The effect of gadolinium DTPA on tissue water compartments in slow- and fast-twitch rabbit muscles

Evaluation of the Blood-Brain Barrier, Demyelination, and Neurodegeneration in Paramagnetic Rim Lesions in Multiple Sclerosis on 7 Tesla MRI

BACKGROUND

Paramagnetic rim lesions (PRLs) are associated with chronic inflammation in multiple sclerosis (MS). 7-Tesla (7T) magnetic resonance imaging (MRI) can evaluate the integrity of the blood-brain barrier (BBB) in addition to the tissue myelination status and cell loss.

PURPOSE

To use MRI metrics to investigate underlying physiology and clinical importance of PRLs.

STUDY TYPE

Prospective.

SUBJECTS

Thirty-six participants (mean-age 47, 23 females, 13 males) of mixed MS subtypes.

FIELD STRENGTH/SEQUENCE

7T, MP2RAGE, MULTI-ECHO 3D-GRE, FLAIR.

ASSESSMENT

Lesion heterogeneity; longitudinal changes in lesion counts; comparison of T1, R2*, and χ; association between baseline lesion types and disease progression (2-3 annual MRI visits with additional years of annual clinical follow-up).

STATISTICAL TESTS

Two-sample t-test, Wilcoxon Rank-Sum test, Pearson's chi-square test, two-group comparison with linear-mixed-effect model, mixed-effect ANOVA, logistic regression. P-values <0.05 were considered significant.

RESULTS

A total of 58.3% of participants had at least one PRL at baseline. Higher male proportion in PRL+ group was found. Average change in PRL count was 0.20 (SD = 2.82) for PRLs and 0.00 (SD = 0.82) for mottled lesions. Mean and median pre-/post-contrast T1 were longer in PRL+ than in PRL-. No differences in mean χ were seen for lesions grouped by PRL (P = 0.310, pre-contrast; 0.086, post-contrast) or PRL/M presence (P = 0.234, pre-contrast; 0.163, post-contrast). Median χ were less negative in PRL+ and PRL/M+ than in PRL- and PRL/M-. Mean and median pre-/post-contrast R2* were slower in PRL+ compared to PRL-. Mean and median pre-/post-contrast R2* were slower in PRL/M+ than in PRL/M-. PRL presence at baseline was associated with confirmed EDSS Plus progression (OR 3.75 [1.22-7.59]) and PRL/M+ at baseline with confirmed EDSS Plus progression (OR 3.63 [1.14-7.43]).

DATA CONCLUSION

Evidence of BBB breakdown in PRLs was not seen. Quantitative metrics confirmed prior results suggesting greater demyelination, cell loss, and possibly disruption of tissue anisotropy in PRLs.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2.

Quantitative susceptibility mapping (QSM) of the cardiovascular system: challenges and perspectives

Quantitative susceptibility mapping (QSM) is a powerful, non-invasive, magnetic resonance imaging (MRI) technique that relies on measurement of magnetic susceptibility. So far, QSM has been employed mostly to study neurological disorders characterized by iron accumulation, such as Parkinson's and Alzheimer's diseases. Nonetheless, QSM allows mapping key indicators of cardiac disease such as blood oxygenation and myocardial iron content. For this reason, the application of QSM offers an unprecedented opportunity to gain a better understanding of the pathophysiological changes associated with cardiovascular disease and to monitor their evolution and response to treatment. Recent studies on cardiovascular QSM have shown the feasibility of a non-invasive assessment of blood oxygenation, myocardial iron content and myocardial fibre orientation, as well as carotid plaque composition. Significant technical challenges remain, the most evident of which are related to cardiac and respiratory motion, blood flow, chemical shift effects and susceptibility artefacts. Significant work is ongoing to overcome these challenges and integrate the QSM technique into clinical practice in the cardiovascular field.

Magnetic susceptibility anisotropy outside the central nervous system

Magnetic-susceptibility-based MRI has made important contributions to the characterization of tissue microstructure, chemical composition, and organ function. This has motivated a number of studies to explore the link between microstructure and susceptibility in organs and tissues throughout the body, including the kidney, heart, and connective tissue. These organs and tissues have anisotropic magnetic susceptibility properties and cellular organizations that are distinct from the lipid organization of myelin in the brain. For instance, anisotropy is traced to the epithelial lipid orientation in the kidney, the myofilament proteins in the heart, and the collagen fibrils in the knee cartilage. The magnetic susceptibility properties of these and other tissues are quantified using specific MRI tools: susceptibility tensor imaging (STI), quantitative susceptibility mapping (QSM), and individual QSM measurements with respect to tubular and filament directions determined from diffusion tensor imaging. These techniques provide complementary and supplementary information to that produced by traditional MRI methods. In the kidney, STI can track tubules in all layers including the cortex, outer medulla, and inner medulla. In the heart, STI detected myofibers throughout the myocardium. QSM in the knee revealed three unique layers in articular cartilage by exploiting the anisotropic susceptibility features of collagen. While QSM and STI are promising tools to study tissue susceptibility, certain technical challenges must be overcome in order to realize routine clinical use. This paper reviews essential experimental findings of susceptibility anisotropy in the body, the underlying mechanisms, and the associated MRI methodologies. Copyright © 2016 John Wiley & Sons, Ltd.

