature-sensitive liposomes : an in vivo proof-of-concept study

Temperature-sensitive liposomes (TSLs) co-encapsulating doxorubicin and 250 mM [Gd(HPDO3A)(H₂O)] were evaluated for HIFU-mediated drug delivery under MR image guidance. In vitro studies showed simultaneous and quantitative release of the drug and the MRI contrast agent from the lumen of the TSLs at 42°C, while no leakage was observed over 1 h at 37°C. In a proof-of-concept study, local hyperthermia has been applied for 30 min in 9L rat tumors using a clinical MR-HIFU system. The local temperature-triggered release of [Gd(HPDO3A)(H₂O)] was monitored with interleaved T₁ mapping of the tumor tissue. A good correlation between the ΔR₁, the uptake of doxorubicin and the gadolinium concentration in the tumor was found, implying that the in vivo release of doxorubicin from TSLs can be probed in situ with the longitudinal relaxation time of the co-released MRI contrast agent.

Diffusion tensor imaging of left ventricular remodeling in response to myocardial infarction in the mouse

The cardiac muscle architecture lies at the basis of the mechanical and electrical properties of the heart, and dynamic alterations in fiber structure are known to be of prime importance in healing and remodeling after myocardial infarction. In this study, left ventricular remodeling was characterized using diffusion tensor imaging (DTI) in a mouse model of myocardial infarction. Myocardial infarction was induced in mice by permanent ligation of the left anterior descending coronary artery. Serial ex vivo DTI measurements were performed 7, 14, 28, and 60 days after ligation. Apparent diffusion coefficient, fractional anisotropy, the three eigenvalues of the diffusion tensor, and the myofiber disarray served as readout parameters. After myocardial infarction, the mouse hearts displayed extreme wall thinning in the infarcted area, which covered large parts of the apex and extended into the free wall up to the equator. Average heart mass increased by 70% 7–60 days after infarction. Histological analysis showed that the infarct at 7 days consisted of unstructured tissue with residual necrosis and infiltration of macrophages and myofibroblasts. At 14 days after infarction, the necrotic tissue had disappeared and collagen fibers were starting to appear. From 28 to 60 days, the infarct had fully developed into a mature scar. DTI parameters showed dynamic changes as a function of time after infarction. The apparent diffusion coefficient in the infarcted region was lower than in remote regions and increased as a function of time after infarction. The fractional anisotropy was higher in the infarcted region and was maximum at 28 days, which was attributed to the development of structured collagen fibers. Myofiber disarray, which was analyzed by considering the alignment of fibers in neighboring voxels, was significantly higher in infarcted regions. DTI provides a valuable non‐destructive tool for characterizing structural remodeling in diseased myocardium. Copyright

SPECT/CT imaging of temperature-sensitive liposomes for MR-image guided drug delivery with high intensity focused ultrasound

The goal of this study was to investigate the blood kinetics and biodistribution of temperature-sensitive liposomes (TSLs) for MR image-guided drug delivery. The co-encapsulated doxorubicin and [Gd(HPDO3A)(H₂O)] as well as the ¹¹¹In-labeled liposomal carrier were quantified in blood and organs of tumor bearing rats. After TSL injection, mild hyperthermia (T=42 °C) was induced in the tumor using high intensity focused ultrasound under MR image-guidance (MR-HIFU). The biodistribution of the radiolabeled TSLs was investigated using SPECT/CT imaging, where the highest uptake of ¹¹¹In-labeled TSLs was observed in the spleen and liver. The MR-HIFU-treated tumors showed 4.4 times higher liposome uptake after 48 h in comparison with controls, while the doxorubicin concentration was increased by a factor of 7.9. These effects of HIFU-treatment are promising for applications in liposomal drug delivery to tumors.

Tumour hyperthermia and ablation in rats using a clinical MR-HIFU system equipped with a dedicated small animal set-up

Purpose: We report on the design, performance, and specifications of a dedicated set‐up for the treatment of rats on a clinical magnetic resonance high intensity focused ultrasound (MR‐HIFU) system. Materials and methods: The small animal HIFU‐compatible 4‐channel MR receiver volume coil and animal support were designed as add‐on to a clinical 3T Philips Sonalleve MR‐HIFU system. Prolonged hyperthermia (T ≈ 42°C, 15 min) and thermal ablation (T = 65°C) was performed in vivo on subcutaneous rat tumours using 1.44 MHz acoustic frequency. The direct treatment effect was assessed with T2‐weighted imaging and dynamic contrast enhanced (DCE‐) MRI as well as histology. Results: The developed HIFU‐compatible coil provided an image quality that was comparable to conventional small animal volume coils (i.e. without acoustic window), and a SNR increase by a factor of 10 as compared to the coil set‐up used for clinical MR‐HIFU therapy. The use of an animal support minimised far field heating and allowed precise regulation of the animal body core temperature, which varied <1°C during treatment. Conclusions: The results demonstrated that, by using a designated set‐up, both controlled hyperthermia and thermal ablation treatment of malignant tumours in rodents can be performed on a clinical MR‐HIFU system. This approach provides all the advantages of clinical MR‐HIFU, such as volumetric heating, temperature feedback control and a clinical software interface for use in rodent treatment. The use of a clinical system moreover facilitates a rapid translation of the developed protocols into the clinic.

