Evaluation of the susceptibility effect on gradient echo phase images in vivo: a sequential study of intracerebral hematoma

Spontaneous Spongiform Brainstem Degeneration in a Young Mouse Lemur (Microcebus murinus) with Conspicuous Behavioral, Motor, Growth, and Ocular Pathologies

Here we report a case of severe growth retardation and neurologic abnormalities in a female gray mouse lemur (Microcebus murinus), a small NHP species for which the genomic sequence recently became available. The female lemur we present here died on postnatal day 125. This lemur had impaired development of motor skills and showed severe ataxia and tremors. In addition, hearing seemed normal whereas ophthalmic examination revealed incipient bilateral cataracts, abnormal pigmentation in the lens of the left eye, and a missing optokinetic nystagmus, which indicated impaired vision. Most prominently, the lemur showed severe growth retardation. Necropsy revealed maldevelopment of the left reproductive organs and unilateral dilation of the right lateral ventricle, which was confirmed on brain MRI. Brain histology further revealed large, bilateral areas of vacuolation within the brainstem, but immunohistochemistry indicated no sign of pathologic prion protein deposition. Full genomic sequencing of the lemur revealed a probably pathologic mutation in LARGE2 of the LARGE gene family, which has been associated with congenital muscular dystrophies. However, potentially functional mutations in other genes were also present. The observed behavioral and motor signs in the presented animal might have been linked to spongiform degeneration and resulting brainstem dysfunction and progressive muscle weakness. The macroscopic developmental abnormalities and ophthalmic findings might be genetic in origin and linked to the mutation in LARGE2.

Overview of quantitative susceptibility mapping

Magnetic susceptibility describes the magnetizability of a material to an applied magnetic field and represents an important parameter in the field of MRI. With the recently introduced method of quantitative susceptibility mapping (QSM) and its conceptual extension to susceptibility tensor imaging (STI), the non-invasive assessment of this important physical quantity has become possible with MRI. Both methods solve the ill-posed inverse problem to determine the magnetic susceptibility from local magnetic fields. Whilst QSM allows the extraction of the spatial distribution of the bulk magnetic susceptibility from a single measurement, STI enables the quantification of magnetic susceptibility anisotropy, but requires multiple measurements with different orientations of the object relative to the main static magnetic field. In this review, we briefly recapitulate the fundamental theoretical foundation of QSM and STI, as well as computational strategies for the characterization of magnetic susceptibility with MRI phase data. In the second part, we provide an overview of current methodological and clinical applications of QSM with a focus on brain imaging. Copyright © 2016 John Wiley & Sons, Ltd.

Acute tumor response to ZD6126 assessed by intrinsic susceptibility magnetic resonance imaging

The effective magnetic resonance imaging (MRI) transverse relaxation rate R(2)* was investigated as an early acute marker of the response of rat GH3 prolactinomas to the vascular-targeting agent, ZD6126. Multigradient echo (MGRE) MRI was used to quantify R(2)*, which is sensitive to tissue deoxyhemoglobin levels. Tumor R(2)* was measured prior to, and either immediately for up to 35 minutes, or 24 hours following administration of 50 mg/kg ZD6126. Following MRI, tumor perfusion was assessed by Hoechst 33342 uptake. Tumor R(2)* significantly increased to 116 +/- 4% of baseline 35 minutes after challenge, consistent with an ischemic insult induced by vascular collapse. A strong positive correlation between baseline R(2)* and the subsequent increase in R(2)* measured 35 minutes after treatment was obtained, suggesting that the baseline R(2)* is prognostic for the subsequent tumor response to ZD6126. In contrast, a significant decrease in tumor R(2)* was found 24 hours after administration of ZD6126. Both the 35-minute and 24-hour R(2)* responses to ZD6126 were associated with a decrease in Hoechst 33342 uptake. Interpretation of the R(2)* response is complex, yet changes in tumor R(2)* may provide a convenient and early MRI biomarker for detecting the antitumor activity of vascular-targeting agents.

Differentiation of calcification from chronic hemorrhage with corrected gradient echo phase imaging

PURPOSE

The purpose of the current study was to prospectively evaluate the role of corrected gradient echo phase imaging in differentiation of calcified granuloma from chronic hemorrhage.

