Somatosensory cortical activation identified by functional MRI in preterm and term infants.

Functional MRI (fMRI) has not previously been used systematically to investigate brain function in preterm infants. We here describe statistically robust and reproducible fMRI results in this challenging subject group using a programmable somatosensory stimulus synchronized with MR image acquisition which induced well-localized positive blood oxygen level dependent (BOLD) responses contralateral to the side of the stimulation in: 11 preterm infants (median post menstrual age 33 weeks and 4 days, range 29+1 to 35+3); 6 control infants born at term gestational age; and 18 infants born preterm (median gestational age at birth 30 weeks and 5 days, range 25+4 to 36+0) but studied at term corrected gestational age. Bilateral signals were identified in 8 of the ex-preterm infants at term age. Anatomical confirmation of appropriate activations was provided with diffusion tensor imaging (DTI) based tractography which identified connecting pathways from the regions of activation through the ipsilateral corticospinal tracts and posterior limb of the internal capsule. These results demonstrate that it is possible to reliably identify positive BOLD signals in the infant brain and that fMRI techniques can also be applied in the study of preterm infants.

A patient care system for early 3.0 Tesla magnetic resonance imaging of very low birth weight infants.

BACKGROUND: Very low birth weight (VLBW) infants (weight <1500 g) are increasingly cared for without prolonged periods of positive pressure ventilation (PPV). AIMS: To develop a system for 3.0 T magnetic resonance (MR) image acquisition from VLBW infants who are not receiving PPV, and to test the clinical stability of a consecutive cohort of such infants. DESIGN: Seventy VLBW infants whose median weight at image acquisition was 940 g (590-1490) underwent brain MR imaging with the developed care system as participants in research. Twenty infants (29%) received nasal continuous positive airway pressure (nCPAP), 28 (40%) received supplemental oxygen by nasal cannulae, and 22 (31%) breathed spontaneously in air during the MR examination. RESULTS: There were no significant adverse events. Seventy-six percent had none or transient self-correcting oxygen desaturations. Desaturations that required interruption of the scan for assessment were less common among infants receiving nCPAP (2/20) or breathing spontaneously in air (2/22), compared with those receiving nasal cannulae oxygen (13/28), p=0.003. Sixty-four (91%) infants had an axillary temperature > or =36 degrees C at completion of the scan (lowest 35.7 degrees C), There was no relationship between weight (p=0.167) or use of nCPAP (p=0.453) and axillary temperature <36 degrees C. No infant became hyperthermic. CONCLUSION: VLBW infants who do not require ventilation by endotracheal tube can be imaged successfully and safely at 3.0 T, including those receiving nCPAP from a customised system.

High b-value diffusion tensor imaging of the neonatal brain at 3T.

BACKGROUND AND PURPOSE: Diffusion-weighted MR imaging studies of the adult brain have shown that contrast between lesions and normal tissue is increased at high b-values. We designed a prospective study to test the hypothesis that diffusion tensor imaging (DTI) obtained at high b-values increases image contrast and lesion conspicuity in the neonatal brain. MATERIALS AND METHODS: We studied 17 neonates, median (range) age of 10 (2-96) days, who were undergoing MR imaging for clinical indications. DTI was performed on a Philips 3T Intera system with b-values of 350, 700, 1500, and 3000 s/mm(2). Image contrast and lesion conspicuity at each b-value were visually assessed. In addition, regions of interest were positioned in the central white matter at the level of the centrum semiovale, frontal and occipital white matter, splenium of the corpus callosum, posterior limb of the internal capsule, and the thalamus. Apparent diffusion coefficient (ADC) and fractional anisotropy (FA) values for these regions were calculated. RESULTS: Isotropic diffusion image contrast and lesion-to-normal-tissue contrast increased with increasing b-value. ADC values decreased with increasing b-value in all regions studied; however, there was no change in FA with increasing b-value. CONCLUSIONS: Diffusion image contrast increased at high b-values may be useful in identifying lesions in the neonatal brain.

