Correlation of MR imaging and histologic findings in mouse melanoma

Single shot echo planar imaging (ssEPI) vs single shot turbo spin echo (ssTSE) DWI of the orbit in patients with ocular melanoma

OBJECTIVES

Diffusion weighted imaging (DWI) has become important for orbital imaging. However, the echoplanar imaging (EPI) DWI has inherent obstacles due to susceptibility to magnetic field inhomogeneities. We conducted a comparative study assessing the image quality of orbits in a patient cohort with uveal melanoma (UM). We hypothesized that single shot turbo spin echo (ssTSE) DWI would have better image quality in terms of less distortion and artifacts and yield better tissue evaluation compared to ssEPI-DWI.

METHODS

ssEPI-DWI and ssTSE-DWI of orbits were obtained from 50 patients with uveal melanoma who were prospectively enrolled in the study. Distortion ratio (DR), signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), diffusion signal properties, and apparent diffusion coefficient (ADC) values were collected and compared between ssEPI-DWI and ssTSE-DWI. Two reviewers evaluated and compared the geometric distortion, susceptibility and ghosting artifacts, resolution, demarcation of ocular mass, and overall quality.

RESULTS

A higher DR was found in ssEPI-DWI compared to ssTSE-DWI (p < 0.001). SNR and CNR were lower for the temporal lobe cortex (p ≤ 0.004), but higher for melanoma in ssEPI-DWI than ssTSE-DWI (p ≤ 0.037). Geometric distortion and artifacts were more common in ssEPI-DWI (p < 0.001). Resolution (p ≤ 0.013) and overall quality (p < 0.001) were better in ssTSE-DWI. Ocular masses were demarcated better on ssEPI-DWI (p ≤ 0.002). Significant negative correlations between T1 and T2 signal intensities (r = -0.369, p ≤ 0.008) and positive correlations between T2 and both DWI signal intensities (r = 0.686 and p < 0.001 for ssEPI-DWI, r = 0.747 and p < 0.001 for ssTSE-DWI) were revealed.

CONCLUSION

With less geometric distortion and susceptibility artifacts, better resolution, and overall quality, ssTSE-DWI can serve as an alternative to ssEPI-DWI for orbital DWI.

ADVANCES IN KNOWLEDGE

ssTSE-DWI can be a better alternative of diffusion imaging of orbits with less susceptibility artifact and geometric distortion compared to ssEPI-DWI.

Molecular imaging probes for multi-spectral optoacoustic tomography

In this review, we discuss recent progress in emerging optoacoustic probes, their mechanisms, applications and challenges for biological imaging using MSOT.

Vaccinia virus-mediated melanin production allows MR and optoacoustic deep tissue imaging and laser-induced thermotherapy of cancer

We reported earlier the delivery of antiangiogenic single chain antibodies by using oncolytic vaccinia virus strains to enhance their therapeutic efficacy. Here, we provide evidence that gene-evoked production of melanin can be used as a therapeutic and diagnostic mediator, as exemplified by insertion of only one or two genes into the genome of an oncolytic vaccinia virus strain. We found that produced melanin is an excellent reporter for optical imaging without addition of substrate. Melanin production also facilitated deep tissue optoacoustic imaging as well as MRI. In addition, melanin was shown to be a suitable target for laser-induced thermotherapy and enhanced oncolytic viral therapy. In conclusion, melanin as a mediator for thermotherapy and reporter for different imaging modalities may soon become a versatile alternative to replace fluorescent proteins also in other biological systems. After ongoing extensive preclinical studies, melanin overproducing oncolytic virus strains might be used in clinical trials in patients with cancer.

Magnetic resonance imaging of tumor necrosis

BACKGROUND

The prognostic and predictive value of magnetic resonance (MR) investigations in clinical oncology may be improved by implementing strategies for discriminating between viable and necrotic tissue in tumors. The purpose of this preclinical study was to investigate whether the extent of necrosis in tumors can be assessed by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and/or T(2)-weighted MR imaging.