Magnetic susceptibility anisotropy of myocardium imaged by cardiovascular magnetic resonance reflects the anisotropy of myocardial filament α-helix polypeptide bonds

BACKGROUND

A key component of evaluating myocardial tissue function is the assessment of myofiber organization and structure. Studies suggest that striated muscle fibers are magnetically anisotropic, which, if measurable in the heart, may provide a tool to assess myocardial microstructure and function.

METHODS

To determine whether this weak anisotropy is observable and spatially quantifiable with cardiovascular magnetic resonance (CMR), both gradient-echo and diffusion-weighted data were collected from intact mouse heart specimens at 9.4 Tesla. Susceptibility anisotropy was experimentally calculated using a voxelwise analysis of myocardial tissue susceptibility as a function of myofiber angle. A myocardial tissue simulation was developed to evaluate the role of the known diamagnetic anisotropy of the peptide bond in the observed susceptibility contrast.

RESULTS

The CMR data revealed that myocardial tissue fibers that were parallel and perpendicular to the magnetic field direction appeared relatively paramagnetic and diamagnetic, respectively. A linear relationship was found between the magnetic susceptibility of the myocardial tissue and the squared sine of the myofiber angle with respect to the field direction. The multi-filament model simulation yielded susceptibility anisotropy values that reflected those found in the experimental data, and were consistent that this anisotropy decreased as the echo time increased.

CONCLUSIONS

Though other sources of susceptibility anisotropy in myocardium may exist, the arrangement of peptide bonds in the myofilaments is a significant, and likely the most dominant source of susceptibility anisotropy. This anisotropy can be further exploited to probe the integrity and organization of myofibers in both healthy and diseased heart tissue.

Microstructural origins of gadolinium-enhanced susceptibility contrast and anisotropy

PURPOSE

MR histology based on magnetic susceptibility can be used to visualize diamagnetic myelin (and its deterioration) in the central nervous system and is facilitated by the application of high magnetic field strengths and paramagnetic contrast agents. Characterizing the effect of these tools will aid in assessing white matter myelin content and microstructure.

METHODS

Image data from six gadolinium-perfused mouse brain specimens were acquired at 2.0, 7.0, and 9.4 Tesla. Magnetic susceptibility contrast was analyzed for its dependence on field strength, gadolinium concentration, and white matter fiber orientation. A model for this contrast is presented based on the three-pool model for white matter.

RESULTS

The specimen data illustrate that white-gray matter susceptibility contrast is field strength independent. White-gray matter contrast improves significantly as a function of gadolinium contrast agent in the tissue, i.e., white matter appears increasingly more diamagnetic relative to gray matter. The simulated data from the model suggest that susceptibility anisotropy of white matter fiber bundles increases nonlinearly as a function of gadolinium concentration due to contrast agent compartmentalization into the extracellular white matter water pool.

CONCLUSION

Using contrast agents in MR histology facilitates white-gray matter susceptibility contrast modulation and the probing of white matter microstructure and orientation.

Lumbar muscle dysfunction during remission of unilateral recurrent nonspecific low-back pain: evaluation with muscle functional MRI

OBJECTIVES

After cessation of a low-back pain (LBP) episode, alterations in trunk muscle behavior, despite recovery from pain, have been hypothesized to play a pathogenic role in the recurrence of LBP. This study aimed to identify the presence of lumbar muscle dysfunction during the remission of recurrent LBP, while performing a low-load trunk-extension movement.

METHODS

Thirteen participants with unilateral recurrent LBP were tested at least 1 month after cessation of the previous LBP episode and were compared with a healthy control group without any history of LBP (n=13). Also, differences between previously painful and nonpainful sides were examined. Muscle functional magnetic resonance imaging, based on quantitative T2-imaging, was used to examine muscle tissue characteristics (T2 rest) and muscle recruitment (T2 shift) during prone trunk extension. The lumbar multifidus, erector spinae, quadratus lumborum, and psoas were bilaterally visualized on 2 lumbar levels using a T2-weighted (spin-echo multicontrast) magnetic resonance imaging sequence.

RESULTS

Linear mixed model analysis revealed a significantly lower T2 rest (P=0.044) and a significantly higher T2 shift (P=0.034) solely for the multifidus in the LBP group compared with the control group. No significant differences between pain sides were found.

DISCUSSION

Lower T2-rest values have been suggested to correlate with a conversion of the multifidus' fiber typing toward the glycolytic muscle spectrum. Elevated T2 shifts correspond with increased levels of metabolic activity in the multifidus in the LBP group, for which several hypotheses can be put forward. Taken together, these findings provide evidence of concurrent alterations in the multifidus structure and activity in individuals with unilateral recurrent LBP, despite being pain free and functionally recovered.

MRI findings of treated bacterial septic arthritis

INTRODUCTION

The purpose of this study was to report the MRI findings that can be encountered in successfully treated bacterial septic arthritis.

MATERIALS AND METHODS

The study included 12 patients (8 male and 4 female; mean age 38 years, range 9-85) with 13 proven cases of bacterial septic arthritis. The joints involved were hip (n = 3), knee (n = 3), shoulder (n = 2), sacroiliac (n = 2), ankle (n = 1), wrist (n = 1), and elbow (n = 1). MRI examinations following surgical debridement and at initiation of antibiotic therapy and after successful treatment were compared for changes in effusion, synovium, bone, and periarticular soft tissues. Imaging findings were correlated with microbiological and clinical findings.

