Pathologic changes of the lateral pterygoid muscle in patients with derangement of the temporomandibular joint disk: objective measures at MR imaging.

BACKGROUND AND PURPOSE: The superior head of the lateral pterygoid muscle (SHLP), which inserts on the anterior disk of the temporomandibular joint (TMJ), can spasm, contracting and exerting forward traction on the disk. This mechanism can lead to anterior displacement. In TMJ dysfunction, it is hypothesized that the SHLP will demonstrate morphologic changes with measurable changes in signal intensity related to atrophy or muscular edema, or both. The goal of this study was to evaluate the lateral pterygoid muscle (LPM) in patients with TMJ dysfunction. MATERIALS AND METHODS: Patients with displacement of the TMJ disk with and without reduction were identified through a review of radiology reports. Absolute measurements of thickness as well as region-of-interest measurements were placed over the 2 heads of the LPM bilaterally on sagittal T1- and T2-weighted images. Statistically significant differences between the superior and inferior heads of the LPM were calculated with use of a 1-tailed Student t test and were correlated with the degree of disk derangement. RESULTS: In patients with disk derangement, significantly increased region-of-interest values on T2- and T1-weighted images were demonstrated within the SHLP. No patients with anatomically normal disks demonstrated a statistically significant difference in region-of-interest values between the superior and inferior heads of the LPMs. CONCLUSION: Correlation between increased region-of-interest values and pathologic alteration of the relationship between the condylar head and disk was identified. In patients with displacement of the anterior disk with and without reduction, region-of-interest values were significantly increased, which indicates abnormal signal intensity involving the superior head of the LPM.

Improved delineation of human brain tumors on MR images using a long-circulating, superparamagnetic iron oxide agent.

The purpose of this study was to corroborate experimental findings that long-circulating, superparamagnetic iron oxide contrast agents accumulate at the margins of human brain tumors, thereby improving their delineation on magnetic resonance (MR) images. This limited clinical study examined a total of four patients with brain tumors (three with primary gliomas and one with metastatic melanoma; n = 8 lesions) who were given a pharmaceutical formulation of a superparamagnetic, ultra-small-particulate iron oxide (USPIO, intravenous dose of 1.1 mg Fe/kg). The agent has a characteristically long plasma half-life and is currently undergoing Phase III clinical trials for liver disease (AMI-227, Advanced Magnetics, Cambridge, MA). MR (conventional spin-echo and gradient-echo) images of the brain were obtained before and 12, 24, and/or 36 hours after administration of the agent, with follow-up several weeks later. Twelve to 36 hours after IV administration of the USPIO, both primary and metastatic brain tumors showed readily detectable increases in signal intensity on T1-weighted spin-echo images. Unlike the pattern of enhancement with a gadolinium (Gd) chelate, which occurred immediately and decreased within hours, that with the USPIO occurred gradually, with a peak at 24 hours, and decreased over several days. Whereas the enhancing tumor margin with the Gd chelate blurred with time due to diffusion of the agent, the margin with the USPIO remained sharp, presumably due to the much lower diffusion coefficient (large size) of the particles and partly because of local endocytosis by tumor cells. Compared with Gd chelates, long-circulating, superparamagnetic iron oxide contrast agents can provide prolonged delineation of the margins of human brain tumors on MR images, which has implications for the targeting of diagnostic biopsies and the planning of surgical resections.

Gadolinium-enhanced MR angiography of the carotid arteries.

To assess the efficacy of gadolinium (Gd)-enhanced magnetic resonance angiography (gdMRA) in overcoming signal dropout artifacts on conventional MRA (cMRA), the authors examined 13 patients with suspected neurovascular stenotic/occlusive lesions on MRA pre- and post-gadolinium enhancement. The sample consisted of 18 internal carotid artery (ICA) les ons (16 extracranial, 2 cranial). In 13 of 16 stenotic vessels, gdMRA better characterized the pathoanatomy of moderate to severe stenotic lesions, changing MRA diagnosis in 3 patients. In zero of three vessels with no flow enhancement by cMRA, the lumen remained nonvisualized on gdMRA. For an ICA lesion at the skull base, enhancement of surrounding normal structures limited the usefulness of the technique. The authors conclude that Gd MRA is a fast, accurate, and convenient noninvasive technique for documenting the morphology and severity of carotid stenotic disease, especially extracranially.

MR imaging and scintigraphy of gene expression through melanin induction.