MATERIAL AND METHODS

Three amelanotic human melanoma xenograft lines differing substantially in tumor necrotic fraction, necrotic pattern, extracellular volume fraction, and blood perfusion were used as experimental models of human cancer. MRI was performed at 1.5 T and a spatial resolution of 0.23 × 0.47 × 2.0 mm(3). Gadolinium diethylene-triamine penta-acetic acid (Gd-DTPA) was used as contrast agent. Plots of Gd-DTPA concentration versus time were generated for each voxel, and three parameters were calculated for each curve: the extracellular volume fraction (ν(e)), the final slope (a), and the Gd-DTPA concentration at one minute after the contrast administration (C(1min)). Parametric images of ν(e), a, C(1min), and the signal intensity in T(2)-weighted images (SI(T2W)) were compared with the histology of the imaged tissue.

RESULTS

The ν(e), a, and C(1min) frequency distributions were significantly different for necrotic and viable tissue in all three tumor lines. By using adequate values of ν(e), a, and C(1min) to discriminate between necrotic and viable tissue, significant correlations were found between the fraction of necrotic tissue assessed by MRI and the fraction of necrotic tissue assessed by image analysis of histological preparations. On the other hand, the SI(T2W) frequency distributions did not differ significantly between necrotic and viable tissue in two of the three tumor lines.

CONCLUSION

Necrotic regions in tumor tissue can be identified in parametric images derived from DCE-MRI series, whereas T(2)-weighted images are unsuitable for detection of tumor necrosis.

Developing MR reporter genes: promises and pitfalls

MR reporter genes have the potential to monitor transgene expression non-invasively in real time at high resolution. These genes can be applied to interrogate the efficacy of gene therapy, to assess cellular differentiation, cell trafficking, and specific metabolic activity, and also assess changes in the microenvironment. Efforts toward the development of MR reporter genes have been made for at least a decade, but, despite these efforts, the field is still in its early developmental stage. This reflects the fact that there are potential pitfalls, caused by the low sensitivity of detection, the need for substrates with their associated undesirable pharmacokinetics, and/or the difficult and, in some cases, delayed interpretation of signal changes. Nevertheless, significant progress has been made during the last few years. Whereas enzyme-based reporters were initially applied to NMR spectroscopic monitoring of changes in phosphor and fluorine metabolism, MRI-based approaches are now emerging that rely on: (1) enzyme-based cleavage of functional groups that block water (proton) exchange or protein binding of MR contrast agents; (2) expression of surface receptors that enable binding of specific MR contrast agents; (3) expression of para- and anti-ferromagnetic (metallo)proteins involved with iron metabolism, such as tyrosinase, transferrin receptor, and ferritin. After an introduction to the basic principles of designing promoters, expression vectors, and cloning of transgenes, a fresh look is provided on the use of reporter genes for optical (including bioluminescent) and nuclear imaging, with which MR reporter genes compete. Although progress in the use of MR reporter genes has been slow, newer strategies that use metalloproteins or alternative contrast mechanisms, with no need for substrates, promise rapid growth potential for this field.

Choroidal amelanotic melanoma in a patient with oculocutaneous albinism

CASE REPORT

We describe the clinical presentation of a choroidal amelanotic melanoma in a 46-year-old woman with oculocutaneous albinism. Clinical aspects, investigations, and management are presented, and findings from computed tomography and magnetic resonance imaging are described. Microscopic findings and histopathological features, demonstrating a spindle B-cell melanoma of the choroid, are also reviewed.

COMMENTS

Magnetic resonance imaging may be helpful in diagnosing amelanotic melanoma in patients with oculocutaneous albinism.

Monitoring acute inflammatory processes in mouse muscle by MR imaging and spectroscopy: a comparison with pathological results

We have studied an animal model of acute local inflammation in muscle induced by Aspergillus fumigatus by using magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS). We have compared our data to those found using histopathology and segmentation maps obtained by the mathematical processing of three-dimensional T2-weighted MRI data via a neural network. The MRI patterns agreed satisfactorily with the clinical and biological evidence of the phases of acute local infection and its evolution towards chronicity. The MRS results show a statistically significant increase in inorganic phosphate and a significant decrease in phosphocreatine levels in the inflamed region. Image segmentation made with a self-organizing, neural-network map yielded a set of ordered representatives that remained constant for all animals during the inflammatory process, allowing a non-invasive, three-dimensional identification and quantification of the inflamed infected regions by MRI.