Magnetic resonance guided high-intensity focused ultrasound mediated hyperthermia improves the intratumoral distribution of temperature-sensitive liposomal doxorubicin

ObjectivesThe aim of this study was to investigate the intratumoral distribution of a temperature-sensitive liposomal carrier and its encapsulated compounds, doxorubicin, and a magnetic resonance (MR) imaging contrast agent after high-intensity focused ultrasound (HIFU)–mediated hyperthermia-induced local drug release. Materials and Methods111In-labeled temperature-sensitive liposomes encapsulating doxorubicin and [Gd(HPDO3A) (H2O)] were injected intravenously in the tail vein of rats (n = 12) bearing a subcutaneous rhabdomyosarcoma tumor on the hind leg. Immediately after the injection, local tumor hyperthermia (2 × 15 minutes) was applied using a clinical 3 T MR-HIFU system. Release of [Gd(HPDO3A)(H2O)] was studied in vivo by measuring the longitudinal relaxation rate R1 with MR imaging. The presence of the liposomal carriers and the intratumoral distribution of doxorubicin were imaged ex vivo with autoradiography and fluorescence microscopy, respectively, for 2 different time points after injection (90 minutes and 48 hours). ResultsIn hyperthermia-treated tumors, radiolabeled liposomes were distributed more homogeneously across the tumor than in the control tumors (coefficient of variationhyp, 90 min = 0.7 ± 0.2; coefficient of variationcntrl, 90 min = 1.1 ± 0.2). At 48 hours after injection, the liposomal accumulation in the tumor was enhanced in the hyperthermia group in comparison with the controls. A change in R1 was observed in the HIFU-treated tumors, suggesting release of the contrast agent. Fluorescence images showed perivascular doxorubicin in control tumors, whereas in the HIFU-treated tumors, the delivered drug was spread over a much larger area and also taken up by tumor cells at a larger distance from blood vessels. ConclusionsTreatment with HIFU hyperthermia not only improved the immediate drug delivery, bioavailability, and intratumoral distribution but also enhanced liposomal accumulation over time. The sum of these effects may have a significant contribution to the therapeutic outcome.

Quantification of Left Ventricular Volumes and Ejection Fraction in Mice Using PET, Compared with MRI

PET has become an important noninvasive imaging technique in cardiovascular research for the characterization of mouse models in vivo. This modality offers unique insight into biochemical changes on a molecular level, with excellent sensitivity. However, morphologic and functional changes may be of equal importance for a thorough assessment of left ventricular (LV) pathophysiology. Although echocardiography and MRI are widely considered the imaging techniques of choice for the assessment of these parameters, their use with PET considerably increases study complexity and decreases cost- and time-efficiency. In this study, a novel method for the additional quantification of LV volumes and ejection fraction (EF) from PET was evaluated using cardiac MRI as the reference method. Methods: The radiolabeled glucose derivative 18F-FDG was injected into 33 mice (6 mice with previous permanent occlusion of the left anterior descending artery [LAD], 15 mice with a temporary 30-min occlusion of the LAD, and 12 mice without previous surgery). 18F-FDG uptake within the LV myocardium was measured using a dedicated small-animal PET scanner. After we reconstructed the images into 16 electrocardiogram (ECG)-gated frames, we determined the LV cavity volumes in end-diastole (EDV) and end-systole (ESV) and the EF using a semiautomatic segmentation algorithm based on elastic surfaces. A 6.3-T cardiac MRI examination was performed in the same animals using an ECG-triggered and respiratory-gated multislice cine sequence. The MR images were segmented with a semiautomatic algorithm using commercially available software. Results: Overall, measurements from PET agreed well with those obtained by MRI. Mean EDV and ESV were slightly overestimated by PET (86 ± 43 μL and 44 ± 42 μL), compared with MRI (73 ± 44 μL and 41 ± 46 μL); mean (±SD) EF was similar (PET, 55 ± 19 μL; MRI, 54 ± 18 μL). Correlation between PET and MRI was excellent for EDV (0.97) and ESV (0.96) and good for EF (0.86). The slope of the regression line was nearly perfect for EDV (0.98) and EF (1.01) and slightly below 1 for ESV (0.90), indicating a good separation of abnormal and normal values with PET. The y-intercept was above zero for EDV (15 μL) and ESV (7 μL) and near to zero for EF (0.2%). Conclusion: The quantification of LV volumes and EF in mice with PET is both efficient and accurate. This method allows for combined molecular and functional imaging of the left ventricle within a single scan, obviating additional sophisticated MRI in many cases.

