A combined solenoid-surface RF coil for high-resolution whole-brain rat imaging on a 3.0 Tesla clinical MR scanner

Mechanically Adjustable 4-Channel RF Transceiver Coil Array for Rat Brain Imaging in a Whole-Body 7 T MR Scanner

Investigations of human brain disorders are frequently conducted in rodent models using magnetic resonance imaging. Due to the small specimen size and the increase in signal-to-noise ratio with the static magnetic field strength, dedicated small-bore animal scanners can be used to acquire high-resolution data. Ultra-high-field (≥7 T) whole-body human scanners are increasingly available, and they can also be used for animal investigations. Dedicated sensors, in this case, radiofrequency coils, are required to achieve sufficient sensitivity for the high spatial resolution needed for imaging small anatomical structures. In this work, a four-channel transceiver coil array for rat brain imaging at 7 T is presented, which can be adjusted for use on a wide range of differently sized rats, from infants to large adults. Three suitable array designs (with two to four elements covering the whole rat brain) were compared using full-wave 3D electromagnetic simulation. An optimized static B1+ shim was derived to maximize B1+ in the rat brain for both small and big rats. The design, together with a 3D-printed adjustable coil housing, was tested and validated in ex vivo rat bench and MRI measurements.

A basic MRI anatomy of the rat brain in coronal sections for practical guidance to neuroscientists

Identification of the brain structures in the magnetic resonance imaging (MRI) of the rat is very important for the experimental work of many neuroscientists. Our intention was to recognize most of the structures without overlapping the MRI sections with the histological templates. Three live rats were used for this study who were examined in a micro-MRI apparatus by performing T2-weighted sequences in serial brain sections. Most of the white matter structures were easily identified, e.g. the anterior commissure, corpus callosum with forceps minor and major, cingulum, external and internal capsules, fornix, stria medullaris and terminalis, cranial nerves, mammillothalamic tract, fasciculus retroflexus, medial and lateral lemniscus, posterior commissure, commissures of the superior and inferior colliculi, medial longitudinal fasciculus, and the cerebral peduncle. Large and small gray matter structures were recognized as well, for example, the anterior olfactory structures, nucleus accumbens, caudate putamen, claustrum, bed nucleus of the stria terminalis, pituitary gland, globus pallidus, amygdala, some midline and intralaminar thalamic nuclei, certain hypothalamic nuclei, hippocampal formation, pineal body, periaqueductal gray matter, lateral and medial geniculate bodies, superior and inferior colliculi, and cranial nerves nuclei. All in all, of the total 160 recognized brain structures, 77 were identified without using the corresponding histological atlases. We believe that our labeled MRI pictures could be an important way for quick orientation for evaluating the effects of the experimental work regarding the rat brain.

A continuous-infusion dynamic MRI model at 3.0 Tesla for the serial quantitative evaluation of microvascular proliferation in an animal model of glioblastoma multiforme

PURPOSE

To develop a continuous-infusion dynamic MRI technique to characterize tumor-associated microvascular proliferation (MVP) in a rat brain model of glioblastoma multiforme.

METHODS

The proposed model assumes effects due to tumor-associated MVP (eg, vascular permeability, K^trans ; intravascular plasma fraction, v_p ) cannot be individually separated and solves for a single parameter (k^vasc ) that quantifies the T₁ -weighted contrast enhancement from dynamic images acquired during continuous contrast agent (CA) infusion. Untreated C6 tumor-bearing animals (N = 6) were serially imaged on postoperative days (PODs) 14 and 18 with a 3 Tesla clinical scanner utilizing a dynamic spatial and temporal resolution of 0.38 × 0.38 × 1.5 mm³ and 3.47 s, respectively.

RESULTS

An association was present between PODs 14 and 18 for median tumor k^vasc (Pearson's r = 0.94, P = 0.0052) and CA concentration ([CA], derived from pre- and postcontrast R₁ maps; r = 0.94, P = 0.0054). On POD 18, there was a voxel-based association between k^vasc and [CA] within each tumor (0.45 < r < 0.82, P < 0.001). However, voxel-based subregions demonstrated a reduced association between k^vasc and [CA] (N = 5; -0.08 < r < 0.22, P > 0.05) or an inverse association (N = 1; r = -0.28, P = 0.001), indicating differences between locations of vascular permeability and subsequent CA pooling in tumors.