In vitro studies of gadolinium-DTPA conjugated with monoclonal antibodies as cancer-specific magnetic resonance imaging contrast agents

The monoclonal antibodies, 9.2.27 against human melanoma cell lines and WM53 against leukemia cell lines, were conjugated with cyclic anhydride gadolinium-diethylenetriaminepenta-acetic acid (Gd-cDTPAa) and used as tumor-specific contrast agents in magnetic resonance imaging (MRI). The data indicate that Gd-DTPA-9.2.27 in solution decreased the T1 relaxation of water protons at 7.0 Tesla (300 MHz) in direct proportion to the gadolinium concentration, and this effect was greater than in Gd-DTPA solutions. These conjugates show high specificity for melanoma and leukemia cell lines. T1 relaxation time at 7.0 Tesla, measured for the melanoma cell line (MM-138) and leukemia cell line (HL-60) after incubation at 37 degrees C for 4 hr, were significantly decreased (approximately 25%) relative to controls. The gadolinium concentration in cells and washing solutions was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). A linear relationship was observed between T1 relaxation rates and gadolinium concentrations obtained by ICP-AES. The ICP-AES results showed no gadolinium uptake in the non-targeted HT-29 colorectal cancer cells.

Antivascular treatment of solid melanoma tumors with bacteriochlorophyll-serine-based photodynamic therapy

We describe here a strategy for photodynamic eradication of solid melanoma tumors that is based on photo-induced vascular destruction. The suggested protocol relies on synchronizing illumination with maximal circulating drug concentration in the tumor vasculature attained within the first minute after administrating the sensitizer. This differs from conventional photodynamic therapy (PDT) of tumors where illumination coincides with a maximal concentration differential of sensitizer in favor of the tumor, relative to the normal surrounding tissue. This time window is often achieved after a delay (3-48 h) following sensitizer administration. We used a novel photosensitizer, bacteriochlorophyll-serine (Bchl-Ser), which is water soluble, highly toxic upon illumination in the near-infrared (lambda max 765-780 nm) and clears from the circulation in less than 24 h. Nude CD1 mice bearing malignant M2R melanotic melanoma xenografts (76-212 mm3) received a single complete treatment session. Massive vascular damage was already apparent 1 h after treatment. Changes in vascular permeability were observed in vivo using contrast-enhanced magnetic resonance imaging (MRI), with the contrast reagent Gd-DTPA, by shortening spin-spin relaxation time because of hemorrhage formation and by determination of vascular macromolecular leakage. Twenty-four hours after treatment a complete arrest of vascular perfusion was observed by Gd-DTPA-enhanced MRI. Histopathology performed at the same time confirmed primary vascular damage with occlusive thrombi, hemorrhage and tumor necrosis. The success rate of cure of over 80% with Bchl-Ser indicates the benefits of the short and effective treatment protocol. Combining the sensitizer administration and illumination steps into one treatment session (30 min) suggests a clear advantage for future PDT of solid tumors.

Malignant melanoma of the sinonasal cavity: explanation of magnetic resonance signal intensities with histopathologic characteristics

PURPOSE

To evaluate the magnetic resonance (MR) findings of malignant melanoma of the sinonasal cavity and compare these findings with those of the histopathological examination.

MATERIALS AND METHODS

The MR images of 11 patients with malignant melanoma of the sinonasal cavity were retrospectively reviewed. Ten patients had primary malignant melanoma of the sinonasal cavity and one had a local recurrence. The imaging findings were evaluated with special attention given to the signal intensity of the tumor, internal characteristics, and growth pattern on MR. Signal intensity and enhancement patterns of the tumors were compared with the histopathological findings.

RESULTS

On T1-weighted image, 6 tumors were hyperintense (5 melanotic and one amelanotic melanoma), and 5 tumors were isointense or hypointense (4 amelanotic and one melanotic). On T2-weighted images, 2 amelanotic tumors showed hyperintensity, and 5 melanotic tumors showed hypointensity. Four tumors (one melanotic and 3 amelanotic) were isointense. Four of the 7 tumors with hyperintensity on T1-weighted images showed patchy, higher-signal intensity areas. In 3 of them, patchy areas of a higher degree of pigmentation were found on histopathological examination. There were multiple linear dark signal intensities on T2-weighted images and/or linear-enhancing areas on enhanced T1-weighted images within the masses in 5 of the 11 patients. These findings could be explained as intratumoral vessels in 4 tumors and fibrous septa in one tumor on histopathological examinations.