RESULTS

Joint effusions were present in all joints at baseline and regressed significantly at follow-up MRI (p = 0.001). Abscesses were present in 5 cases (38 %), and their sizes decreased significantly at follow-up (p = 0.001). Synovial enhancement and thickening were observed in all joints at both baseline and follow-up MRI. Myositis/cellulitis was present in 10 cases (77 %) at baseline and in 8 cases (62 %) at follow-up MRI. Bone marrow edema was present in 10 joints (77 %) at baseline and persisted in 8 joints (62 %). Bone erosions were found in 8 joints (62 %) and persisted at follow-up MRI in all cases.

CONCLUSION

The sizes of joint effusions and abscesses appear to be the factors with the most potential for monitoring therapy for septic arthritis, since both decreased significantly following successful treatment. Synovial thickening and enhancement, periarticular myositis/cellulitis, and bone marrow edema can persist even after resolution of the infection.

Magnetic resonance and confocal imaging of solute penetration into the lens reveals a zone of restricted extracellular space diffusion

It has been proposed that in the absence of blood supply, the ocular lens operates an internal microcirculation system that delivers nutrients to internalized fiber cells faster and more efficiently than would occur by passive diffusion alone. To visualize the extracellular space solute fluxes potentially generated by this system, bovine lenses were organ cultured in artificial aqueous humor (AAH) for 4 h in the presence or absence of two gadolinium-based contrast agents, ionic Gd3+, or a chelated form of Gd3+, Gd-diethylenetriamine penta-acetic acid (Gd-DTPA; mol mass = 590 Da). Contrast reagent penetration into the lens core was monitored in real time using inversion recovery-spin echo (IR-SE) magnetic resonance imaging (MRI), while steady-state accumulation of [Gd-DTPA]−2 was also determined by calculating T1 values. After incubation, lenses were fixed and cryosectioned, and sections were labeled with the membrane marker wheat germ agglutinin (WGA). Sections were imaged by confocal microscopy using standard and reflectance imaging modalities to visualize the fluorescent WGA label and gadolinium reagents, respectively. Real-time IR-SE MRI showed rapid penetration of Gd3+ into the outer cortex of the lens and a subsequent bloom of signal in the core. These two areas of signal were separated by an area in the inner cortex that limited entry of Gd3+. Similar results were obtained for Gd-DTPA, but the penetration of the larger negatively charged molecule into the core could only be detected by calculating T1 values. The presence of Gd-DTPA in the extracellular space of the outer cortex and core, but its apparent absence from the inner cortex was confirmed using reflectance imaging of equatorial sections. In axial sections, Gd-DTPA was associated with the sutures, suggesting these structures provide a pathway from the surface, across the inner cortex barrier to the lens core. Our studies have revealed inner and outer boundaries of a zone within which a narrowing of the extracellular space restricts solute diffusion and acts to direct fluxes into the lens core via the sutures.

Differentiation between deep and superficial fibers of the lumbar multifidus by magnetic resonance imaging

The purpose of this study was to investigate the differentiation in muscle tissue characteristics and recruitment between the deep and superficial multifidus muscle by magnetic resonance imaging. The multifidus is a very complex muscle in which a superficial and deep component can be differentiated from an anatomical, biomechanical, histological and neuromotorial point of view. To date, the histological evidence is limited to low back pain patients undergoing surgery and cadavers. The multifidus muscles of 15 healthy subjects were investigated with muscle functional MRI. Images were taken under three different conditions: (1) rest, (2) activity without pain and (3) activity after experimentally induced low back muscle pain. The T2 relaxation time in rest and the shift in T2 relaxation time after activity were compared for the deep and superficial samples of the multifidus. At rest, the T2 relaxation time of the deep portion was significantly higher compared to the superficial portion. Following exercise, there was no significant difference in shift in T2 relaxation time between the deep and superficial portions, and in the pain or in the non-pain condition. In conclusion, this study demonstrates a higher T2 relaxation time in the deep portion, which supports the current assumption that the deep multifidus has a higher percentage of slow twitch fibers compared to the superficial multifidus. No differential recruitment has been found following trunk extension with and without pain induction. For further research, it would be interesting to investigate a clinical LBP population, using this non-invasive muscle functional MRI approach.

Temporal left atrial lesion formation after ablation of atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) ablation uses radiofrequency (RF) energy to induce thermal damage to the left atrium (LA) in an attempt to isolate AF circuits. This injury can be seen using delayed enhancement magnetic resonance imaging (DE-MRI).

OBJECTIVE

The purpose of this study was to describe DE-MRI findings of the LA in the acute and chronic stages postablation.

METHODS

Twenty-five patients were scanned at two time points postablation. The first group (n = 10) underwent DE-MRI at 24 hours and at 3 months. The second group (n = 16) was scanned at 3 months and at 6 or 9 months. One patient had three scans (24 hours, 3 months, 9 months) and was included in both groups. The location and extent of enhancement were then analyzed between both groups.