METHOD

Eighty-five patients with single/multiple calcifications and hemorrhages irrespective of their location were studied with corrected gradient echo phase imaging. In all the cases, CT was used as the gold standard for the presence/absence of calcification.

RESULTS

All calcified lesions showed positive phase, whereas chronic hemorrhages showed negative phase in all cases. Five calcified lesions showed no phase shift at TE =15 ms and positive shift at TE = 35 ms. Heterogeneous phase shift was observed in three calcified lesions at TE = 35 ms; all three lesions showed positive phase shift at TE = 15 ms. There was no site-specific problem in differentiation of calcification from chronic hemorrhage including in the basal ganglia.

CONCLUSION

We conclude that calcified granuloma can be easily differentiated from chronic hemorrhage with corrected gradient echo phase imaging, which may obviate the need for CT for its confirmation.

Macroscopic susceptibility in ultra high field MRI

PURPOSE

Magnetic susceptibility provides the basis for functional studies and image artifacts in MRI. In this work, magnetic susceptibility and the associated artifacts were analyzed at 8 T in phantoms and in the human head.

METHOD

A mineral oil phantom was constructed in which three cylindrical air-filled tubes were inserted. This phantom was analyzed with gradient-recalled echo and SE imaging techniques acquired using varying TEs and receiver bandwidths. To visualize the presence of magnetic susceptibility artifacts in the head at 8 T, near axial, coronal, and sagittal GE images were also acquired from human volunteers.

RESULTS

The use of gradient-recalled echo imaging resulted in the production of significant magnetic susceptibility artifacts. These artifacts could be readily visualized in phantom samples containing air-filled cylindrical tubes. In the human head, susceptibility artifacts produced significant image distortion in the skull base region. In this area, susceptibility artifacts often resulted in the complete loss of MR signal. Magnetic susceptibility artifacts were manifested as bands of varying signal intensity in the frontal lobe and temporal bone region. In addition, they produced clear distortions in the appearance of brain vasculature and seemed to accentuate the relative size of venous structures within the brain.

CONCLUSION

When using gradient-recalled echo imaging in combination with relatively long TE values, magnetic susceptibility artifacts can be severe at 8 T. These artifacts could be reduced by increasing receiver bandwidths and by lowering effective TEs. As ultra high field MRI provides a fertile ground for the study of susceptibility artifacts in MRI, improvements obtained at this field strength will have a direct impact on studies performed at lower field strengths.

Temperature monitoring of interstitial thermal tissue coagulation using MR phase images

The temperature-dependent water proton frequency shift was investigated for temperature monitoring of interstitial thermal coagulation. A procedure for on-line temperature calculation was developed, and errors due to temperature-dependent susceptibility were investigated by finite element analysis and reference measurements. The temperature coefficient of magnetic susceptibility and proton chemical shift were determined for brain tissue and other substances. With the proposed procedure, the location of isotherms could be well visualized during laser-induced interstitial coagulation in vitro and in vivo. Systematic errors caused by magnetic susceptibility changes with temperature depend strongly on the characteristics of the heat source and can exceed susceptibility effects caused by physiologic tissue changes. For the laser applicators discussed here, however, a first order compensation for this effect was found to be satisfactory, because it reduces the absolute error to the range of +/- 1 degrees C. The proposed method represents a very promising approach for monitoring of the interstitial thermal coagulation.

Measurements of magnetic field variations in the human brain using a 3D-FT multiple gradient echo technique

A magnetic resonance 3DFT multiple gradient-echo technique was used for measurements of the proton spectrum for each voxel in the measured slice. Water, fat, magnetic field and T2 distributions in the head of a normal volunteer and a patient with intracerebral hematoma were computed. Magnetic field variations caused by the head were calculated after correction for the static magnetic field inhomogeneity. Large local magnetic field variations up to 3 ppm were found in the human brain near interfaces between air or bone and brain tissues and 0.5 ppm between hematoma and brain tissue. Information about magnetic field variations could be useful for shimming procedures in vivo and for correcting artifacts in imaging and spectroscopy.

