Biomedical subjects

Jim M Wild

Publications and source records attributed to Jim M Wild.

At least 19 recordsLinked to original sources

PubMed2007

Rapid lung volumetry using ultrafast dynamic magnetic resonance imaging during forced vital capacity maneuver: correlation with spirometry.

INTRODUCTION: Dynamic magnetic resonance imaging (MRI) has the potential for rapid noninvasive evaluation of changes in lung volume. The aim of this study was to perform rapid lung volumetry using ultrafast dynamic MRI to capture a forced vital capacity (FVC) maneuver. MATERIALS AND METHODS: Nine healthy volunteers underwent 2-dimensional spoiled gradient echo imaging in coronal and sagittal planes during FVC maneuvers. An elliptical model of the axial cross section of the lungs was used to generate rapid volume-time curves. Spirometric indices were correlated with MR volumetry findings. RESULTS: Total lung volume calculated from static MRI correlated well with the dynamic MR scans (r = 0.83; P < 0.01). Spirometric indices (first second of forced expiration and FVC) calculated from our MR volumetry technique correlated well with conventional spirometry (P < 0.01). CONCLUSION: The technique provides a means of sampling lung volume change during the rapid subsecond movements that take place during a FVC maneuver.

Adult↗

PubMed2006

Radiation-damping effects in a birdcage resonator with hyperpolarised 3He gas NMR at 1.5 T.

The presence and diagnosis of radiation damping could have major implications in NMR experiments with hyperpolarised gases, where accurate knowledge of the flip angle is imperative. In this work radiation damping was observed and investigated in a low-pass birdcage resonator (Q=250) with samples of hyperpolarised 3He at 1.5 T. With an initially highly polarised (P=38%) sample of 3He in a spherical cell, the observed FID had a distorted line shape with a spectral line width that was three times that of the same sample in a virtually depolarised state (1 Hz line width for P<1%). Moreover a linear relation between the sample's magnetisation (M0) and the line width of the spectrum was observed which is indicative of radiation damping. With highly polarised samples, significant radiation damping was observed and the effect was a lower than expected rate of depletion of M0 in RF flip angle calibration experiments, which led to significant underestimate of the RF flip angle. To our knowledge this is the first report of radiation damping in a birdcage resonator with samples hyperpolarised or otherwise. Experimental observation of radiation damping could be used as means of measuring coil efficiency as an alternative to the geometrical filling factor (eta) the definition of which is open to question for a birdcage resonator. Estimates of the birdcage filling factor from the measured damping time constants (eta(RD)=0.4%) are compared to those derived from electromagnetic energy ratios (eta(E)=1.6%) and metallic sphere frequency shift methods (eta(fs)=1.4%). These figures are much lower than the simple volume geometrical upper limit of eta(v)=3.7% derived from the ratio of cell volume to total coil volume (shield included). The physical explanation for this shortfall is that the bulk of the magnetic energy stored in the birdcage is spatially distributed predominantly between the rungs and the shield, and not in the coil centre where the sample is placed and where the B1+ field has its highest spatial homogeneity.

Artifacts↗

PubMed2006

Assessment of lung disease in children with cystic fibrosis using hyperpolarized 3-Helium MRI: comparison with Shwachman score, Chrispin-Norman score and spirometry.

This study assesses the feasibility of hyperpolarized 3-Helium MRI in children with cystic fibrosis (CF) and correlates the findings with standard clinical parameters based on chest radiograph (CXR) and pulmonary function tests (PFT). An uncontrolled, observational study in eighteen children with cystic fibrosis aged 5 - 17 years (median 12.1 years), with different severity of disease was carried out. All subjects underwent routine clinical assessment including PFT and standard auxology; CXR was obtained and Shwachman and Chrispin-Norman scores calculated. Hyperpolarized 3-He magnetic resonance imaging (MRI) was carried out using a spin-exchange polarizer and a whole body 1.5 T scanner. Ventilation distribution images were obtained during a 21-second breath-hold and scored according to previously defined criteria. Spearman's non-parametric correlations test was performed to assess for statistical significance at the p<0.05 level. The children tolerated the procedure well. No desaturation events were observed during 3-He MRI. A significant, albeit moderate, correlation was found between MRI score and FEV1% predicted (r=-0.41; p=0.047) and FVC% predicted (r=-0.42; p=0.04), while there were trends of correlations between Shwachman score and MRI score (r=-0.38; p=0.06) and Shwachman score and FEV1% predicted (r=0.39; p=0.055). The feasibility of hyperpolarized 3-He MRI in children with CF was demonstrated. MRI appears to be able to demonstrate functional lung changes, although correlations with routine clinical tests are only moderate to poor. This non-ionising radiation technique could be useful for monitoring lung disease and assessing therapy in this patient population.

