[show abstract][hide abstract] ABSTRACT: We compared overexpression of the magnetotactic bacterial gene MagA with the modified mammalian ferritin genes HF + LF, in which both heavy and light subunits lack iron response elements. Whereas both expression systems have been proposed for use in non-invasive, magnetic resonance (MR) reporter gene expression, limited information is available regarding their relative potential for providing gene-based contrast. Measurements of MR relaxation rates in these expression systems are important for optimizing cell detection and specificity, for developing quantification methods, and for refinement of gene-based iron contrast using magnetosome associated genes. We measured the total transverse relaxation rate (R2*), its irreversible and reversible components (R2 and R2', respectively) and the longitudinal relaxation rate (R1) in MDA-MB-435 tumor cells. Clonal lines overexpressing MagA and HF + LF were cultured in the presence and absence of iron supplementation, and mounted in a spherical phantom for relaxation mapping at 3 Tesla. In addition to MR measures, cellular changes in iron and zinc were evaluated by inductively coupled plasma mass spectrometry, in ATP by luciferase bioluminescence and in transferrin receptor by Western blot. Only transverse relaxation rates were significantly higher in iron-supplemented, MagA- and HF + LF-expressing cells compared to non-supplemented cells and the parental control. R2* provided the greatest absolute difference and R2' showed the greatest relative difference, consistent with the notion that R2' may be a more specific indicator of iron-based contrast than R2, as observed in brain tissue. Iron supplementation of MagA- and HF + LF-expressing cells increased the iron/zinc ratio approximately 20-fold, while transferrin receptor expression decreased approximately 10-fold. Level of ATP was similar across all cell types and culture conditions. These results highlight the potential of magnetotactic bacterial gene expression for improving MR contrast.
[show abstract][hide abstract] ABSTRACT: Purpose: Attenuation correction for whole-body PET/MRI is challenging. Most commercial systems compute the attenuation map from MRI using a four-tissue segmentation approach. Bones, the most electron-dense tissue, are neglected because they are difficult to segment. In this work, the authors build on this segmentation approach by adding bones using a registration technique and assessing its performance on human PET images.Methods: Twelve oncology patients were imaged with FDG PET/CT and MRI using a Turbo-FLASH pulse sequence. A database of 121 attenuation correction quality CT scans was also collected. Each patient MRI was compared to the CT database via weighted heuristic measures to find the "most similar" CT in terms of body geometry. The similar CT was aligned to the MRI with a deformable registration method. Two MRI-based attenuation maps were computed. One was a standard four-tissue segmentation (air, lung, fat, and lean tissue) using basic image processing techniques. The other was identical, except the bones from the aligned CT were added. The PET data were reconstructed with the patient's CT-based attenuation map (the silver standard) and both MRI-based attenuation maps. The relative errors of the MRI-based attenuation corrections were computed in 14 standardized volumes of interest, in lesions, and over whole tissues. The squared Pearson correlation coefficient was also calculated over whole tissues. Statistical testing was done with ANOVAs and paired t-tests.Results: The MRI-based attenuation correction ignoring bone had relative errors ranging from -37% to -8% in volumes of interest containing bone. By including bone, the magnitude of the relative error was reduced in all cases ( p < 0.001), ranging from -3% to 4%. Further, the relative error in volumes of interest adjacent to bone was improved from a mean of -7.5% to 2% ( p < 0.001). In the other seven volumes of interest, including bone reduced the magnitude of relative error in three cases ( p < 0.001), had no effect in three cases, and increased relative error in one case. There was no statistically significant difference in the relative error in lesions. Over whole tissues, including bone slightly increased relative error in lung from 7.7% to 8.0% ( p = 0.002), in fat from 8.5% to 9.2%( p < 0.001), and in lean tissue from -2.1% to 2.6% ( p < 0.001), but reduced the magnitude of relative error in bone from -14.6% to 1.3% ( p < 0.001). The correlation coefficient was essentially unchanged in all tissues regardless of whether bone was included or not.Conclusions: The approach to include bones in MRI-based attenuation maps described in this work improves quantification of whole-body PET images in and around bony anatomy. The reduction in error is often large (tens of percents), and could alter image interpretation and subsequent patient care. Changes in other parts of the PET image are minimal and likely not of clinical significance.