PURPOSE: To determine whether an expression vector that encodes for human tyrosinase, the key enzyme in the melanin production pathway, can be used to image gene expression with magnetic resonance (MR) imaging and scintigraphy. MATERIALS AND METHODS: Mouse fibroblasts and human embryonal kidney cells were transfected with an expression vector that contained a complete complementary DNA sequence that encodes the human tyrosinase gene (pcDNA3tyr). Transfected cells were assayed for messenger RNA presence, melanin staining, and indium-111 binding; scintigraphy and MR imaging were performed. RESULTS: Transfected cells contained tyrosinase messenger RNA and stained positively for melanin. Transfected cells had a higher In-111 binding capacity than nontransfected cells, a difference readily detectable with scintigraphy. MR imaging showed transfected cells to have markedly higher signal intensity after gene transfer than nontransfected cells. CONCLUSION: Gene transfer and expression in cell culture can be detected with MR imaging and scintigraphy. The proposed strategy of using an imaging marker gene may have a substantial effect on the noninvasive imaging of gene therapy.

Paramagnetic metal scavenging by melanin: MR imaging.

PURPOSE: To quantitate the binding of metals to synthetic melanin in vitro, which is believed to be the reason why melanotic melanomas are hyperintense on T1-weighted magnetic resonance (MR) images, and to test whether such binding by natural melanin can be detected in cultured melanoma cells in vivo with MR imaging. MATERIALS AND METHODS: Seven synthetic metallomelanins were prepared and their metal contents and relaxivities determined. Melanotic PC1A and amelanotic B16 melanoma cells were incubated with increasing concentrations of iron. MR images of synthetic melanin and cell phantoms were obtained. RESULTS: The iron-binding capacities and relaxivities of the different synthetic metallomelanins varied considerably, which reflects the heterogeneous structure of melanin and the complexity of its binding of metals. Nevertheless, the MR signal intensities of the synthetic melanin and cell phantoms show marked increases that scale, respectively, with increasing iron content and iron concentration in the incubation medium. CONCLUSION: Melanotic melanomas are hyperintense on T1-weighted images because of paramagnetic metal scavenging. This observation has implications for the interpretation of MR images, the improved detection of melanomas, and the development of imaging marker genes.

MR imaging of phagocytosis in experimental gliomas.

PURPOSE: To determine whether phagocytosis can be observed in vivo in glioma cells. MATERIALS AND METHODS: Rat C6 glioma cells were studied in culture and after intracerebral implantation into 13 rats. Monocrystalline iron oxide nanoparticles (MION), a model marker of phagocytosis, was administered intravenously to tumor-bearing rats at 2-20 mg of iron per kilogram. Magnetic resonance (MR) imaging was performed at multiple time points. RESULTS: Glioma cells in culture showed uptake of MION in amounts of up to 10 ng of iron per 10(6) cells, corresponding to approximately 50,000 particles per cell. Fluorescently labeled MION was found to be located primarily in tubular lysosomes. Intracerebral gliomas showed characteristic changes in signal intensity at MR imaging that peaked 12 hours after administration of MION and lasted up to 5 days; these changes corresponded to uptake and subsequent biodegradation of MION by tumor cells. CONCLUSION: Phagocytosis of glioma cells can be detected in vivo with iron oxide-enhanced MR imaging, and this may permit accurate delineation of tumor margins.

Cerebral iron oxide distribution: in vivo mapping with MR imaging.

PURPOSE: To map the distribution of an iron oxide label in the central nervous system with in vivo magnetic resonance (MR) imaging. MATERIALS AND METHODS: Unilateral osmotic disruption of the blood-brain barrier (BBB) in rats (n = 40) was followed by injection of monocrystalline iron oxide nanoparticles (MION) into the carotid artery. MR images (1.5 T) were obtained in and ex vivo, and results were correlated with histologic section-matched iron maps. RESULTS: A mean of 0.2% of the injected MION was found in the brain 24 hours after unilateral osmotic disruption of the BBB. The spatial distribution of iron oxide within the brain correlated with areas known to have high relative perfusion. Iron was found in cell bodies and dendrites of cortical neurons and astrocytes and in the interstitial space. The threshold in concentration for detection of MION in the brain was 62.2 ng Fe/mm2. CONCLUSION: MR imaging is well suited to noninvasive in vivo mapping of the intracerebral iron oxide distribution.

Determinants of in vivo MR imaging of slow axonal transport.

PURPOSE: To investigate specific surface characteristics of magnetic contrast agents based on a monocrystalline iron oxide nanoparticle (MION) that may determine their uptake and/or transport by axons. MATERIALS AND METHODS: MION were modified to have a range of surface charges or were covalently linked to wheat germ agglutinin (WGA), a neurotropic protein. Each agent was injected directly into the sciatic nerves or femoral arteries of rats (n = 22), and magnetic resonance (MR) images were obtained several days later. The imaging results then were correlated with results at postmortem histologic examination. RESULTS: Substantial uptake and/or transport by axons occurred only after intraneural injection and only if the agent had a strong surface charge or was covalently linked to WGA. The sciatic nerves appeared as uniformly hypointense structures having lengths proportional to the time from injection to imaging, and the calculated transport rates (4-7 mm/d) were consistent with slow axonal transport. Numerous Schwann cells and macrophages acquired large fractions of the injected agents and contributed substantially to the imaging results. CONCLUSION: Those characteristics of MION-based contrast agents that promote efficacy after intraneural injection may impede delivery to the nerve after intraarterial injection.

