Posture-dependent human 3He lung imaging in an open-access MRI system: initial results

Brain imaging with portable low-field MRI

The advent of portable, low-field MRI (LF-MRI) heralds new opportunities in neuroimaging. Low power requirements and transportability have enabled scanning outside the controlled environment of a conventional MRI suite, enhancing access to neuroimaging for indications that are not well suited to existing technologies. Maximizing the information extracted from the reduced signal-to-noise ratio of LF-MRI is crucial to developing clinically useful diagnostic images. Progress in electromagnetic noise cancellation and machine learning reconstruction algorithms from sparse k-space data as well as new approaches to image enhancement have now enabled these advancements. Coupling technological innovation with bedside imaging creates new prospects in visualizing the healthy brain and detecting acute and chronic pathological changes. Ongoing development of hardware, improvements in pulse sequences and image reconstruction, and validation of clinical utility will continue to accelerate this field. As further innovation occurs, portable LF-MRI will facilitate the democratization of MRI and create new applications not previously feasible with conventional systems.

Tackling SNR at low-field: a review of hardware approaches for point-of-care systems

OBJECTIVE

To review the major hardware components of low-field point-of-care MRI systems which affect the overall sensitivity.

METHODS

Designs for the following components are reviewed and analyzed: magnet, RF coils, transmit/receive switches, preamplifiers, data acquisition system, and methods for grounding and mitigating electromagnetic interference.

RESULTS

High homogeneity magnets can be produced in a variety of different designs including C- and H-shaped as well as Halbach arrays. Using Litz wire for RF coil designs enables unloaded Q values of ~ 400 to be reached, with body loss representing about 35% of the total system resistance. There are a number of different schemes to tackle issues arising from the low coil bandwidth with respect to the imaging bandwidth. Finally, the effects of good RF shielding, proper electrical grounding, and effective electromagnetic interference reduction can lead to substantial increases in image signal-to-noise ratio.

DISCUSSION

There are many different magnet and RF coil designs in the literature, and to enable meaningful comparisons and optimizations to be performed it would be very helpful to determine a standardized set of sensitivity measures, irrespective of design.

Application of a 2D frequency encoding sectoral approach to hyperpolarized 129Xe MRI at low field

Inhaled hyperpolarized ¹²⁹Xe MRI is a non-invasive and radiation risk free lung imaging method, which can directly measure the business unit of the lung where gas exchange occurs: the alveoli and acinar ducts (lung function). Currently, three imaging approaches have been demonstrated to be useful for hyperpolarized ¹²⁹Xe MR in lungs: Fast Gradient Recalled Echo (FGRE), Radial Projection Reconstruction (PR), and spiral/cones. Typically, non-Cartesian acquisitions such as PR and spiral/cones require specific data post-processing, such as interpolating, regridding, and density-weighting procedures for image reconstruction, which often leads to smoothing effects and resolution degradation. On the other hand, Cartesian methods such as FGRE are not short-echo time (TE) methods; they suffer from imaging gradient-induced diffusion-weighting of the k-space center, and employ a significant number of radio-frequency (RF) pulses. Due to the non-renewable magnetization of the hyperpolarized media, the use of a large number of RF pulses (FGRE/PR) required for full k-space coverage is a significant limitation, especially for low field (<0.5 T) hyperpolarized gas MRI. We demonstrate an ultra-fast, purely frequency-encoded, Cartesian pulse sequence called Frequency-Encoding Sectoral (FES), which takes advantage of the long T₂* of hyperpolarized ¹²⁹Xe gas at low field strength (0.074 T). In contrast to PR/FGRE, it uses a much smaller number of RF pulses, and consequently maximizes image Signal-to-Noise Ratio (SNR) while shortening acquisition time. Additionally, FES does not suffer from non-uniform T₂* decay leading to image blurring; a common issue with interleaved spirals/cones. The Cartesian k-space coverage of the proposed FES method does not require specific k-space data post-processing, unlike PR/FGRE and spiral/cones methods. Proton scans were used to compare the FES sequence to both FGRE and Phase Encoding Sectoral, in terms of their SNR values and imaging efficiency estimates. Using FES, proton and hyperpolarized ¹²⁹Xe images were acquired from a custom hollow acrylic phantom (0.04L) and two normal rats (¹²⁹Xe only), utilizing both single-breath and multiple-breath schemes. For the ¹²⁹Xe phantom images, the apparent diffusion coefficient, T₁, and T₂* relaxation maps were acquired and generated. Blurring due to the T₂* decay and B₀ field variation were simulated to estimate dependence of the image resolution on the duration of the data acquisition windows (i.e. sector length), and temperature-induced resonance frequency shift from the low field magnet hardware.