Adolescent↗

PubMed2006

Steady-state free precession with hyperpolarized 3He: experiments and theory.

The magnetization response of hyperpolarized 3He gas to a steady-state free precession (SSFP) sequence was simulated using matrix product operators. The simulations included the effects of flip angle (alpha), sequence timings, resonant frequency, gas diffusion coefficient, imaging gradients, T1 and T2. Experiments performed at 1.5 T, on gas phantoms and with healthy human subjects, confirm the predicted theory, and indicate increased SNR with SSFP through use of higher flip angles when compared to optimized spoiled gradient echo (SPGR). Simulations and experiments show some compromise to the SNR and some point spread function broadening at high alpha due to the incomplete refocusing of transverse magnetization, caused by diffusion dephasing from the readout gradient. Mixing of gas polarization levels by diffusion between slices is also identified as a source of signal loss in SSFP at higher alpha through incomplete refocusing. Nevertheless, in the sample experiments, a SSFP sequence with an optimized flip angle of alpha=20 degrees, and 128 sequential phase encoding views, showed a higher SNR when compared to SPGR (alpha=7.2 degrees) with the same bandwidth. Some of the gas sample experiments demonstrated a transient signal response that deviates from theory in the initial phase. This was identified as being caused by radiation damping interactions between the large initial transverse magnetization and the high quality factor (Q=250) birdcage resonator. In 3He NMR experiments, performed without imaging gradients, diffusion dephasing can be mitigated, and the effective T2 is relatively long (1 s). Under these circumstances the SSFP sequence behaves like a CPMG sequence with sinalpha/2 weighting of SNR. Experiments and simulations were also performed to characterize the off-resonance behaviour of the SSFP HP 3He signal. Characteristic banding artifacts due to off-resonance harmonic beating were observed in some of the in vivo SSFP images, for instance in axial slices close to the diaphragm where B0 inhomogeneity is highest. Despite these artifacts, a higher SNR was observed with SSFP in vivo when compared to the SPGR sequence. The trends predicted by theory of increasing SSFP SNR with increasing flip angle were observed in the range alpha=10-20 degrees without compromise to image quality through blurring caused by excessive k-space filtering.

Computer Simulation↗

PubMed2006

Hyperpolarized 3helium magnetic resonance ventilation imaging of the lung in cystic fibrosis: comparison with high resolution CT and spirometry.

The purpose of this study was to compare hyperpolarized 3helium magnetic resonance imaging (3He MRI) of the lungs in adults with cystic fibrosis (CF) with high-resolution computed tomography (HRCT) and spirometry. Eight patients with stable CF prospectively underwent 3He MRI, HRCT, and spirometry within 1 week. Three-dimensional (3D) gradient-echo sequence was used during an 18-s breath-hold following inhalation of hyperpolarized 3He. Each lung was divided into six zones; 3He MRI was scored as percentage ventilation per lung zone. HRCT was scored using a modified Bhalla scoring system. Univariate (Spearman rank) and multivariate correlations were performed between 3He MRI, HRCT, and spirometry. Results are expressed as mean+/-SD (range). Spirometry is expressed as percent predicted. There were four men and four women, mean age = 31.9+/-9 (20-46). Mean forced expiratory volume in 1 s (FEV)1 = 52%+/-29 (27-93). Mean 3He MRI score = 74%+/-25 (55-100). Mean HRCT score = 48.8+/-24 (13.5-83). The correlation between 3He MRI and HRCT was strong (R = +/-0.89, p < 0.001). Bronchiectasis was the only independent predictor of 3He MRI; 3He MRI correlated better with FEV1 and forced vital capacity (FVC) (R = 0.86 and 0.93, p < 0.01, respectively) than HRCT (R = +/-0.72 and +/-0.81, p < 0.05, respectively). This study showed that 3He MRI correlates strongly with structural HRCT abnormalities and is a stronger correlate of spirometry than HRCT in CF.

Adult↗

PubMed2006

Parallel imaging of hyperpolarized helium-3 with simultaneous slice excitation.