RESULTS

The median change in LA wall injury between 24 hours and 3 months was -6.38% (range -11.7% to 12.58%). The median change in LA wall injury between 3 months and later follow-up was +2.0% (range -4.0% to 6.58%). There appears to be little relationship between the enhancement at 24 hours and 3 months (R(2) = 0.004). In contrast, a strong correlation is seen at 3 months and later follow-up (R(2) = 0.966). Qualitative comparison revealed a stronger qualitative relationship between MRI findings at 3 months and later follow-up than at 24 hours and 3 months.

CONCLUSION

RF-induced scar appears to have formed by 3 months postablation. At 24 hours postablation, DE-MRI enhancement appears consistent with a transient inflammatory response rather than stable LA scar formation.

Meat quality assessment using biophysical methods related to meat structure

This paper overviews the biophysical methods developed to gain access to meat structure information. The meat industry needs reliable meat quality information throughout the production process in order to guarantee high-quality meat products for consumers. Fast and non-invasive sensors will shortly be deployed, based on the development of biophysical methods for assessing meat structure. Reliable meat quality information (tenderness, flavour, juiciness, colour) can be provided by a number of different meat structure assessment either by means of mechanical (i.e., Warner-Bratzler shear force), optical (colour measurements, fluorescence) electrical probing or using ultrasonic measurements, electromagnetic waves, NMR, NIR, and so on. These measurements are often used to construct meat structure images that are fusioned and then processed via multi-image analysis, which needs appropriate processing methods. Quality traits related to mechanical properties are often better assessed by methods that take into account the natural anisotropy of meat due to its relatively linear myofibrillar structure. Biophysical methods of assessment can either measure meat component properties directly, or calculate them indirectly by using obvious correlations between one or several biophysical measurements and meat component properties. Taking these calculations and modelling the main relevant biophysical properties involved can help to improve our understanding of meat properties and thus of eating quality.

Use of imaging to assess normal and adaptive muscle function

Physical therapists must be able to determine the activity and passive properties of the musculoskeletal system in order to accurately plan and evaluate therapeutic measures. Discussed in this article are imaging methods that not only allow for the measurement of muscle activity but also allow for the measurement of cellular processes and passive mechanical properties noninvasively and in vivo. The techniques reviewed are T1- and T2-weighted magnetic resonance (MR) imaging, MR spectroscopy, cine-phase-contrast MR imaging, MR elastography, and ultrasonography. At present, many of these approaches are expensive and not readily available in physical therapy clinics but can be found at medical centers. However, there are ways of using these techniques to provide important knowledge about muscle function. This article proposes creative ways in which to use these techniques as evaluative tools.

Steroid Myopathy: Evaluation of Fiber Atrophy with T2 Relaxation Time—Rabbit and Human Study

PURPOSE

To determine whether muscle fiber atrophy associated with steroid myopathy can be detected with T2 relaxation time.

MATERIALS AND METHODS

Animal and human studies were approved by the ethics committee. Informed consent was obtained. Twelve rabbits were divided into a group that received 3 mg/kg of triamcinolone subcutaneously each day for 10 consecutive days (n = 6) and a control group that received saline (n = 6). Magnetic resonance (MR) imaging was performed before and after treatment. T2 and fat deposition ratio (FDR) of soleus and gastrocnemius muscles before and after treatment and between control rabbits and rabbits treated with steroids were compared by using two-way repeated analysis of variance and Bonferroni post hoc test to evaluate effects of steroid treatment. After imaging, rabbits were sacrificed. Extracellular space ratio (ECSR) and fiber diameter were examined. Correlation among T2, ECSR, and diameter of type 2 muscle fibers was analyzed with a Pearson correlation test with Bonferroni correction in gastrocnemius to determine factors affecting T2. In humans, T2 relaxation time and FDR of both muscles were compared between volunteers not treated with steroids and patients treated with steroids by using an unpaired t test to evaluate the effects of steroids.

RESULTS

In rabbits, T2 of gastrocnemius muscle was significantly (P < .01) longer after steroid treatment than before steroid treatment and was also significantly (P < .01) longer than after saline administration. T2 of the gastrocnemius showed no significant difference in control rabbits before or after saline administration or in control rabbits and rabbits before steroid administration. T2 of the soleus muscle or FDR of either muscle showed no significant difference. There was a significant correlation (P < .01) among T2, ECSR, and diameter of type 2 muscle fibers in the gastrocnemius. In humans, T2 of the gastrocnemius was significantly (P < .01) longer in patients than in volunteers. T2 of the soleus or FDR of either muscle showed no significant difference.

CONCLUSION

Muscle fiber atrophy associated with steroid myopathy is detectable as prolongation of T2 relaxation time in the gastrocnemius muscle; the authors believe prolongation of T2 relaxation time is mainly due to increased ECSR reflecting type 2 muscle fiber atrophy.

Alterations of the proton-T2 time in relaxed skeletal muscle induced by passive extremity flexions

PURPOSE

To demonstrate reciprocal changes of the apparent proton-T2 time in the biceps and triceps due to passive contraction and extension of the muscle fibers.

MATERIALS AND METHODS

The contraction state of the upper arm muscles of six healthy volunteers was passively changed by alternating the forearm position between the straight-arm position and an elbow flexion of 90 degrees. The relaxation of the muscle during passive contraction and extension was measured with the use of muscle electromyography (EMG) experiments. Spin-echo (SE) MRI with increasing echo times (TEs) of 12-90 msec was used to acquire the averaged signal decay of the segmented biceps and triceps. The apparent T2 was deduced using monoexponential least-square fitting.