In vivo T1 and T2 relaxation time maps of brain tissue, skeletal muscle, and lipid measured in healthy volunteers at 50 mT

PURPOSE

Low-field (B₀ < 0.1 T) MRI has generated much interest as a means of increased accessibility via reduced cost and improved portability compared to conventional clinical systems (B₀ ≥ 1.5 Tesla). Here we measure MR relaxation times at 50 mT and compare results with commonly used models based on both in vivo and ex vivo measurements.

METHODS

Using 3D turbo spin echo readouts, T₁ and T₂ maps of the human brain and lower leg were acquired on a custom-built 50 mT MRI scanner using inversion-recovery and multi-echo-based sequences, respectively. Image segmentation was performed based on a histogram analysis of the relaxation times.

RESULTS

The average T₁ times of gray matter, white matter, and cerebrospinal fluid (CSF) were 327 ± 10 ms, 275 ± 5 ms, and 3695 ± 287 ms, respectively. Corresponding values of T₂ were 102 ± 6 ms, 102 ± 6 ms, and 1584 ± 124 ms. T₁ times in the calf muscle were measured to be 171 ± 11 ms and were 130 ± 5 ms in subcutaneous and bone marrow lipid. Corresponding T₂ times were 39 ± 2 ms in muscle and 90 ± 13 ms in lipid.

CONCLUSIONS

For tissues except for CSF, the measured T₁ times are much shorter than reported at higher fields and generally lie within the range of different models in the literature. As expected, T₂ times are similar to those seen at typical clinical field strengths. Analysis of the relaxation maps indicates that segmentation of white and gray matter based purely on T₁ or T₂ will be quite challenging at low field given the relatively small difference in relaxation times.

In vivo 3D brain and extremity MRI at 50 mT using a permanent magnet Halbach array

Purpose

To design a low‐cost, portable permanent magnet‐based MRI system capable of obtaining in vivo MR images within a reasonable scan time.

Methods

A discretized Halbach permanent magnet array with a clear bore diameter of 27 cm was designed for operation at 50 mT. Custom‐built gradient coils, RF coil, gradient amplifiers, and RF amplifier were integrated and tested on both phantoms and in vivo.

Results

Phantom results showed that the gradient nonlinearity in the y‐direction and z‐direction was less than 5% over a 15‐cm FOV and did not need correcting. For the x‐direction, it was significantly greater, but could be partially corrected in postprocessing. Three‐dimensional in vivo scans of the brain of a healthy volunteer using a turbo spin‐echo sequence were acquired at a spatial resolution of 4 × 4 × 4 mm in a time of about 2 minutes. The T₁‐weighted and T₂‐weighted scans showed a good degree of tissue contrast. In addition, in vivo scans of the knee of a healthy volunteer were acquired at a spatial resolution of about 3 × 2 × 2 mm within 12 minutes to show the applicability of the system to extremity imaging.

Conclusion

This work has shown that it is possible to construct a low‐field MRI unit with hardware components costing less than 10 000 Euros, which is able to acquire human images in vivo within a reasonable data‐acquisition time. The system has a high degree of portability with magnet weight of approximately 75 kg, gradient and RF amplifiers each 15 kg, gradient coils 10 kg, and spectrometer 5 kg.