Medical Physics 08/2013; 40(8):082509. · 2.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: The bacterial gene MagA imparts magnetic properties to mammalian cells and provides a basis for cell tracking by magnetic resonance imaging (MRI). In a mouse model of tumor growth from transplanted cells, we used repetitive MRI to demonstrate the in vivo imaging potential of MagA expression relative to a modified ferritin overexpression system, lacking regulation through iron response elements (HF + LF).
Subcutaneous tumor xenografts were monitored weekly from days 2 to 34 post-injection. Small animal MRI employed balanced steady-state free precession. Imaging was correlated with tumor histology using hematoxylin, Prussian Blue, Ki-67, and BS-1 lectin.
Tumor heterogeneity with respect to tissue morphology and magnetic resonance (MR) contrast was apparent within a week of cell transplantation. In MagA- and HF + LF-expressing tumors, MR contrast enhancement was recorded up to day 20 post-injection and 0.073-cm(3) tumor volumes. MagA-expressing tumors showed increases in both quantity and quality of MR contrast as measured by fractional void volume and contrast-to-noise ratio, respectively. MR contrast in both MagA- and HF + LF-expressing tumors was maximal by day 13, doubling fractional void volume 1 week ahead of controls.
MagA- and HF + LF-expressing tumor xenografts augment MR contrast after 1 week of growth. MagA expression increases MR contrast within days of cell transplantation and provides MR contrast comparable to HF + LF. MagA has utility for monitoring cell growth and differentiation, with potential for in vivo detection of reporter gene expression using MRI.
Molecular imaging and biology: MIB: the official publication of the Academy of Molecular Imaging 07/2013; · 2.47 Impact Factor
[show abstract][hide abstract] ABSTRACT: The hybridization of positron emission tomography (PET) and magnetic resonance imaging (MRI) within a single imaging bore is a major advance in noninvasive imaging. Intrinsic co-registration of metabolic/molecular probe imaging with morphological, functional, and tissue imaging presents new opportunities for disease characterization.
Sarcoidosis is a multisystem inflammatory disease hall- marked by inflammation, noncaseating granuloma formation, and organ dysfunction. Cardiac involvement accounts for up to 25% of disease-related mortality and is conventionally diagnosed with the Japanese Ministry criteria. However, studies using cardiac PET and MRI suggest a robust capacity to identify cardiac involvement—PET through identification of active inflammation and MRI through identification of mature fibrosis or scar.
In this report, we describe the first clinical use of simultaneous PET-MRI to assist in the diagnosis of cardiac disease: active cardiac sarcoidosis.
[show abstract][hide abstract] ABSTRACT: The proton MAS NMR spectra in MCM-41 at low hydration levels (less than hydration amounting to one water molecule per surface hydroxyl group) show complex proton resonance peak structures, with hydroxyl proton resonances seen in dry MCM-41 disappearing as water is introduced into the pores and new peaks appearing, representing water and hydrated silanol groups. Surface hydroxyl group-water molecule chemical exchange and chemical shift averaging brought about by a water molecule visiting different surface hydrogen bonding sites have been proposed as possible causes for the observed spectral changes. In this report a simple model based on chemical shift averaging, due to the making and breaking of hydrogen bonds as water molecules move on the MCM-41 surface, is shown to fully reproduce the NMR spectra, both as a function of hydration and temperature. Surface proton-water proton chemical exchange is not required in this model at low hydration levels.
Solid State Nuclear Magnetic Resonance 11/2012; · 2.10 Impact Factor
[show abstract][hide abstract] ABSTRACT: Gastrocnemius muscle phosphocreatine ([PCr]) and hydrogen ion ([H(+)]) were measured using (31)P-magnetic resonance spectroscopy during repeated bouts of 10-s heavy-intensity (HI) exercise and 5-s rest compared with continuous (CONT) HI exercise. Recreationally active male subjects (n = 7; 28 yr ± 9 yr) performed on separate occasions 12 min of isotonic plantar flexion (0.75 Hz) CONT and intermittent (INT; 10-s exercise, 5-s rest) exercise. The HI power output in both CONT and INT was set at 50% of the difference between the power output associated with the onset of intracellular acidosis and peak exercise determined from a prior incremental plantar flexion protocol. Intracellular concentrations of [PCr] and [H(+)] were calculated at 4 s and 9 s of the work period and at 4 s of the rest period in INT and during CONT exercise. [PCr] and [H(+)] (mean ± SE) were greater at 4 s of the rest periods vs. 9 s of exercise over the course of the INT exercise bout: [PCr] (20.7 mM ± 0.6 vs. 18.7 mM ± 0.5; P < 0.01); [H(+)] (370 nM ± 13.50 vs. 284 nM ± 13.6; P < 0.05). Average [H(+)] was similar for CONT vs. INT. We therefore suggest that there is a glycolytic contribution to ATP recovery during the very short rest period (<5 s) of INT and that the greater average power output of CONT did not manifest in greater [H(+)] and greater glycolytic contribution compared with INT exercise.