Hyperpolarized (HP) gas imaging of the lungs is an ideal potential application for parallel imaging. This is due to the fact that there is limited scan time (breath hold of 20 s) and limited non-renewable polarization. Reduced phase encode parallel imaging is demanding on hardware in that it requires multiple receivers. In this work, simultaneous parallel acquisition of hyperpolarized (HP) 3He images from multiple slices was demonstrated in phantoms and in vivo using a simultaneous slice excitation method, at a field strength of 1.5 T. The pulse sequence allows simultaneous acquisition of n slices per RF excitation, thus reducing the number of RF pulses needed to fully cover a given volume with multi-slicing. Unlike conventional parallel imaging, this method does not require prior reference scan information, which would consume some of the finite longitudinal polarization in lung ventilation studies with HP gas.

Artifacts↗

PubMed2005

Combined helium-3/proton magnetic resonance imaging measurement of ventilated lung volumes in smokers compared to never-smokers.

PURPOSE: To use a combination of helium-3 (3-He) magnetic resonance imaging (MRI) and proton single-shot fast spin echo (SSFSE) to compare ventilated lung volumes in groups of "healthy" smokers, smokers diagnosed with moderate chronic obstructive pulmonary disease (COPD), and never-smokers. MATERIALS AND METHODS: All study participants were assessed with spirometry prior to imaging. 3-He images were collected during an arrested breath hold, after inhaling a mixture of 200 mL of hyperpolarized 3-He/800 mL of N2. Proton SSFSE images were acquired after inhaling 1 liter of room air. The ventilated volume for each study participant was calculated from the 3-He images, and a ratio was calculated to give a percentage ventilated lung volume. RESULTS: Never-smokers exhibited a 90% mean ventilated volume. The mean ventilated lung volumes for healthy smokers and smokers diagnosed with COPD were 75.2% and 67.6%, respectively. No correlation with spirometry was demonstrated for either of the smoking groups. CONCLUSION: Combined 3-He/Proton SSFSE MRI of the lungs is a noninvasive method, using nonionizing radiation, which demonstrates ventilated airspaces and enables the calculation of ventilated lung volumes. This method appears to be sensitive to early obstructive changes in the lungs of smokers.

Adult↗

PubMed2005

Quantitative analysis of regional airways obstruction using dynamic hyperpolarized 3He MRI-preliminary results in children with cystic fibrosis.

PURPOSE: To investigate regional airways obstruction in patients with cystic fibrosis (CF) with quantitative analysis of dynamic hyperpolarized (HP) (3)He MRI. MATERIALS AND METHODS: Dynamic radial projection MRI of HP (3)He gas was used to study respiratory dynamics in a group of eight children with CF. Signal kinetics in a total of seven regions of interest (ROIs; three in each lung, and one in the trachea) were compared with the results of spirometric pulmonary function tests (PFTs). The tracheal signal intensity was used as a form of "input function" to normalize for input flow effects. RESULTS: A pattern of low flow rate in the upper lobes was observed. When the flow measurements from the peripheral ROIs were averaged to obtain an index of flow in the peripheral lung, a good correlation was found (P = 3.74 x 10(-5)) with the forced expired volume in one second (FEV1). CONCLUSION: These results suggest that a quantitative measurement of localized airways obstruction in the early stages of CF may be obtained from dynamic (3)He MRI by using the slope of the signal rise as a measure of air flow into the peripheral lung. This study also demonstrates that children can cooperate well with the (3)He MRI technique.

Adolescent↗

PubMed2005

3D volume-localized pO2 measurement in the human lung with 3He MRI.

A method for 3D volume-localized quantification of pO2 in the lungs is presented that uses repetitive frame 3D gradient-echo imaging of (3)He. The method was demonstrated by experiments on (3)He phantoms containing known concentrations of O(2) and in vivo on a group of three healthy human volunteers. The results were compared with those obtained by equivalent 2D thin-slice and 2D projection methodologies, and were found to be consistent with published results from the 2D projection methodologies (pO(2) = 0.09-0.18 bar). Studies performed on the same subject, on three separate occasions, demonstrated a repeatability of pO(2) measurement to within 14% using the 3D technique. Experimental differences between the 2D and 3D methods were substantiated with theoretical and numerical analyses of the signal decay, which took into account the effects of out-of-slice diffusion as a source of error in the thin-slice 2D experiments. It is shown that the 2D thin-slice technique systematically underestimates pO2 when there is significant gas diffusion (factor of 4 underestimate for D = 0.9 cm(2)s(-1) representative of free (3)He in air).