A rapid method for labelling CD4+ T cells with ultrasmall paramagnetic iron oxide nanoparticles for magnetic resonance imaging that preserves proliferative, regulatory and migratory behaviour in vitro.

A number of techniques have been developed to track the migration of T cells in vivo, but they all suffer significant shortcomings, including the examination of selected organs rather than the organism as a whole--thus precluding longitudinal studies--or limitations imposed by poor spatial resolution and the application of ionizing radiation. By conjugating the HIV tat peptide to ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles in a reaction yielding a mean valence of 45, a magnetic resonance (MR) contrast agent was synthesised that allowed T cells to be efficiently labelled within just 5 min. The USPIO nanoparticles were incorporated into both the cytoplasm and nucleus of labelled cells, which retained normal in vitro proliferative responses to a polyclonal stimulus; suppressive responses mediated by labelled CD4(+) CD25(+) regulatory T cells; chemotactic responses to the chemokine CXCL-12; and transmigration of an activated endothelial monolayer. We believe that this rapid, efficient and essentially non-toxic approach to labelling both murine and human T cells for MRI holds considerable promise, paving the way for the wider immunological application of this exciting technology.

Technical report: Magnetic resonance direct thrombus imaging at 3 T field strength in patients with lower limb deep vein thrombosis: a feasibility study.

AIM: To investigate the feasibility of imaging lower limb deep vein thrombosis using magnetic resonance imaging (MRI) at 3.0 T magnetic field strength with an optimized a T1 magnetization prepared rapid gradient echo technique (MP-RAGE) in patients with normal volunteers as controls. MATERIALS AND METHODS: Patients with deep vein thrombosis (n = 4), thrombophlebitis (n = 2) and healthy volunteers (n = 9) were studied. MRI of the distal thigh and upper calf was performed at 3.0 T with MP-RAGE using two pre-pulses to suppress blood and fat (flip angle 15 degrees, echo time 5 ms, and repetition time 10 ms). A qualitative analysis was performed for detection of thrombi and image quality. Contrast-to-noise ratios were determined in thrombosed and patent veins. RESULTS: Thrombi were clearly visible as high-signal intensity structures with good suppression of the anatomical background. A blinded reader accurately diagnosed 15 out of 16 cases. The contrast-to-noise ratio measurements showed a positive contrast of thrombus over background muscle 16.9 (SD 4.3, 95% CI: 12.5-21.3) and a negative contrast of the lumen to muscle in patent veins of normal volunteers -7.8 (SD 4.3, 95% CI: -11.1 to -4.5), with p = 0.0015. CONCLUSION: Thrombi generate high signal intensity at 3.0 T allowing for their direct visualization if flowing blood, stationary blood and fat are sufficiently suppressed. This preliminary data supports the development of these techniques for other vascular applications.

A comparison of MR cholangiopancreatography at 1.5 and 3.0 Tesla.

Clinical MR systems operating at 3.0 Tesla have the potential to significantly improve spatial resolution due to the boost in intrinsic signal to noise ratio. However, body imaging at these field strengths presents a number of technical challenges. We performed a prospective pilot study in which 10 patients underwent an MR cholangiopancreatography (MRCP) examination consecutively on 1.5 and 3.0 Tesla systems (both Philips Intera). An axial half Fourier segmented turbo spin echo (HASTE) sequence and a coronal thick-slab 2D turbo-spin echo (TSE) sequence were compared on both systems. A reader measured the signal intensity (SI) ratios of common bile duct (CBD): liver, and CBD: fat on HASTE images and CBD: liver on the TSE images. A second reader performed a qualitative analysis of the intrahepatic and extrahepatic biliary anatomy. Quantitative data was compared using the paired t-test and qualitative data with the paired Wilcoxon signed rank test with p < 0.05. The quantitative analysis of the HASTE sequences showed a slightly higher signal intensity ratio (CBD:liver) at 3.0 Tesla compared with 1.5 Tesla (8.1 vs 5.6, p = 0.002). No significant difference was found between the SI ratios of (CBD:fat) on HASTE images or (CBD:liver) on TSE images. The qualitative analysis showed superior image quality of 3.0 Tesla over 1.5 Tesla images on both HASTE (31 vs 25, p = 0.032), and TSE sequences (34 vs 28, p = 0.043). This pilot study shows that MRCP is feasible at 3.0 Tesla with some improvement in image quality and signal characteristics. Further development may be achieved with sequence optimization and improved coil design.