Journal of Applied Physiology 05/2012; 113(3):410-7. · 3.48 Impact Factor
[show abstract][hide abstract] ABSTRACT: Present attenuation-correction algorithms in whole-body PET/MRI do not consider variations in lung density, either within or between patients; this may adversely affect accurate quantification. In this work, a technique to incorporate patient-specific lung density information into MRI-based attenuation maps is developed and compared with an approach that assumes uniform lung density.
Five beagles were scanned with (18)F-FDG PET/CT and MRI. The relationship between MRI and CT signal in the lungs was established, allowing the prediction of attenuation coefficients from MRI. MR images were segmented into air, lung, and soft tissue and converted into attenuation maps, some with constant lung density and some with patient-specific lung densities. The resulting PET images were compared by both global metrics of quantitative fidelity (accuracy, precision, and root mean squared error) and locally with relative error in volumes of interest.
A linear relationship was established between MRI and CT signal in the lungs. Constant lung density attenuation maps did not perform as well as patient-specific lung density attenuation maps, regardless of what constant density was chosen. In particular, when attenuation maps with patient-specific lung density were used, precision, accuracy, and root mean square error improved in lung tissue. In volumes of interest placed in the lungs, relative error was significantly reduced from a minimum of 12% to less than 5%. The benefit extended to tissues adjacent to the lungs but became less important as distance from the lungs increased.
A means of using MRI to infer patient-specific attenuation coefficients in the lungs was developed and applied to augment whole-body MRI-based attenuation maps. This technique has been shown to improve the quantitative fidelity of PET images in the lungs and nearby tissues, compared with an approach that assumes uniform lung density.
Journal of Nuclear Medicine 05/2012; 53(6):977-84. · 5.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: Previous work has shown an association between restricted wrist range of motion (ROM) and upper extremity musculoskeletal disorders in computer users. We compared the prevalence of MRI-identified wrist abnormalities and wrist ROM between asymptomatic and symptomatic computer users.
MR images at 1.5 T of both wrists were obtained from 10 asymptomatic controls (8 F, 2 M) and 14 computer users (10 F, 4 M) with chronic wrist pain (10 bilateral; 4 right-side). Maximum wrist range of motion in flexion and radioulnar deviation was measured with an electrogoniometer.
Extraosseous ganglia were identified in 66.6% of asymptomatic wrists and in 75% of symptomatic wrists. Intraosseous ganglia were identified in 45.8% of asymptomatic wrists and in 75% of symptomatic wrists, and were significantly (p < .05) larger in the symptomatic wrists. Distal ECU tendon instability was identified in 58.4% of both asymptomatic and symptomatic wrists. Dominant wrist flexion was significantly greater in the asymptomatic group (68.8 ± 6.7 deg.) compared to the symptomatic group (60.7 ± 7.3 deg.), p < .01. There was no significant correlation between wrist flexion and intraosseous ganglion burden (p = .09)
This appears to be the first MRI study of wrist abnormalities in computer users.This study demonstrates that a variety of wrist abnormalities are common in computer users and that only intraosseous ganglia prevalence and size differed between asymptomatic and symptomatic wrists. Flexion was restricted in the dominant wrist of the symptomatic group, but the correlation between wrist flexion and intraosseous ganglion burden did not reach significance. Flexion restriction may be an indicator of increased joint loading, and identifying the cause may help to guide preventive and therapeutic interventions.