CONCLUSION

Malignant melanoma of the sinonasal cavity shows characteristic MR signal intensity, which is mainly attributable to the degree and distribution of melanin pigmentation, and partly attributable to hemorrhage within the mass. The linear, low-signal intensity on T2-weighted images or enhanced lines are intratumoral vessels or fibrous septa.

Light-dependent oxygen consumption in bacteriochlorophyll-serine-treated melanoma tumors: on-line determination using a tissue-inserted oxygen microsensor

Successful application of anticancer therapy, and especially photodynamic therapy (PDT) mediated by type II (PDTII) processes, depends on the oxygen content within the tumor before, during and after treatment. The high consumption of oxygen during type II PDT imposes constraints on therapy strategies. Although rates of oxygen consumption and repletion during PDTII were suggested by theoretical studies, direct measurements have not been reported. Application of a novel oxygen sensor allowed continuous and direct in situ measurements (up to a depth of 8-9 mm from the tumor surface and for several hours) of temporal variations in the oxygen partial pressure (pO2) during PDT. Highly pigmented M2R mouse melanoma tumors implanted in CD1 nude mice were treated with bacteriochlorophyll-serine (Bchl-Ser; a new photodynamic reagent) and were subjected to fractionated illumination (700 < lambda < 900 nm) at a fluence rate of 12 mW cm-2. This illumination led to total oxygen depletion with an average consumption rate of 7.2 microM(O2) s-1. Spontaneous reoxygenation (at an average rate of 2.5 microM(O2)/s) was observed during the following dark period. These rates are in good agreement with theoretical considerations (Foster et al., Radiat. Res. 126, 296, 1991 and Henning et al., Radiat. Res. 142, 221, 1995). The observed patterns of oxygen consumption and recovery during prolonged periods of light/dark cycles were interpreted in terms of vasculature damage and sensitizer clearance. The presented data support the previously suggested advantages of fractionated illumination for type II photodynamic processes.

Serine conjugates of chlorophyll and bacteriochlorophyll: photocytotoxicity in vitro and tissue distribution in mice bearing melanoma tumors

Chlorophyll (Chl) and bacteriochlorophyll (Bchl) have been made water soluble by transesterification with serine (Ser) at the propionyl residue and tested as potential reagents for photodynamic therapy (PDT). Photocytotoxicity of the conjugates Chl-Ser and Bchl-Ser in M2R mouse melanoma was tested in cell cultures. Tissue uptake and clearance of the photosensitizers in CD1 nude and C57B1 mice implanted with M2R tumors are described. Photocytotoxicity in cell cultures was determined microscopically and by [3H]thymidine incorporation. The LD50 values in vitro were 0.05-0.1 microM for both sensitizers while that of the commercially available hematoporphyrin derivative (HPD, Photosan) was over 100 times higher for the same light intensity (45 mW/cm2). Pigment concentrations were determined fluorometrically in acetone extracts of the tissues of interest at different times after intraperitoneal injection of 20 mg pigment/kg body weight. The distribution pattern of Chl-Ser in the different tissues resembled that reported for Photofrin, chlorin and bacteriochlorin derivatives. Clearance from normal tissues was essentially completed within 16 h for Bchl-Ser and 72 h for Chl-Ser with mean half-lives (t 1/2) of about 2 and 7 h, respectively. In contrast, the clearance rates of these pigments and their metabolites from melanoma tumor tissue were significantly longer: t 1/2 = 20 h for Chl-Ser and 15 h for Bchl-Ser and metabolites. The clearance rates showed biphasic or single exponential decay patterns in normal tissues and in tumors, respectively. Cumulatively the high phototoxicity, simple mode of delivery and fast tissue clearance rates reported here suggest that polar conjugates of Chl and Bchl promise to be highly effective PDT reagents.