RESULTS

The median T2 alterations in biceps and triceps among all volunteers were found to be 1.2 and -1.3 msec in the straight and bent forearm positions, respectively. The confidence intervals (0.5 to 1.7 msec in biceps, and -2.6 to -1.1 msec in triceps) clearly indicate that proton-T2 in MR images is significantly (P < 0.05) prolonged with muscle contraction.

CONCLUSION

The observed increase of the proton-T2 time was correlated with a passive contraction of skeletal muscle fibers. This passive effect can be attributed to changes in the intracellular water mobility corresponding to the well-known "active" T2 increase that occurs after stimulation of muscle.

Myocardial late enhancement in contrast-enhanced cardiac MRI: distinction between infarction scar and non-infarction-related disease

OBJECTIVE

Our objective was to assess and compare the patterns of late enhancement (LE) in contrast-enhanced cardiac MRI caused by myocardial infarction and different myocardial diseases that are not related to ischemic infarction.

MATERIALS AND METHODS

A total of 811 consecutive contrast-enhanced cardiac MRI studies performed for different indications were reviewed for left ventricular myocardial LE after gadopentetate dimeglumine administration. MRI studies were performed on a 1.5-T scanner using an inversion recovery turbo FLASH sequence (TR/TE, 8/4 msec; flip angle, 25 degrees). The LE pattern of ischemic infarction scar was compared with that in nonischemic myocardial disease.

RESULTS

LE was found in 421 (52%) patients. In all patients with myocardial infarction, LE included the subendocardial layer. Nineteen patients without history of myocardial infarction and angiographically excluded coronary artery disease showed different patterns of LE caused by myocarditis, sarcoidosis, arrhythmogenic right ventricular dysplasia, cardiomyopathy, endomyocardial fibrosis, and iatrogenic scars after biopsy, ablation of septal hypertrophy, and myocardial laser revascularization.

CONCLUSION

LE in contrast-enhanced cardiac MRI is not specific for ischemic infarction. LE in ischemic infarction always involves the subendocardial layer, whereas it does not necessarily do so in other myocardial diseases. Therefore, if LE omit the subendocardial layer, different nonischemic myocardial diseases have to be considered. The pattern of LE might be helpful for the differential diagnosis of myocardial disease and in distinguishing it from ischemic disease.

Effect of muscular contraction on magnetization transfer detected at 1.5 T

The effect of rigor (formation of actomyosin complexes) on magnetization transfer was observed with a 1.5 T clinical magnetic resonance (MR) imaging system. The magnetization transfer ratio of chemically skinned (calcium-sensitive) muscle fiber preparations increased much more in a rigor state than in a relaxed state, while that of calcium-insensitive fiber preparations and solutions showed no difference. These results suggest that the formation of actomyosin complexes increased the magnetization transfer ratio. A clinical MR system is not only effective for medical imaging, but also has the potential to demonstrate physiological characteristics.

Measurements of extracellular volume fraction and capillary permeability in tissues using dynamic spin-lattice relaxometry: studies in rabbit muscles

Dynamic MR longitudinal R(1) relaxometry after administration of a gadolinium contrast bolus (Gd-DTPA) has been used for in vivo measurements of the extracellular volume fraction (v) and the capillary permeability (k min(-1)) in rabbit muscles to distinguish between red slow- and white fast-twitch muscle fiber types. For this purpose a protocol imaging sequence has been used which allows fast R(1) measurements during the contrast agent uptake. Physiological tissue parameters, k and v, were obtained by computing procedures assuming a simplified monoexponential plasma model. These were shown to be about twice as large in the slow-twitch semimembranosous proprius muscle (SP), containing 100% oxidative type-I fiber, that in the fast-twitch rectus femorus muscle (RF), containing only 6% type-I fiber type. The capillary permeability has been found to be 0.25 +/- 0.02 min(-1) for the (SP) and 0.10 +/- 0.01 min(-1) for the (RF). Similarly, the extracellular volume fractions were 0.189 +/- 0.015 and 0.082 +/- 0.006 respectively, in close agreement with literature data and experimental results obtained by invasive radionuclide measurements. For the pool of the 10 studied animals, no significant variation among animals was observed in the extracellular volume fraction and the capillary permeability for the different muscle fiber types. The dynamic relaxometry method used is easy to implement on conventional MR imagers and has potential applications in muscle diseases. The method has also potential applications for tissue characterization based on extracellular volume and capillary permeability quantification. In particular, the method can be used for the evaluation of tumors and their responses to therapies.

MRI of acute myocarditis: a comprehensive approach based on various imaging sequences

STUDY OBJECTIVES

To explore the diagnostic performance of MRI for the diagnosis of acute myocarditis, using a comprehensive imaging approach.

DESIGN AND SETTINGS

Twenty patients with myocarditis and 7 age-matched and gender-matched control subjects underwent comprehensive MRI. Magnetic resonance (MR) examinations included axial T2-weighted sequences, precontrast and postcontrast ECG-gated T1-weighted sequences in axial and short heart axis, cine-MRI, and serial dynamic turbo fast low-angle shot (turboFLASH) acquisitions in the short axis following Gd injection for a period of 2 min. Precontrast and postcontrast images were postprocessed using subtraction. Two observers read all images qualitatively and quantitatively. Myocardial enhancement was compared between patients and control subjects.