[show abstract][hide abstract] ABSTRACT: The pattern of sexual differentiation of the human brain is not well understood, particularly at the early stages of development when intense growth and multiple maturational phenomena overlap and interrelate. A case-control study of 20 preterm males and females matched for age was conducted. Three-dimensional images were acquired with 3 T MRI. The cerebral volume and the cortical folding area (FA), defined as the surface area of the interface between cortical gray and white matter, were compared between males and females. Females had smaller cerebra than males even after removing the influence of overall size differences between the subjects. The cortical FA increased in relation to volume by a power of 4/3 in both groups. Females had larger cortical FA compared with males with similar cerebral volumes. The study provides in vivo evidence of sexually dimorphic early human brain development. The relatively more "compact" female model may well relate to sex differences in neural circuitry and cognitive domains.
Pediatric Research 09/2009; 66(5):551-5. · 2.67 Impact Factor
[show abstract][hide abstract] ABSTRACT: Molecular imaging with magnetic resonance imaging (MRI) may benefit from the ferrimagnetic properties of magnetosomes, membrane-enclosed iron biominerals whose formation in magnetotactic bacteria is encoded by multiple genes. One such gene is MagA, a putative iron transporter. We have examined expression of MagA in mouse neuroblastoma N2A cells and characterized their response to iron loading and cellular imaging by MRI. MagA expression augmented both Prussian blue staining and the elemental iron content of N2A cells, without altering cell proliferation, in cultures grown in the presence of iron supplements. Despite evidence for iron incorporation in both MagA and a variant, MagAE137V, only MagA expression produced intracellular contrast detectable by MRI at 11 Tesla. We used this stable expression system to model a new sequence for cellular imaging with MRI, using the difference between gradient and spin echo images to distinguish cells from artifacts in the field of view. Our results show that MagA activity in mammalian cells responds to iron supplementation and functions as a contrast agent that can be deactivated by a single point mutation. We conclude that MagA is a candidate MRI reporter gene that can exploit more fully the superior resolution of MRI in noninvasive medical imaging.
[show abstract][hide abstract] ABSTRACT: In this study, we examined muscle metabolic and acid-base status during incremental wrist extension exercise in the forearm of individuals with work-related myalgia (WRM). Eighteen women employed in full-time occupations involving repetitive forearm labor were recruited in this cross-sectional study. Nine of these women were diagnosed with WRM, while the other nine had no previous WRM history and were used as age-matched controls (Con). Phosphorus-31 magnetic resonance spectroscopy ((31)P-MRS) was used to noninvasively monitor the intracellular concentrations of phosphocreatine ([PCr]) and inorganic phosphate ([P(i)]) as well as intracellular pH (pH(i)) status during exercise in WRM and Con. We observed a 38% decreased work capacity in WRM compared with Con [0.18 W (SD 0.03) vs. 0.28 W (SD 0.10); P = 0.007]. Piecewise linear regression of the incremental exercise data revealed that the onset of a faster decrease in pH(i) (i.e., the pH threshold, pHT) and the onset of a faster increase in log([P(i)]/[PCr]) (i.e., the phosphorylation threshold, PT) occurred at a 14% relatively lower power output in WRM [pHT: 45.2% (SD 5.3) vs. 59.0% (SD 4.6), P < 0.001; PT: 44.8% (SD 4.3) vs. 57.8% (SD 3.1), P < 0.001; % of peak power output, Con vs. WRM, respectively]. Monoexponential modeling of the kinetics of [PCr] and pH(i) recovery following exercise demonstrated a slower (P = 0.005) time constant (tau) for [PCr] in WRM [113 s (SD 25)] vs. Con [77 s (SD 23)] and a slower (P = 0.007) tau for pH(i) in WRM [370 s (SD 178)] vs. Con [179 s (SD 52)]. In conclusion, our results suggest that WRM is associated with an increased reliance on nonoxidative metabolism. Possible mechanisms include a reduction in local muscle blood flow and perfusion, an increased ATP cost of force production, or both.