Metastatic melanoma: correlation of MRI characteristics and histopathology

To correlate the findings on MRI with histopathology of metastatic melanoma, MRI was performed on 29 patients with 36 lesions, using spin-echo and inversion recovery sequences. Histopathologic examination of lesions was performed within 4 weeks of imaging. Lesions were categorized according to cell type and were also evaluated for the presence and extent of melanin, iron, and necrosis. These data were then correlated with the signal intensities of the lesions. Enhancement of lesions after injection of intravenous gadolinium was calculated and correlated to vascularity of the tumor. Melanin was present more frequently in lesions appearing hyperintense or with mixed signal intensity (12/15) than in those appearing hypo- or isointense (6/21) on the T1-weighted sequence. This trend was significant (P = .013). Also, more lesions appearing mixed, ie, having both hypo- and hyperintense components, contained melanin (15/23), as opposed to lesions that appeared to be only hyperintense (3/13) on the STIR sequence. There was no clear association between signal intensity and melanin content on the T2-weighted sequence. There was no significant association between the signal intensities on the MR images and the iron content, tumor size, or tumor cell type of these lesions. There was no clear association between enhancement after gadolinium injection and vascularity, as assessed by histology. The authors concluded in this study that T1 shortening and hypointensity on the STIR sequence seen in patients with metastatic melanoma are most closely related to the melanin content of the tumor.

Detection of necrosis in human tumour xenografts by proton magnetic resonance imaging

Tumours with necrotic regions have an inadequate blood supply and are expected to differ from well-vascularised tumours in response to treatment. The purpose of the present work was to investigate whether proton magnetic resonance imaging (MRI) might be used to detect necrotic regions in tumours. MR images and histological sections from individual tumours of three different amelanotic human melanoma xenograft lines (BEX-t, HUX-t, SAX-t) were analysed in pairs. MRI was performed at 1.5 T using two spin-echo pulse sequences, one with a repetition time (TR) of 600 ms and echo times (TEs) of 20, 40, 60 and 80 ms and the other with a TR of 2000 ms and TEs of 20, 40, 60 and 80 ms. Spin-lattice relaxation time (T1), spin-spin relaxation time (T2) and proton density (N0) were calculated for each volume element corresponding to a pixel. Synthetic MR images, pure T1, T2 and N0 images and spin-echo images with chosen values for TR and TE were generated from these data. T1, T2 and N0 distributions of tumour subregions, corresponding to necrotic regions and regions of viable tissue as defined by histological criteria, were also generated. T1 and T2 were significantly shorter in the necrotic regions than in the regions of viable tissue in all tumours. These differences were sufficiently large to allow the generation of synthetic spin-echo images showing clear contrast between necrosis and viable tissue. Maximum contrast was achieved with TRs within the range 2800-4000 ms and TEs within the range 160-200 ms. Necrotic tissue could also be distinguished from viable tissue in pure T1 and T2 images. Consequently, the possibility exists that MRI might be used for detection of necrotic regions in tumours and hence for prediction of tumour treatment response.

Effect of melanin on phosphorus T1S in human melanoma xenografts studied by 31P MRS

31P MRS resonance ratios of tumors depend on the T1S of the phosphorus compounds. The objective of the 31P MRS study reported here was to investigate whether the phosphorus T1S of melanomas are influenced by the presence of melanin. One amelanotic (COX-t) and one melanotic (ROX-t) human melanoma xenograft line were studied at two different tumor volumes: 200 and 1000 mm3. 31P MRS was performed in nonanaesthetized mice at 4.7 T. The T1S were measured by using the superfast inversion recovery technique. Fraction of necrotic tissue in the tumors was determined by histological examination. The ROX-t tumors showed shorter T1S than the COX-t tumors at a volume of 200 mm3, where the fraction of necrotic tissue in the tumors was insignificant. The difference was similar in magnitude for all resonances. The T1S were not significantly different for COX-t and ROX-t at a volume of 1000 mm3, where the tumors of both lines had developed significant necrosis. The phosphorus T1S of melanomas without necrosis can be shortened significantly by the presence of melanin. The magnitude of the T1 shortening is similar for all major compounds. 31P MRS resonance ratios of melanomas are not altered significantly by correcting for effects of partial saturation.

In vivo detection of a boron-neutron-capture agent in melanoma by proton observed 1H-10B double resonance

The Cesium salt of BSSB (Cs4B24H22S2), a common boron-neutron-capture-therapy (BNCT) agent, was injected into M2R mouse melanoma xenografts, and detected in vivo by 1H-observed, 10B-edited NMR spectroscopy. The technique of spin-echo difference spectroscopy, in which a proton spin-echo is detected following the alternating presence and absence of a 10B 180 degrees pulse was used. This method provides much higher sensitivity than direct 10B NMR detection, and should thus be suitable for in vivo detection in patients about to undergo BNCT treatment, where the infused agents are 95% isotopically enriched in 10B.