Smoothelin-B Deficiency Results in Reduced Arterial Contractility, Hypertension, and Cardiac Hypertrophy in Mice

Background— Smoothelins are actin-binding proteins that are abundantly expressed in healthy visceral (smoothelin-A) and vascular (smoothelin-B) smooth muscle. Their expression is strongly associated with the contractile phenotype of smooth muscle cells. Analysis of mice lacking both smoothelins (Smtn-A/B−/− mice) previously revealed a critical role for smoothelin-A in intestinal smooth muscle contraction. Here, we report on the generation and cardiovascular phenotype of mice lacking only smoothelin-B (Smtn-B−/−). Methods and Results— Myograph studies revealed that the contractile capacity of the saphenous and femoral arteries was strongly reduced in Smtn-B−/− mice, regardless of the contractile agonist used to trigger contraction. Arteries from Smtn-A/B−/− compound mutant mice exhibited a similar contractile deficit. Smtn-B−/− arteries had a normal architecture and expressed normal levels of other smooth muscle cell–specific genes, including smooth muscle myosin heavy chain, &agr;-smooth muscle actin, and smooth muscle-calponin. Decreased contractility of Smtn-B−/− arteries was paradoxically accompanied by increased mean arterial pressure (20 mm Hg) and concomitant cardiac hypertrophy despite normal parasympathetic and sympathetic tone in Smtn-B−/− mice. Magnetic resonance imaging experiments revealed that cardiac function was not changed, whereas distension of the proximal aorta during the cardiac cycle was increased in Smtn-B−/− mice. However, isobaric pulse wave velocity and pulse pressure measurements indicated normal aortic distensibility. Conclusions— Collectively, our results identify smoothelins as key determinants of arterial smooth muscle contractility and cardiovascular performance. Studies on mutations in the Smtn gene or alterations in smoothelin levels in connection to hypertension in humans are warranted.

Evaluation of manual and automatic segmentation of the mouse heart from CINE MR images

To compare global functional parameters determined from a stack of cinematographic MR images of mouse heart by a manual segmentation and an automatic segmentation algorithm.

Magnetic resonance imaging of regional cardiac function in the mouse

In this paper we introduce an improved harmonic phase (HARP) analysis for complementary spatial modulation of magnetization (CSPAMM) tagging of the mouse left ventricular wall, which enables the determination of regional displacement fields with the same resolution as the corresponding CINE anatomical images. CINE MRI was used to measure global function, such as the ejection fraction. The method was tested on two healthy mouse hearts and two mouse hearts with a myocardial infarction, which was induced by a ligation of the left anterior descending coronary artery. We show that the regional displacement fields can be determined. The mean circumferential strain for the left ventricular wall of one of the healthy mice was −0.09 ± 0.04 (mean ± standard deviation), while for one of the infarcted mouse hearts strains of −0.02 ± 0.02 and −0.10 ± 0.03 were found in the infarcted and remote regions, respectively.

Multiparametric MRI analysis for the evaluation of MR-guided high intensity focused ultrasound tumor treatment

For the clinical application of high intensity focused ultrasound (HIFU) for thermal ablation of malignant tumors, accurate treatment evaluation is of key importance. In this study, we have employed a multiparametric MRI protocol, consisting of quantitative T1, T2, ADC, amide proton transfer (APT), T1ρ and DCE‐MRI measurements, to evaluate MR‐guided HIFU treatment of subcutaneous tumors in rats. K‐means clustering using all different combinations of the endogenous contrast MRI parameters (feature vectors) was performed to segment the multiparametric data into tissue populations with similar MR parameter values. The optimal feature vector for identification of the extent of non‐viable tumor tissue after HIFU treatment was determined by quantitative comparison between clustering‐derived and histology‐derived non‐viable tumor fractions. The highest one‐to‐one correspondence between these clustering‐based and histology‐based non‐viable tumor fractions was observed for the feature vector {ADC, APT‐weighted signal} (R2 to line of identity (R2y=x) = 0.92) and the strongest agreement was seen 3 days after HIFU (R2y=x = 0.97). To compare the multiparametric MRI analysis results with conventional HIFU monitoring and evaluation methods, the histology‐derived non‐viable tumor fractions were also quantitatively compared with non‐perfused tumor fractions (derived from the level of contrast enhancement in the DCE‐MRI measurements) and 240 CEM tumor fractions (i.e. thermal dose > 240 cumulative equivalent minutes at 43 °C). The correlation between histology‐derived non‐viable tumor fractions directly after HIFU and the 240 CEM fractions was high, but not significant. The non‐perfused fractions overestimated the extent of non‐viable tumor tissue directly after HIFU, whereas an underestimation was observed 3 days after HIFU.