Sequential MR signal change of the thrombus in the false lumen of thrombosed aortic dissection

The evolution of thrombus in the false lumen was investigated in 14 patients with thrombosed aortic dissection, by reviewing the findings acquired at a total of 21 magnetic resonance imaging (MRI) examinations performed between 2 and 146 days after the onset. On electrocardiographic gated 1.5-Tesla MRI, T1-(TR/TE = 860 +/- 190 ms/17-40 ms) and T2-(TR/TE = 1620 +/- 240 ms/70-80 ms) weighted spin echo and gradient echo images were obtained, and the signal intensity of the thrombus on these images was evaluated independently by two observers. The density of the thrombus was also evaluated using computed tomography (CT) images obtained at a total of 54 examinations. On both T1- and T2-weighted images, the thrombus showed signal iso- or hypointensity compared to that of skeletal muscle during the first several days after the onset and, thereafter, showed signal intensity similar to that of fat tissue. It is suggested that the low signal intensity of the thrombus observed during the initial period after the onset was caused by the presence of deoxyhemoglobin and the high intensity observed thereafter was caused by methemoglobin. Focal discrepancy of the signal intensities within two parts of the lumen on spin echo images was observed in 7 patients, and a low-intensity layer on the surface of the thrombus inside the false lumen was observed on gradient echo images in 5 of these 7 patients. This characteristic MR signal change of the thrombus in the false lumen of thrombosed aortic dissection provides useful information concerning the age of the thrombus and in the differential diagnosis of the thrombus from a mural thrombus of aortic aneurysm.

Magnetic resonance imaging study on the effect of levemopamil on the size of intracerebral hemorrhage in rats

BACKGROUND AND PURPOSE

Beneficial effects of calcium antagonists in cerebral ischemia and trauma have been attributed in part to improved cerebral blood flow. Enhancement of cerebral blood flow, however, could aggravate the pathological situation if brain injury is associated with intracerebral hemorrhage. In this study we used high-field magnetic resonance imaging in an animal model of intracerebral hemorrhage to determine noninvasively the effect of the calcium and serotonin antagonist levemopamil [international nonproprietary name for (S)-emopamil] when infused in a dose (6 mg/kg) that is known to increase cerebral blood flow.

METHODS

Intracerebral hemorrhage was induced in rats by stereotaxic microinfusion of collagenase into the caudate putamen. Two series of experiments were performed. (1) Levemopamil was intravenously infused 30 minutes after intracerebral infusion of collagenase (0.05 U), which represents the time of intracranial bleeding. Another group of animals was given heparin (55 IU.kg-1.min-1) to evaluate the capability of this animal model to demonstrate drug-induced worsening of intracerebral hemorrhage. (2) The effects of hyperacute infusion of levemopamil (30 minutes after infusion of 0.5 U of collagenase) were compared with those of a 2-hour delayed administration. In both experimental settings, the extent of intracerebral hemorrhage was determined by T1-weighted magnetic resonance images (spin-echo; repetition time, 400 milliseconds; echo time, 23 milliseconds) taken in vivo in a coronal and a transverse brain plane 24 hours after collagenase infusion.

RESULTS

(1) Hemorrhagic brain areas measured 10.1 +/- 2.9 mm2, 8.5 +/- 2.1 mm2, and 18.8 +/- 2.5 mm2 in the coronal brain plane (10 mm anterior to the interaural line) of control, levemopamil-, and heparin-infused rats, respectively (8 animals per group, mean +/- SD). In the transverse brain plane (6 mm dorsal to the interaural line) the hemorrhagic area was 11.5 +/- 3.6 mm2, 9.7 +/- 2.4 mm2, and 19.9 +/- 3.3 mm2 in control, levemopamil-, and heparin-infused rats, respectively. (2) Animals with 2-hour delayed levemopamil infusion displayed intracerebral hemorrhage similar in size to that of control rats. (3) Neither small nor large hemorrhagic lesions were increased by levemopamil.

CONCLUSIONS

Aggravation of intracerebral hemorrhage was not observed by magnetic resonance imaging in levemopamil-infused animals. However, infusion of heparin caused a significant (P < .05), almost twofold increase in the size of intracerebral hemorrhage. These results justify clinical trials with levemopamil in cerebral disorders such as stroke, brain trauma, and peritumoral brain edema, which may be accompanied by intracerebral hemorrhage from the beginning or where transition to intracerebral hemorrhage may occur.