CONCLUSION

The continuous-infusion method may provide a quantitative measure for characterizing and monitoring tumor-associated MVP. Magn Reson Med 78:1824-1838, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Fragment Length of Circulating Tumor DNA

Malignant tumors shed DNA into the circulation. The transient half-life of circulating tumor DNA (ctDNA) may afford the opportunity to diagnose, monitor recurrence, and evaluate response to therapy solely through a non-invasive blood draw. However, detecting ctDNA against the normally occurring background of cell-free DNA derived from healthy cells has proven challenging, particularly in non-metastatic solid tumors. In this study, distinct differences in fragment length size between ctDNAs and normal cell-free DNA are defined. Human ctDNA in rat plasma derived from human glioblastoma multiforme stem-like cells in the rat brain and human hepatocellular carcinoma in the rat flank were found to have a shorter principal fragment length than the background rat cell-free DNA (134–144 bp vs. 167 bp, respectively). Subsequently, a similar shift in the fragment length of ctDNA in humans with melanoma and lung cancer was identified compared to healthy controls. Comparison of fragment lengths from cell-free DNA between a melanoma patient and healthy controls found that the BRAF V600E mutant allele occurred more commonly at a shorter fragment length than the fragment length of the wild-type allele (132–145 bp vs. 165 bp, respectively). Moreover, size-selecting for shorter cell-free DNA fragment lengths substantially increased the EGFR T790M mutant allele frequency in human lung cancer. These findings provide compelling evidence that experimental or bioinformatic isolation of a specific subset of fragment lengths from cell-free DNA may improve detection of ctDNA.

During cell death, DNA that is not contained within a membrane (i.e., cell-free DNA) enters the circulation. Detecting cell-free DNA originating from solid tumors (i.e., circulating tumor DNA, ctDNA), particularly solid tumors that have not metastasized, has proven challenging due to the relatively abundant background of normally occurring cell-free DNA derived from healthy cells. Our study defines the subtle but distinct differences in fragment length between normal cell-free DNA and ctDNA from a variety of solid tumors. Specifically, ctDNA was overall consistently shorter than the fragment length of normal cell-free DNA. Subsequently, we showed that a size-selection for shorter cell-free DNA fragments increased the proportion of ctDNA within a sample. These results provide compelling evidence that development of techniques to isolate a subset of cell-free DNA consistent with the ctDNA fragment lengths described in our study may substantially improve detection of non-metastatic solid tumors. As such, our findings may have a direct impact on the clinical utility of ctDNA for the non-invasive detection and diagnosis of solid tumors (i.e., the “liquid biopsy”), monitoring tumor recurrence, and evaluating tumor response to therapy.

Fast bound pool fraction imaging of the in vivo rat brain: association with myelin content and validation in the C6 glioma model

Cross-relaxation imaging (CRI) is a quantitative magnetic resonance technique that measures the kinetic parameters of magnetization transfer between protons bound to water and protons bound to macromolecules. In this study, in vivo, four-parameter CRI of normal rat brains (N=5) at 3.0 T was first directly compared to histology. The bound pool fraction, f, was strongly associated with myelin density (Pearson's r=0.99, p<0.001). The correlation persisted in separate analyses of gray matter (GM; r=0.89, p=0.046) and white matter (WM; r=0.97, p=0.029). Subsequently, a new time-efficient approach for solely capturing the whole-brain parametric map of f was proposed, validated with histology, and used to estimate myelin density. Since the described approach for the rapid acquisition of f applied constraints to other CRI parameters, a theoretical analysis of error was performed. Estimates of f in normal and pathologic tissue were expected to have <10% error. A comparison of values for f obtained from the traditional four-parameter fit of CRI data versus the proposed rapid acquisition of f was within this expected margin for in vivo rat brain gliomas (N=4; mean±SE; 3.9±0.2% vs. 4.0±0.2%, respectively). In both whole-brain f maps and myelin density maps, replacement of normal GM and WM by proliferating and invading tumor cells could be readily identified. The rapid, whole-brain acquisition of the bound pool fraction may provide a reliable method for detection of glioma invasion in both GM and WM during animal and human imaging.