Magic angle imaging of the achilles tendon in patients with chronic tendonopathy.

AIMS: To assess the Achilles tendon in patients with chronic tendonopathy using magnetic resonance (MR) magic angle imaging, and to compare the appearances and uptake of contrast medium in abnormal tendons with those in normal tendons. MATERIAL AND METHODS: Eight patients with chronic Achilles tendonopathy and five normal controls were examined with the long axis of the tendon placed at 55 degrees and at 0 degrees to the main magnetic field. Conventional two-dimensional (2D) multi-slice images were obtained and T1 values were calculated before, and for up to 1h after the administration of intravenous gadodiamide. Both the unenhanced appearance and the pattern of enhancement in the tendon were compared. RESULTS: In the patients with tendonopathy, high signal intensity areas were evident on the short T1 inversion recovery (STIR) images obtained at 55 degrees in all tendons. Contrast medium enhancement was seen in six tendons and was most obvious on the images obtained at the magic angle. This was initially focal and then spread more diffusely within the tendon. After contrast medium administration, T1 values were significantly reduced in the tendonopathy group compared with normal controls (p<0.01). On the late post-contrast medium images obtained at 55 degrees, enhancement was evident in most of the tendon and correlated well with high signal intensity seen on STIR images. CONCLUSION: The use of magic angle MR imaging improved the demonstration of signal changes in the Achilles tendon in chronic tendonopathy. The STIR images obtained at the magic angle showed more obvious signal change than those obtained at 0 degrees. The changes due to enhancement were much more evident on images obtained at 55 degrees than at 0 degrees. The uptake of contrast medium was greater in the patients than in normal controls.

SMASH navigators.

The additional data acquired when using multiple receiver coils is commonly used to improve SNR or reduce acquisition times. It may also be used to remove image artifacts by selectively replacing corrupt data. In the present study, a correction scheme is presented based on simultaneous acquisition of spatial harmonics (SMASH) that enables detection and correction of motion artifacts caused by 2D translations. Newly measured data is compared with predictions from previously measured data by making negative and positive spatial harmonics. Differences are attributed to motion occurring in the interval between the acquisition of separate phase encode lines and correction parameters are determined. Two types of rigid body motion are considered: 1) object and coil array move, and 2) object only moves, since each causes different phase errors in k-space. Simulation, phantom, and volunteer experiments demonstrate the validity of the technique.

Detection and elimination of motion artifacts by regeneration of k-space.

A method has been developed using techniques from partially parallel imaging (PPI) to detect localized inconsistencies in k-space that are caused by certain types of motion. The inconsistent data are discarded and consistent data regenerated from the remaining data using PPI techniques. The price is a small decrease in signal-to-noise ratio (SNR) and additional postprocessing. An iterative scheme is presented which does not require separately acquired coil sensitivity information for the PPI reconstructions. This method has been found to reduce artifact levels in phantom and in vivo test studies.

Combination of signals from array coils using image-based estimation of coil sensitivity profiles.

It is well established that the optimal unbiased way to combine image data from array coils is a pixel-by-pixel sum of coil signals, with each signal weighted by the individual coil sensitivity at the location of the pixel. A pragmatic alternative combines the images from the coils as the square root of the sum of squares (SOS), which can reduce the signal-to-noise ratio (SNR) and introduce bias. This work describes how to replace coil sensitivity by an image-derived quantity that enables close to optimal signal combination up to a global intensity scaling. Typical scaling is by an individual coil sensitivity or a linear or SOS combination of the sensitivities of some or all of the coils in the array. The method decreases signal bias, improves SNR when coils have unequal noise levels, and can reduce image artifacts. It can produce phase-corrected data, which eliminates bias completely. In addition, the method allows images from arrays that include highly localized coils, such as a prostate coil and external pelvic array, to be combined with near-optimal SNR and an intensity modulation that makes them easier to view.