Journal of Applied Physiology 12/2008; 106(4):1198-206. · 3.48 Impact Factor
[show abstract][hide abstract] ABSTRACT: Near-infrared spectroscopy (NIRS) offers the ability to assess brain function at the bedside of critically ill neonates. Our group previously demonstrated a persistent reduction in the cerebral metabolic rate of oxygen (CMRO(2)) after hypoxia-ischemia (HI) in newborn piglets. The purpose of this current study was to determine the causes of this reduction by combining NIRS with magnetic resonance spectroscopy (MRS) to measure high-energy metabolites and diffusion-weighted imaging to measure cellular edema. Nine piglets were exposed to 30 min of HI and nine piglets served as controls. Proton and phosphorous MRS spectra, apparent diffusion coefficient (ADC) maps, and CMRO(2) measurements were collected periodically before and for 5.5 h after HI. A significant decrease in CMRO(2) (26 +/- 7%) was observed after HI. Incomplete recovery of nucleotide triphosphate concentration (8 +/- 3% <controls) and reduced ADC (16 +/- 5%) suggested mitochondrial dysfunction. However, CMRO(2) did not correlate with any metabolite concentration during the last 3 h of the recovery period, and no significant changes were found in phosphocreatine and lactate levels. Therefore, the CMRO(2) decrease is likely a combination of impaired mitochondrial function and reduced energy demands during the acute phase, which has been previously observed in the mature brain.
Pediatric Research 10/2008; 65(2):181-7. · 2.67 Impact Factor
[show abstract][hide abstract] ABSTRACT: Optimization of magnetization-prepared rapid gradient-echo (MP-RAGE) sequence variations for maximum white matter (WM) versus gray matter (GM) contrast in neonates at 3T was investigated. Numerical simulations were applied to optimize and compare three contrast preparation modules and to assess the effect of phase encoding (PE) order on contrast between WM and thin cortical GM layers. Simulations predict that a new sequence, which combines both T(1)- and T(2)-weighting into the contrast preparation and utilizes an interleaved elliptical-spiral PE order, should provide the strongest contrast between neonatal WM and cortical GM. This sequence was compared to a conventional MP-RAGE acquisition (i.e., T(1)-weighted preparation, centric PE order) for in vivo imaging of seven preterm newborn infants. Regional measurements of the contrast-to-noise ratio (CNR) between WM and GM demonstrated an increase of 50-70% (depending on GM region) using the new sequence, in good agreement with theoretical predictions. This improved contrast resulted in superior WM versus GM discrimination in intensity-based brain tissue segmentations.
Magnetic Resonance in Medicine 06/2008; 59(5):1190-6. · 3.27 Impact Factor
[show abstract][hide abstract] ABSTRACT: Abnormalities in cerebral blood flow (CBF) are believed to play a significant role in the development of major neonatal neuropathologies. One approach that would appear ideal for measuring CBF in this fragile age group is arterial spin labeling (ASL) since ASL techniques are noninvasive and quantitative. The purpose of this study was to assess the accuracy of a pulsed ASL method implemented on a 3-T scanner dedicated to neonatal imaging. Cerebral blood flow was measured in nine neonatal piglets, the ASL-CBF measurements were acquired at two inversion times (TI) (1,200 and 1,700 ms), and CBF was measured by perfusion computed tomography (pCT) for validation. Perfusion CT also provided images of cerebral blood volume, which were used to identify large blood vessels, and contrast arrival time, which were used to assess differences in arterial transit times between gray and white matter. Good agreement was found between gray matter CBF values from pCT (76+/-1 ml/min per 100 g) and ASL at TI=1,700 ms (73+/-1 ml/min per 100 g). At TI=1,200 ms, ASL overestimated CBF (91+/-2 ml/min per 100 g), which was attributed to substantial intravascular signal. No significant differences in white matter CBF from pCT and ASL were observed (average CBF=60+/-1 ml/min per 100 g), nor was there any difference in contrast arrival times for gray and white matter (0.95+/-0.04 and 0.99+/-0.03 s, respectively), which suggests that the arterial transit times for ASL were the same in this animal model. This study verified the accuracy of the implemented ASL technique and showed the value of using pCT to study other factors that can affect ASL-CBF measurements.
Magnetic Resonance Imaging 06/2008; 26(4):543-53. · 2.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: Occupational computer use has been associated with upper extremity musculoskeletal disorders (UEMSDs), but the etiology and pathophysiology of some of these disorders are poorly understood. Various theories attribute the symptoms to biomechanical and/or psychosocial stressors. The results of several clinical studies suggest that elevated antagonist muscle tension may be a biomechanical stress factor. Affected computer users often exhibit limited wrist range of motion, particularly wrist flexion, which has been attributed to increased extensor muscle tension, rather than to pain symptoms. Recreational or domestic activities requiring extremes of wrist flexion may produce injurious stress on the wrist joint and muscles, the symptoms of which are then exacerbated by computer use. As these activities may involve a variety of forearm postures, we examined whether changes in forearm posture have an effect on pain reports during wrist flexion, or whether pain would have a limiting effect on flexion angle.