The roles of neuromelanin, binding of metal ions, and oxidative cytotoxicity in the pathogenesis of Parkinson's disease: a hypothesis.

A characteristic feature of both Parkinson's disease (idiopathic paralysis agitans) and normal aging is loss of pigmented neurons in the substantia nigra. This has been found to correlate with the accumulation of neuromelanin and with oxidative stress in this brain region, but a clear association between these factors has not been established. Based on our recent demonstration that neuromelanin is a true melanin, containing bound metal ions in situ, we present a general model for its accumulation in vivo and the hypotheses (1) that it has a cytoprotective function in the sequestration of redox-active metal ions under normal conditions but (2) that it has a cytotoxic role in the pathogenesis of Parkinson's disease. Thus, neuromelanin accumulates normally through the autooxidation of catecholamines and serves tightly to bind redox-active metal ions, processes which would accelerate under conditions of intracellular or extracellular oxidative stress. Based on the known properties of melanin, however, neuromelanin also has the potential for exacerbating oxidative stress, eg by generating H2O2 when it is intact or by releasing redox-active metal ions if it loses its integrity; these reactions also would modulate the reactivity of the neuromelanin. By overwhelming intracellular antioxidative defense mechanisms, such a positive-feedback cycle could turn a condition of chronic or repeated oxidative stress in vulnerable neurons into an acute crisis, leading to cellular death. If the cumulative stress in duration and/or degree is severe enough, neuronal depletion could be sufficient to cause Parkinson's disease during life. One possible trigger for this cascade is suggested by the increased nigral iron contents in postmortem parkinsonian brains and the correlation of this disease with urban living where exposure to heavy metal ions is high: the saturation of neuromelanin with redox-active metal ions. Parkinson's disease therefore may be a form of accelerated aging in the substantia nigra associated with environmental toxins in which neuromelanin has a central, active role.

MR imaging of slow axonal transport in vivo.

Three magnetopharmaceuticals based on a monocrystalline iron oxide nanocompound (MION) are evaluated as potential contrast agents for demonstrating axonal transport in vivo by magnetic resonance (MR) imaging. One agent has a strong positive charge, one has a strong negative charge, and the third is covalently linked to wheat germ agglutinin, a plant lectin with a high affinity for axon terminals. All three agents were tagged with rhodamine, and fluorescence microscopy was used to determine their fate after administration and to validate the imaging results. Following injection into or near various neural structures in the motor and visual systems of rats, MR images were obtained at multiple times up to 11 days later, and the imaged tissues were processed for subsequent histological examination. Similar results were obtained with all three agents. Axonal transport was not seen by MR imaging or fluorescence microscopy when the agents were injected into the calf muscles, the vitreous of the eye, or the superior colliculus. However, bidirectional axonal transport was shown unequivocally by both methods after injection directly into the site of a focal crush injury to the sciatic nerve. The nerve, which otherwise is isointense with surrounding tissues on MR images, appeared as a uniformly hypointense structure having a length approximately in proportion to the time from injection to imaging. By 11 days, the course of the nerve was traceable from its component roots in the cauda equina to its bifurcation into the tibial and common peroneal nerves in the leg. A transport rate of about 5 mm/day was calculated, which is consistent with the mechanism of slow transport. MION-based magnetopharmaceuticals thus can be used to demonstrate slow axonal transport, and thereby visualize peripheral nerves, in vivo by MR imaging.

The minocycline-induced thyroid pigment and several synthetic models: identification and characterization by electron paramagnetic resonance spectroscopy.