PATIENTS

Myocardial involvement was focal in 6 patients examined within 1 week from clinical onset, and diffuse in the remaining 14 patients examined later.

RESULTS

Qualitatively, contrast-enhanced T1-weighted subtracted images had 100% sensitivity and specificity for myocardial involvement. Postcontrast T1-weighted images were able to discriminate the early phase (nodular enhancement) from the later phase of myocarditis (diffuse enhancement). Quantitatively, myocardial enhancement was 56% +/- 3.2% in patients, vs 29% +/- 3.1% in control subjects using T1-weighted MRI (p < 0.0001). Serial turboFLASH images displayed greater myocardial enhancement between 25 s and 120 s in patients than in control subjects (p < 0.0001); however, there was marked enhancement of skeletal muscles in both early and late stages of myocarditis compared to control subjects (p < 0.0001).

CONCLUSION

On the basis of subtracted cardiac-gated T1-weighted images and serial postinjection turboFLASH images, our study shows that myocarditis is largely, at least in the early stages, a focal process in the myocardium. It also provides evidence of transient skeletal muscle involvement, which may actually be useful for diagnosis.

Changes in orcine muscle water characteristics during growth--an in vitro low-field NMR relaxation study

This study investigates the effects of developmental stage and muscle type on the mobility and distribution of water within skeletal muscles, using low-field (1)H-NMR transverse relaxation measurements in vitro on four different porcine muscles (M. longissimus dorsi, M. semitendinosus, M. biceps femoris, M. vastus intermedius) from a total of 48 pigs slaughtered at various weight classes between 25 kg and 150 kg. Principal component analysis (PCA) revealed effects of both slaughter weight and muscle type on the transverse relaxation decay. Independent of developmental stage and muscle type, distributed exponential analysis of the NMR T(2) relaxation data imparted the existence of three distinct water populations, T(2b), T(21), and T(22), with relaxation times of approximately 1-10, 45-120, and 200-500 ms, respectively. The most profound change during muscle growth was a shift toward faster relaxation in the intermediate time constant, T(21). It decreased by approx. 24% in all four muscle types during the period from 25 to 150 kg live weight. Determination of dry matter, fat, and protein content in the muscles showed that the changes in relaxation time of the intermediate time constant, T(21), during growth should be ascribed mainly to a change in protein content, as the protein content explained 77% of the variation in the T(21) time constant. Partial least squares (PLS) regression revealed validated correlations in the region of 0.58 to 0.77 between NMR transverse relaxation data and muscle development for all the four muscle types, which indicates that NMR relaxation measurements may be used in the prediction of muscle developmental stage.

Effects of aging on muscle T2 relaxation time: difference between fast- and slow-twitch muscles

RATIONALE AND OBJECTIVES

To determine whether the T2 relaxation time of skeletal muscle is affected by aging and to compare the effects of aging between fast- and slow-twitch muscles in a human study. To investigate the mechanisms of age-related changes in T2 relaxation time in an animal (mouse) study.

METHODS

T2 relaxation times of the soleus (slow-twitch, rich in type I fiber) and gastrocnemius (fast-twitch, rich in type II fiber) muscles were examined in 59 healthy human subjects, 22 to 76 years of age, by clinical magnetic resonance imaging. In mice, T2 relaxation times, fat ratios, and extracellular space ratios (extracellular space/intracellular plus extracellular space) of the spinalis (fast-twitch, rich in type II fiber) muscles were also examined (group of 7 old mice, 24-26 months; group of 7 young mice, 8-10 weeks).

RESULTS

In the human study, the T2 relaxation time of the gastrocnemius muscle increased significantly with aging (r = 0.53, P < 0.01) while that of the soleus muscle did not. In the animal study, the T2 relaxation time of the spinalis muscle was significantly longer (P < 0.05) and the extracellular space ratio of the spinalis muscle significantly wider (P < 0.01) in old than in young mice. No significant difference in fat ratio was observed between old and young mice. A significant, positive correlation was seen between the extracellular space ratio and T2 relaxation time (r = 0.84, P < 0.01).

CONCLUSIONS

The T2 relaxation time of fast-twitch muscle increases with aging, due mainly to increased extracellular space, reflecting age-related type II fiber atrophy.

Osmotic effects on the T2 relaxation decay of in vivo muscle

Saline solutions are commonly employed as a vehicle for drugs administered intramuscularly. In this study, in vivo measurements of spin-spin relaxation (T2) processes by magnetic resonance imaging (MRI) were performed to investigate the distribution of water in rat masseter muscle tissue after intramuscular injection of saline solutions of varying tonicity. Prior to saline injection, image-based T2 relaxation decay of muscle was monoexponential. After injection of saline, the T2 relaxation decay became multiexponential. Non-negative least squares (NNLS) analysis of the decay curves revealed two relaxation components: a fast component (T2 = 20-40 ms) and a slow component (T2 = 150-400 ms), which are assigned to intra- and extracellular water protons, respectively. Injection of hypertonic saline solutions significantly increased the extracellular water component in muscle tissue compared to isotonic saline solutions, an effect which lasted for more than 60 min. These findings suggest that MRI techniques may be useful to investigate the effect of hyper- or hypotonic solutions on muscle tissue in vivo.