We measured maximum active wrist flexion using a goniometer with the forearm supported in the prone, neutral, and supine postures. Data was obtained from 5 subjects with UEMSDs attributed to computer use and from 13 control subjects.
The UEMSD group exhibited significantly restricted wrist flexion compared to the control group in both wrists at all forearm postures with the exception of the non-dominant wrist with the forearm prone. In both groups, maximum active wrist flexion decreased at the supine forearm posture compared to the prone posture. No UEMSD subjects reported an increase in pain symptoms during testing.
The UEMSD group exhibited reduced wrist flexion compared to controls that did not appear to be pain related. A supine forearm posture reduced wrist flexion in both groups, but the reduction was approximately 100% greater in the UEMSD group. The effect of a supine forearm posture on wrist flexion is consistent with known biomechanical changes in the distal extensor carpi ulnaris tendon that occur with forearm supination. We infer from these results that wrist extensor muscle passive tension may be elevated in UEMSD subjects compared to controls, particularly in the extensor carpi ulnaris muscle. Measuring wrist flexion at the supine forearm posture may highlight flexion restrictions that are not otherwise apparent.
[show abstract][hide abstract] ABSTRACT: Development of a composite material phantom, comprised of polyvinyl alcohol cryogel (PVA-C) and an agarose additive, to effectively mimic the magnetic resonance relaxation times (T1 and T2) of neonatal white matter (WM) and gray matter (GM) at 3.0 T.
Samples of PVA-C with and without agarose were prepared with 1 cycle of freezing/thawing. Measurements of T1 and T2, at 3.0 T, were performed on the samples at temperatures ranging from 20 degrees C to 40 degrees C.
A sample temperature of 40 degrees C was required to achieve a T1 value sufficiently long to represent neonatal WM. At this temperature, neonatal WM relaxation times required 3% PVA-C with 0.3% agarose, whereas gray matter relaxation times required 8% PVA-C with 1.4% agarose.
By adjusting the sample temperature, polyvinyl alcohol concentration, and agarose concentration, the relaxation times of neonatal brain tissues can be obtained using this composite material.
[show abstract][hide abstract] ABSTRACT: The apparent diffusion coefficient changes with time after hypoxic-ischemic brain injury. In this study, we quantitatively examined the relationship between the apparent diffusion coefficient and postnatal age for neonates with hypoxic-ischemic encephalopathy and poor outcome, and determined the postnatal age at which these values cannot be distinguished from those of neonates without hypoxic-ischemic encephalopathy (pseudonormalization time). Diffusion-weighted brain images were obtained from clinical scans of term neonates with hypoxic-ischemic encephalopathy and poor outcome (12 neonates, 23 scans) and from control subjects (30 neonates, 31 scans). The correlation between apparent diffusion coefficient and postnatal age was investigated for several brain regions. Pseudonormalization times were determined (1) from the intersection of the regression lines for the hypoxic-ischemic encephalopathy and control groups, as well as (2) from intrasubject apparent diffusion coefficient changes between two scans within a small subgroup. Pseudonormalization times from the regression ranged from 8.3 +/- 1.9 days to 10.1 +/- 2.1 days. Slightly (approximately 1 day) longer values were obtained from the intrasubject analysis. The results suggest that, although abnormally decreased apparent diffusion coefficient values may be evident from approximately 2 days to almost 1 week of postnatal age, abnormally elevated values may not be apparent until late in the second week of life.