A distinctive side-effect of exposure to minocycline is black pigmentation of the thyroid gland. Previous studies have identified an association between this side-effect and the ability of minocycline to competitively inhibit thyroid peroxidase, but extensive histochemical analyses have resulted in ambiguous definitions of the pigment. Electron paramagnetic resonance spectroscopy, an especially effective technique for investigating melanins, is used in the present study to show that the thyroid pigment is well-modeled by synthetic pigments generated by oxidation of minocycline in vitro, thus indicating that it is a polymeric product due to oxidation of minocycline in vivo by thyroid peroxidase. The results demonstrate that pigments derived from minocycline comprise a novel class of pigments with redox and paramagnetic characteristics which are melanin-like in most respects yet are fundamentally unique. They also indicate that the thyroid pigment contains a large quantity of Fe bound tightly in situ, a finding with important pathophysiological implications in view of the redox properties of the pigment. The binding of Fe , however, also may provide a basis for noninvasively detecting the presence of pigmentation by magnetic resonance imaging of the thyroid. Other results of this study show that homogenizing tissues during the purification of any natural pigment can cause contamination of the pigment by extraneous metal ions and that subsequent incubation in hot acid, although most effective in removing metal ions and hydrolyzing tissue proteins, leads to degradation of melanin. By comparison, a purification procedure utilizing incubation in acid at room temperature generally is well-suited for identifying and characterizing natural melanins by electron paramagnetic resonance spectroscopy, but is inadequate for the thyroid pigment.

Purified human neuromelanin, synthetic dopamine melanin as a potential model pigment, and the normal human substantia nigra: characterization by electron paramagnetic resonance spectroscopy.

Neuromelanin is a poorly understood pigment that accumulates in catecholaminergic neurons during normal aging. Electron paramagnetic resonance spectroscopy, an especially effective technique for investigating melanins, is used in the present study to show unambiguously that neuromelanin is a melanin; however, it is not well modeled by synthetic dopamine melanin and thus is an atypical melanin. Some of the unusual features of neuromelanin can be explained by postulating two distinct sources for its free radicals, the dominant one possibly derived from a precursor containing sulfur. Examination of human substantia nigra by electron paramagnetic resonance spectroscopy during the purification of neuromelanin also demonstrates, contrary to some other studies, that a portion of the paramagnetic metal ions in this tissue are bound to the pigment in situ. Combined with previous histochemical data, these observations have implications for the mechanism through which neuromelanin accumulates in vivo and are consistent with its having a cytoprotective function under normal conditions, but a cytotoxic role at advanced ages and in patients with Parkinson's disease. Other results of this study show that homogenizing tissues during the purification of any natural pigment may cause contamination of the pigment by extraneous metal ions and that subsequent incubation in hot acid, though most effective in removing metal ions and hydrolyzing proteins, leads to degradation of melanin. A purification procedure using incubation in acid at room temperature, however, is well suited for identifying and characterizing unknown natural pigments by electron paramagnetic resonance spectroscopy.

A standardized test for the identification and characterization of melanins using electron paramagnetic resonance (EPR) spectroscopy.

Melanins are complex, incompletely understood polymeric pigments that historically have been difficult to investigate with common chemical, histochemical, and physicochemical techniques. Because these pigments uniquely contain a stable population of organic free radicals, electron paramagnetic resonance (EPR) spectroscopy is a particularly effective method for studying them, and a set of qualitative EPR criteria has been established for their identification. However, a number of practical problems have arisen in applying these criteria to identify and characterize unknown pigments in relatively scarce pathological specimens, indicating that a standardized approach is needed. As reported here, a standardized EPR test for melanin based on the EPR criteria has been developed, guided by the requirements that it be sensitive, accurate, simple, and easy to interpret. It has been evaluated using the well-characterized synthetic melanin prepared by alkaline autooxidation of 5,6-dihydroxyphenylalanine (Dopa) and initially applied to the identification and characterization of an unknown pigment purified from an unusual malignant lung tumor.

Sources of the increased longitudinal relaxation rates observed in melanotic melanoma. An in vitro study of synthetic melanins.

Researchers have suggested that the increased longitudinal relaxation rates (1/T1) of solvent water protons often found in melanoma result either from the paramagnetism of stable free radicals occurring in melanin or from that of methemoglobin in nonacute hemorrhagic regions of the tumor. However, field-cycling relaxometry and model solutions of synthetic melanin produced data which show that free radicals in melanin do not contribute significantly to 1/T1; instead, aggregation of melanin into macromolecular particles and binding of biologically-common paramagnetic metal ions (ie Fe3+, Mn2+, and Cu2+) to melanin effectively do increase 1/T1. These data have been combined with published histochemical data on melanin-containing tissues, while disregarding any additional effect related to hemorrhage. The result indicates that in melanoma the expected contribution of melanin-bound Fe3+ to 1/T1, at typical imaging fields, predominates under estimated in vivo conditions; furthermore, the total contribution from all sources, specifically due to the presence of melanin, is sufficient to account for reported measurements of 1/T1 in melanoma. Comparing the latter results with published data on T1 relaxation in model solutions of methemoglobin suggests that co-existing regions of nonacute microhemorrhage also may contribute significantly to 1/T1 under certain conditions. Finally, the implications for 1/T2 of melanin occurring in vivo within discrete melanosomes is discussed.