Monitoring blood oxygen state in muscle microcirculation with transverse relaxation

Oxygen uptake from the microcirculation is a direct measure of tissue function. Magnetic resonance is capable of detecting differences between oxygenated and deoxygenated blood due to the paramagnetic properties of deoxyhemoglobin. At the level of the microcirculation, however, imaging methods cannot directly visualize the vessels. Instead, bulk MR parameters are investigated for their ability to monitor blood oxygen saturation (%O(2)) changes in the microcirculation of tissue, specifically skeletal muscle. Experiments in an in vitro model verified the feasibility of detecting changes in exponential decay signals, and also verified the prediction of only two distinct decay components. Experiments in a rabbit model demonstrate that T(2)' and monoexponential T(2) decay are not sensitive to blood oxygen changes, but that the long-T(2) component in a biexponential fit is correlated to the blood oxygen state. Assuming a two-pool model for water protons in muscle, and with knowledge of the T(2)-%O(2) relation, estimates of the microcirculation blood oxygen state can be made with some reasonable assumptions. Magn Reson Med 45:662-672, 2001.

Rapid relaxation times measurements by MRI: an in vivo application to contrast agent modeling for muscle fiber types characterization

This paper is a description of a simulation method to evaluate the contrast in NMR imaging and its aim is to help to optimize the use of contrast media in clinical imaging. Indeed, there is a need to define objective criteria in order to choose among several contrast media the ones that are the most effective and to define their optimal conditions of use, such as: the dose to be injected, the required time after injection to obtain the best enhancement and the optimal imaging sequence parameter values. The method is based on NMR signal simulation in the presence of contrast media and requires the fast measurement of the T1 and T2 relaxation times to obtain the dynamic relaxometry variation of tissues after contrast injection. In this work the fast imaging techniques that are to be described enable the measurement of T1 and T2 with a 30sec temporal resolution on 128*256 matrix images. The accuracy of the method was assessed in rabbit muscles after the injection of two gadolinium chelates (Gd-DTPA and Gd-DOTA) with the aim of improving the in vivo characterization of fast-twitch and slow-twitch muscle fiber types. The simulation results were in close agreement with contrast image analysis and showed, for relevant clinical doses, a small efficacy for both chelates. The interest of the proposed simulation method lies in the fact that it enables to objectively compare the efficacy of different contrast agents, to forecast the efficacy of a given contrast reagent and to define the optimal dose and the optimal imaging sequence parameters that give the best contrast. This simulation method obviates numerous prior experiments to evaluate the benefit expected from different contrast media. The method, which has been evaluated here for muscle investigations is applicable to any tissue analysis and can help to guide the best condition of use of contrast agents in MR imaging.

Contrast media-enhanced magnetic resonance imaging visualizes myocardial changes in the course of viral myocarditis

BACKGROUND

The course of tissue changes in acute myocarditis in humans is not well understood. Diagnostic tools currently available are unsatisfactory. We tested the hypothesis that inflammation is reflected by signal changes in contrast-enhanced magnetic resonance imaging (MRI).

METHODS AND RESULTS

We assessed 44 consecutive patients with symptoms of acute myocarditis. Nineteen patients met the inclusion criteria revealing ECG changes, reduced myocardial function, elevated creatine kinase, positive troponin T, serological evidence for acute viral infection, exclusion of coronary heart disease, and positive antimyosin scintigraphy. We studied these patients on days 2, 7, 14, 28, and 84 after the onset of symptoms. We obtained ECG-triggered, T1-weighted images before and after application of 0.1 mmol/kg gadolinium. We measured the global relative signal enhancement of the left ventricular myocardium related to skeletal muscle and compared it with measurements in 18 volunteers. The global relative enhancement was higher in patients on days 2 (4.8+/-0.3 [mean+/-SE] versus 2.5+/-0.2; P<.0001); 7 (4.7+/-0.5, P<.0001); 14 (4.6+/-0.5, P<.0002); and 28 (3.9+/-0.4, P=.009) but not on day 84 (3.1+/-0.3; P=NS). On day 2, the enhancement was focal, whereas at later time points, the enhancement was diffuse. In patients with evidence of ongoing disease, the values remained elevated.

CONCLUSIONS

Acute myocarditis evolves from a focal to a disseminated process during the first 2 weeks after onset of symptoms. Contrast media-enhanced MRI visualizes the localization, activity, and extent of inflammation and may serve as a powerful noninvasive diagnostic tool in acute myocarditis.

Characterization in vivo of muscle fiber types by magnetic resonance imaging

Magnetic resonance imaging has been used to characterize muscle fiber types. Here, T1 and T2 values were determined in pure slow-twitch and fast-twitch rabbit muscles and in rabbit muscles with mixed fiber types. The muscles with high proportions of oxidative slow-twitch fibers had higher T2 values than the others. Echo time, orientation of muscle fibers in B0, and moving spins had no effect on relaxation parameters. The results are discussed in terms of slow myosin isoform content and oxidative metabolism.