Adult↗

PubMed2005

Emphysematous changes and normal variation in smokers and COPD patients using diffusion 3He MRI.

INTRODUCTION: This study aims to quantify global and regional changes of diffusive motion of 3He gas within the lung, as determined by hyperpolarized 3He MR apparent diffusion coefficient (ADC) measurement, in non-smokers, smokers and chronic obstructive pulmonary disease (COPD) patients. METHODS: Age-matched groups of six healthy non-smokers, five healthy smokers and five patients with COPD. The experiments were performed with approval from the local Research Ethics Committee. Diffusion imaging was performed following hyperpolarized 3He gas inhalation, producing ADC maps. Mean and standard deviation of the ADCs were used to compare the subject groups and assess regional variations within individuals. RESULTS: The intra-individual standard deviation of ADC in the healthy smokers was significantly larger than that of the non-smoking group (P < 0.02). Compared to the non-smoking group, COPD patients had significantly higher mean and standard deviation of ADC (P < 0.01). The mean ADC in the anterior half of the chest was systematically higher than in the posterior half in the healthy non-smoking subject group. DISCUSSION: This study suggests that there are regional trends in the ADC values of healthy volunteers that may have implications for the clinical interpretation of ADC values. Less homogeneous ADC values have been detected in asymptomatic smokers, indicative of damage to the distal air spaces.

Aged↗

PubMed2005

Measurements and modeling of long range 3He diffusion in the lung using a "slice-washout" method.

In healthy lung tissue, pulsed-gradient-spin-echo (PGSE) methods reveal apparent diffusion coefficients (ADC) of the order 0.20 cm2 s(-1); for diffusion times of approximately 2 ms. For these short diffusion times the ADC is only sensitive to structures approximately (2Dt)1/2 approximately 0.6mm in size. Recent work, using magnetic tagging of the longitudinal magnetization has revealed much smaller ADC values for longer length scales. In this work, the in vivo ADC from within the air-spaces, was measured using a new technique. The signal from a series of images was analyzed from a slice that was repeatedly imaged. Diffusion tends to "top-up" the non-renewable polarization within the slice, which leads to a non-exponential decay in image signal. Image data were compared to 1D finite-difference simulations of diffusion to calculate a long range ADC value. The results yield values of the order 0.034 cm2 s(-1), which are nearly an order of magnitude smaller than those reported by PGSE measurements at shorter diffusion times.

Algorithms↗

PubMed2005

Hyperpolarized 3-helium magnetic resonance imaging to probe lung function.

Understanding pulmonary pathophysiology has increasing implications for imaging modalities. Although it was sufficient to perform high-resolution computed tomography in the past, the impetus now is on providing quantitative and functional lung data. Magnetic resonance imaging, which was traditionally difficult to perform in the lungs, has developed into a promising technology. One of the main areas of interest is the use of hyperpolarized noble gases, such as 3-He and 129-Xe, which enable high-definition lung imaging that includes information on lung microstructure. It is possible to obtain three-dimensional information on essential pulmonary processes, such as ventilation, oxygen uptake, and spirometry, which offers new insight into lung pathophysiology. This article focuses on the novel aspects of hyperpolarized 3-He magnetic resonance imaging.

Helium↗

PubMed2004

MRI of helium-3 gas in healthy lungs: posture related variations of alveolar size.

PURPOSE: To probe the variation of alveolar size in healthy lung tissue as a function of posture using diffusion-weighted helium-3 hyperpolarized gas imaging. MATERIALS AND METHODS: Measurements of the helium-3 apparent diffusion coefficient (ADC) were made on six healthy subjects. These were used to show the variation of alveolar size between the lowermost dependent regions of the lung compared to the uppermost regions of the lung in four postures: supine, prone, left-lateral decubitus, and right-lateral decubitus. RESULTS: The distribution of acinar size in the lungs was found to be heterogeneous, and influenced by lung orientation. In nearly all postures, the ADC was significantly higher in the non-dependent uppermost regions of the lung compared to the dependent lowermost regions of the lung; the greatest variation was found in the left-lateral decubitus position. The difference in ADC between uppermost and lowermost regions was on average 0.012 cm(2)second(-1), which represents 20% of the average ADC value for the whole lung. A systematic decrease in ADC from the apex of the lung to the base was also found, which corresponds to an inherent gradient in alveolar size. CONCLUSION: The posture dependent variations in ADC were attributed to compression of the parenchyma under its own weight and the mass of the heart.