An investigation into the use of sensitivity-encoded techniques to increase temporal resolution in dynamic contrast-enhanced breast imaging.

Gadolinium-enhanced dynamic magnetic resonance (MR) imaging is playing an increasingly important diagnostic role in patients with breast cancer. Because of the multi-focal nature of the disease, it is mandatory to cover all of both breasts, not only in the initial scan, but also at subsequent follow-up. This requires volume acquisitions with a temporal resolution limited to 60-80 seconds, which is insufficient to clearly discriminate malignant from benign rates of contrast uptake. In this work, we performed sensitivity-encoded imaging using a commercially available four-channel breast coil (MRI Devices Corporation) on a commercial 0.5-T scanner with moderate gradient performance to give increased temporal resolution in these dynamic contrast-enhanced scans. A two-fold increase in temporal resolution was readily achievable with this coil. Image reconstruction was robust and image quality was assessed qualitatively to be good. We also investigated higher speed-up factors using two directions of sensitivity-encoded reduction and discussed some of the potential artifacts associated with such imaging.

Catheter tracking for MR fluoroscopy: design of a transmit/receive coil for use with a nasogastric tube.

A coil tuned to 21.3 MHz was incorporated into a nasogastric tube and used as a marker of tube position during magnetic resonance (MR) imaging in a 0.5-T scanner. Catheter tracking was investigated with the coil used in both transmit/receive and in receive-only modes. Data acquired from this coil were overlaid on images obtained using the body coil of the scanner. Visualization of the full length of the catheter with local high signal at the tip was achieved with a temporal resolution of approximately 1 second. J. Magn. Reson. Imaging 2001;13:127-130.

Use of multicoil arrays for separation of signal from multiple slices simultaneously excited.

Increased acquisition efficiency has been achieved by exciting several slices simultaneously. The mixed data were unfolded to produce separate slices using the spatial encoding information inherent in a multicoil receiver system. Each coil yields a linear combination of signals from all excited slices weighted by the sensitivity of each coil. A matrix inversion provides a solution to unfold these images.

Microstructured magnetic materials for RF flux guides in magnetic resonance imaging.

Magnetic resonance imaging and spectroscopy systems use coils, either singly or as arrays, to intercept radio-frequency (RF) magnetic flux from regions of interest, often deep within the body. Here, we show that a new magnetic material offers novel possibilities for guiding RF flux to the receiver coil, permitting a clear image to be obtained where none might otherwise be detectable. The new material contains microstructure designed according to concepts taken from the field of photonic band gap materials. In the RF range, it has a magnetic permeability that can be produced to specification while exhibiting negligible direct-current magnetism. The latter property is vital to avoid perturbing the static and audio-frequency magnetic fields needed to obtain image and spectral data. The concept offers a new paradigm for the manipulation of RF flux in all nuclear magnetic resonance systems.

Elimination of magnetic field foldover artifacts in MR images.

Foldover artifacts arise when the same imaging frequency occurs both at a desired location within a slice and at another location within the sensitive region of the radiofrequency (RF) coil. Foldover artifacts can be caused by nonlinearity in the gradient system and by inhomogeneity in B(0). This study investigates an approach in which an extra RF receiver coil and a postprocessing method are used to identify and remove foldover artifacts.

Design and use of internal receiver coils for magnetic resonance imaging.

This review describes coils for MRI that are inserted into the body through natural orifices. It covers the design and implementation of small internal receiver coils for use in the pelvis and gastrointestinal tract. Normal anatomy delineated by the high resolution obtained by using these coils and the appearances in a number of disease states for each clinical application are described.