Relationships between fiber composition and NMR measurements in human skeletal muscle

The purpose of this study was to examine the relationships between the relative contents of phosphocreatine (PCr), inorganic phosphate (Pi), beta-adenosine triphosphate (ATP), and transverse relaxation time (T2) with fiber composition, which determined histochemically in the human skeletal muscle. The vastus lateralis muscles of 28 volunteers were subjected to phosphorus nuclear magnetic resonance (31P NMR) spectroscopy, magnetic resonance imaging (MRI) and muscle biopsy. Muscle fibers were divided into type I and type II fibers using myosin ATPase stain. A wide range of fiber composition levels were observed in the subjects (27.3-74.6% type I fibers). The PCr/ATP, Pi/ATP and (PCr + Pi)/ATP ratios were positively related to the percentage of type II fibers (r = 0.695, p < 0.001, r = 0.429, p < 0.05 and r = 0.773, p < 0.001, respectively). There was no correlation between fiber composition and the PCr/Pi ratio (r 0.127, n.s.) or intracellular pH (r = 0.305, n.s.). Moreover, no correlation was found between T2 and fiber type (r = 0.144, n.s.). These results suggest that 31P NMR can detect the differences in relative content of phosphates between type I and type II fibers, thereby noninvasively evaluating fiber composition in human skeletal muscle.

Pulsed NMR relaxometry of striated muscle fibers

The longitudinal (T1) and transverse (T2) proton (1H) nuclear magnetic resonance (NMR) relaxation of fast- and slow-twitch muscle fibers are examined using rat muscle tissues in which one fiber type predominates. Both continuum and discrete exponential component fits are made to Carr-Purcell-Meiboom-Gill (CPMG) and inversion recovery pulse sequence measurements. In addition, experiments which illustrate the large sources of variability that have led to apparent conflicts in the literature are presented. Based on the results of this study, unique NMR features that distinguish fast- and slow-twitch muscle fibers are presented. The feasibility of differentiating fast- and slow-twitch muscle fibers using magnetic resonance (MR) imaging is briefly discussed.

Regional 1H transverse magnetization studies in perfused rabbit kidney

A Carr-Purcell-Meiboom-Gill imaging sequence consisting of 128 echoes is used to extract transverse magnetization decay curves (TDCs) at 1.9 T from 1.7 x 1.7 x 5-mm3 voxels within the cortex, outer medulla, and inner medulla of perfused rabbit kidneys. The spatially localized TDCs within each tissue type are found to be better approximated by biexponential, as opposed to monoexponential, functions. The biexponential parameters characterizing the TDCs demonstrate an improved degree of tissue specificity over that available from monoexponential analyses. The fraction of the quickly relaxing TDC component and the relaxation rate of this component are observed to decrease from cortex to inner medulla. A two-site exchange analysis is used to convert biexponential TDC parameters into water volume fractions and exchange rates. The exchange rates between the fast and slowly relaxing pools increased from cortex to inner medulla. All exchange rates were less than 1.5 Hz, indicating a relatively slow water exchange process. The imaging methods and subsequent analyses offer the potential to generate unconventional MR images with tissue contrast dependent upon water compartmentation and exchange.

Observation of rat hind limb skeletal muscle during arterial occlusion and reperfusion by 31P MRS and 1H MRI

31P NMR spectra and 1H MR T1- and T2-weighted spin-echo images were concurrently observed in rat hind limb during arterial occlusion and following reperfusion. With arterial occlusion, phosphocreatine level decreased and inorganic phosphate (Pi) level increased in 31P NMR spectra. Intracellular pH's dropped as a function of time. Beta-ATP started to decrease in three hours. In six hours after the occlusion, any peaks other than Pi were scarcely detected. The signal intensities in the 1H MR images increased homogeneously in both T1- and T2-weighted conditions, but the changes were more profound with T2-weighted images. After the release of the arterial occlusion, the 31P NMR spectra recovered to the preischemic state in several hours. The 1H MR images during reperfusion showed characteristic heterogenous pattern. The signal intensities in the anterior tibial muscle and the gastrocnemius muscle remained high in T1-weighted condition and the intensities further increased in T2-weighted condition, while those in other parts returned to the preischemic level. These changes were found to be irreversible even 12 hr after the release. The high signal intensities suggested the increase of water in the extracellular compartment induced by so-called reperfusion injury. Multinuclear analysis using in vivo NMR was valuable to consecutively detect time-dependent and location-specific response in skeletal muscle during ischemia and reperfusion.

1D spectroscopic imaging with rf echo planar (SIRFEN) methods

A recently developed rf echo planar imaging method has been modified to rapidly generate spectroscopic information along one in-plane axis and spatial information along the other. The method allows the production of one-dimensional chemical shift images (1D CSIs) in acquisition times of 18 sec or less. A specific phase-encode-reordering algorithm provides convenient manipulation of T2 weighting, yielding partial suppression of short T2 species like muscle water. The method is demonstrated in phantoms and in vivo with 1D CSIs of human brain and limbs. Abnormal fat distribution is demonstrated in the calf of a patient with aggressive fibromatosis. The advantages of short acquisition times obtainable with SIRFEN are offset by limited spectral resolution, suggesting that primary applications will be confined to rapid spatial mapping of major spectral components.