Adult↗

PubMed2004

Functional MRI of the lung using hyperpolarized 3-helium gas.

Lung imaging has traditionally relied on x-ray methods, since proton MRI is limited to some extent by low proton density in the lung parenchyma and static field inhomogeneities in the chest. The relatively recent introduction of MRI of hyperpolarized noble gases has led to a rapidly evolving field of pulmonary MRI, revealing functional information of the lungs, which were hitherto unattainable. This review article briefly describes the physical background of the technology, and subsequently focuses on its clinical applications. Four different techniques that have been used in various human investigations are discussed: ventilation distribution, ventilation dynamics, and small airway evaluation using diffusion imaging and oxygen uptake assessment.

Administration, Inhalation↗

PubMed2004

Comparison between 2D and 3D gradient-echo sequences for MRI of human lung ventilation with hyperpolarized 3He.

Images of hyperpolarized 3He were acquired during breath-hold in four healthy volunteers with the use of an optimized 3D gradient-echo sequence. The images were compared with existing 2D gradient-echo methods. The average SNR from a 13-mm-thick slice in the peripheral lung was 1.4 times greater with 3D. In the airways the average SNR was 1.7 times greater with 3D. The higher SNR of 3D was particularly evident when regions of unimpeded gas diffusion, such as the major airways, were imaged with thin slices. This is because diffusion dephasing due to the slice-encoding gradient is minimized with a 3D sequence. The in vivo experimental findings were substantiated with experiments on phantoms of free gas, which showed more than four times the SNR with 3D compared to 2D. Theoretical simulations of the 2D and 3D k-space filters were also performed to predict the SNR and spatial resolution observed in the experimental images.

Computer Simulation↗

PubMed2004

Finite-difference simulations of 3He diffusion in 3D alveolar ducts: comparison with the "cylinder model".

Time-dependent measurements of 3He diffusion in the lung could provide an accurate method to quantify alveolar length scales and the progression of diseases such as emphysema. However, the apparent diffusion coefficient (ADC) presents a complex problem to model and solve analytically. Here, finite-difference methods were used to simulate diffusion in 3D alveolar ducts. The results were compared to the only available analytical model--the "cylinder model"--from which it is possible to estimate the average radii of the alveolar ducts from in vivo data. The trend in data observed from simulations was found to agree well with the cylinder model. However, the cylinder model always overestimated the average radii of the simulated alveolar ducts. The simulations also demonstrated that the measurement of the longitudinal ADC (along the alveolar ducts) should be sensitive to early emphysematous changes, whereas the measured radii should be far less sensitive.

Computer Simulation↗

PubMed2004

Investigating 3He diffusion NMR in the lungs using finite difference simulations and in vivo PGSE experiments.

Finite difference simulations have been used to model (3)He gas diffusion in simulated lung tissue. The technique has the advantage that a wide range of structural models and diffusion-sensitizing gradient waveforms can be investigated, for which analytical methods would otherwise be virtually impossible. Results from simulations and in vivo pulsed-gradient-spin-echo (PGSE) experiments show that the apparent diffusion coefficient (ADC) is a function of diffusion time and gradient strength, and suggests diffusion is locally anisotropic. The simulations have been compared to recent work on an analytical model that characterizes lung tissue as a series of independent cylinders. The results presented may have clinical implications for (3)He ADC measurements in assessing lung diseases such as chronic-obstructive-pulmonary-disease.

Adult↗

PubMed2003

MRI for the diagnosis of pulmonary embolism.

Pulmonary embolism (PE) is one of the most frequently encountered clinical emergencies. The diagnosis often involves multiple diagnostic tests, which need to be carried out rapidly to assist in the safe management of the patient. Recent strides in computed tomography (CT) have made big improvements in patient management and efficiency of diagnostic imaging. This review article describes the developments in magnetic resonance (MR) techniques for the diagnosis of acute PE. Techniques include MR angiography (MRA) and thrombus imaging for direct clot visualization, perfusion MR, and combined perfusion-ventilation MR. As will be demonstrated, some of these techniques are now entering the clinical arena, and it is anticipated that MR imaging (MRI) will have an increasing role in the initial diagnosis and follow-up of patients with acute PE.

Acute Disease↗