[show abstract][hide abstract] ABSTRACT: Segmented echoplanar imaging (EPI) is a potentially valuable acquisition method for neonatal diffusion-weighted imaging (DWI) due to the lower acoustic noise levels as well as reduced blurring and distortion associated with it, as compared with single-shot EPI. Reduced acoustic noise may be important for the safety of neonates. However, little information regarding the efficacy of segmented EPI motion correction schemes is available for the neonatal population. We quantitatively assessed the efficacy of a postprocessing technique for motion artifact reduction involving phase correction by nonlinear optimization, alone and in combination with a novel method of utilizing a second data set (referred to as segment data swapping). These methods were applied to three-directional eight-segment echoplanar DW images obtained from 13 sedated neonates and to nine-directional DW images from 3 unsedated neonates. For comparison, the efficacy of the nonlinear optimization method was also evaluated in four adults. Motion correction efficacy was quantified using the motion artifact-to-signal ratio (ASR). The median, 70th percentile and 90th percentile ASR values obtained from neonatal three-directional DWI using nonlinear optimization alone were 2.8%, 4.6% and 9.6%, respectively. Efficacy improved (P<.005), particularly in dealing with the images most difficult to correct, when the phase correction by numerical optimization was combined with segment data swapping (median ASR=1.9%, 70th percentile ASR=2.7%, 90th percentile ASR=4.3%). Similar results were obtained for nine-directional diffusion tensor imaging. Nonlinear optimization alone applied to adult images showed significantly (P<.001) lower ASR values (median ASR=0.9%, 70th percentile ASR=2.1%, 90th percentile ASR=4.1%), demonstrating the greater challenge in DWI of neonates with segmented EPI. In conclusion, phase correction by nonlinear optimization provides effective motion correction for neonatal DW eight-segment EPI, especially when used in conjunction with segment data swapping.
Magnetic Resonance Imaging 11/2007; 25(9):1283-91. · 2.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: Three-dimensional (3D) magnetic resonance imaging (MRI) has shown great potential for studying the impact of prematurity and pathology on brain development. We have investigated the potential of optimized T1-weighted 3D magnetization-prepared rapid gradient-echo imaging (MP-RAGE) for obtaining contrast between white matter (WM) and gray matter (GM) in neonates at 3 T. Using numerical simulations, we predicted that the inversion time (TI) for obtaining strongest contrast at 3 T is approximately 2 s for neonates, whereas for adults, this value is approximately 1.3 s. The optimal neonatal TI value was found to be insensitive to reasonable variations of the assumed T1 relaxation times. The maximum theoretical contrast for neonates was found to be approximately one third of that for adults. Using the optimized TI values, MP-RAGE images were obtained from seven neonates and seven adults at 3 T, and the contrast-to-noise ratio (CNR) was measured for WM versus five GM regions. Compared to adults, neonates exhibited lower CNR between cortical GM and WM and showed a different pattern of regional variation in CNR. These results emphasize the importance of sequence optimization specifically for neonates and demonstrate the challenge in obtaining strong contrast in neonatal brain with T1-weighted 3D imaging.
Magnetic Resonance Imaging 10/2007; 25(8):1162-70. · 2.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: The effects of prior moderate- and prior heavy-intensity exercise on the subsequent metabolic response to incremental exercise were examined. Healthy, young adult subjects (n = 8) performed three randomized plantar-flexion exercise tests: 1) an incremental exercise test (approximately 0.6 W/min) to volitional fatigue (Ramp); 2) Ramp preceded by 6 min of moderate-intensity, constant-load exercise below the intracellular pH threshold (pHT; Mod-Ramp); and 3) Ramp preceded by 6 min of heavy-intensity, constant-load exercise above pHT (Hvy-Ramp); the constant-load and incremental exercise periods were separated by 6 min of rest. (31)P-magnetic resonance spectroscopy was used to continuously monitor intracellular pH, phosphocreatine concentration ([PCr]), and inorganic phosphate concentration ([P(i)]). No differences in exercise performance or the metabolic response to exercise were observed between Ramp and Mod-Ramp. However, compared with Ramp, a 14% (SD 10) increase (P < 0.01) in peak power output (PPO) was observed in Hvy-Ramp. The improved exercise performance in Hvy-Ramp was accompanied by a delayed (P = 0.01) onset of intracellular acidosis [Hvy-Ramp 60.4% PPO (SD 11.7) vs. Ramp 45.8% PPO (SD 9.4)] and a delayed (P < 0.01) onset of rapid increases in [P(i)]/[PCr] [Hvy-Ramp 61.5% PPO (SD 12.0) vs. Ramp 45.1% PPO (SD 9.1)]. In conclusion, prior heavy-intensity exercise delayed the onset of intracellular acidosis and enhanced exercise performance during a subsequent incremental exercise test.
Journal of Applied Physiology 05/2007; 102(5):1799-805. · 3.48 Impact Factor