Accelerated 129 Xe MRI morphometry of terminal airspace enlargement: Feasibility in volunteers and those with alpha-1 antitrypsin deficiency

PURPOSE

Multi-b diffusion-weighted hyperpolarized inhaled-gas MRI provides imaging biomarkers of terminal airspace enlargement including ADC and mean linear intercept (L_m ), but clinical translation has been limited because image acquisition requires relatively long or multiple breath-holds that are not well-tolerated by patients. Therefore, we aimed to accelerate single breath-hold 3D multi-b diffusion-weighted ¹²⁹ Xe MRI, using k-space undersampling in imaging direction using a different undersampling pattern for different b-values combined with the stretched exponential model to generate maps of ventilation, apparent transverse relaxation time constant ( T 2 ∗ ), ADC, and L_m values in a single, short breath-hold; accelerated and non-accelerated measurements were directly compared.

METHODS

We evaluated multi-b (0, 12, 20, 30, and 45.5 s/cm² ) diffusion-weighted ¹²⁹ Xe T 2 ∗ /ADC/morphometry estimates using acceleration factor (AF = 1 and 7) and multi-breath sampling in 3 volunteers (HV), and 6 participants with alpha-1 antitrypsin deficiency (AATD).

RESULTS

For the HV subgroup, mean differences of 5%, 2%, and 8% were observed between fully sampled and undersampled k-space for ADC, L_m , and T 2 ∗ values, respectively. For the AATD subgroup, mean differences were 9%, 6%, and 12% between fully sampled and undersampled k-space for ADC, L_m and T 2 ∗ values, respectively. Although mean differences of 1% and 4.5% were observed between accelerated and multi-breath sampled ADC and L_m values, respectively, mean ADC/L_m estimates were not significantly different from corresponding mean ADC^M /L_m ^M or mean ADC^A /L_m ^A estimates (all P > 0.60 , ^A = undersampled and ^M = multi-breath sampled).

CONCLUSIONS

Accelerated multi-b diffusion-weighted ¹²⁹ Xe MRI is feasible at AF = 7 for generating pulmonary ADC and L_m in AATD and normal lung.

Very-low-field MRI of laser polarized xenon-129

We describe a homebuilt MRI system for imaging laser-polarized xenon-129 at a very low holding field of 2.2mT. A unique feature of this system was the use of Maxwell coils oriented at so-called "magic angles" to generate the transverse magnetic field gradients, which provided a simple alternative to Golay coils. We used this system to image a laser-polarized xenon-129 phantom with both a conventional gradient-echo and a fully phase-encoded pulse sequence. In other contexts, a fully phase-encoded acquisition, also known as single-point or constant-time imaging, has been used to enable distortion-free imaging of short-T₂^∗ species. Here we used this technique to overcome imperfections associated with our homebuilt MRI system while also taking full advantage of the long T₂^∗ available at very low field. Our results demonstrate that xenon-129 image quality can be dramatically improved at low field by combining a fully phase-encoded k-space acquisition with auxiliary measurements of system imperfections including B₀ field drift and gradient infidelity.

Biomedical imaging with hyperpolarized noble gases

Hyperpolarized noble gases (HNGs), polarized to approximately 50% or higher, have led to major advances in magnetic resonance (MR) imaging of porous structures and air-filled cavities in human subjects, particularly the lung. By boosting the available signal to a level about 100 000 times higher than that at thermal equilibrium, air spaces that would otherwise appear as signal voids in an MR image can be revealed for structural and functional assessments. This review discusses how HNG MR imaging differs from conventional proton MR imaging, how MR pulse sequence design is affected and how the properties of gas imaging can be exploited to obtain hitherto inaccessible information in humans and animals. Current and possible future imaging techniques, and their application in the assessment of normal lung function as well as certain lung diseases, are described.

Influência das variações da postura sentada na função respiratória: revisão de literatura

INTRODUÇÃO: Os efeitos da postura sobre a função respiratória têm motivado pesquisas com o objetivo de rastrear alterações nos mecanismos de adaptação à mudança da postura corporal. A importância desse conhecimento está em compreender como essas alterações podem interferir na função respiratória de indivíduos saudáveis e em condições especiais, como obesos e gestantes. OBJETIVO: Realizar uma revisão bibliográfica para descrever o conhecimento produzido sobre as alterações da função respiratória em diferentes posturas corporais, em especial na postura sentada. MÉTODOS: Foram definidos os conceitos-chave da pesquisa: postura, postura sentada, testes respiratórios e função respiratória ou pulmonar; em seguida determinou-se o período de pesquisa que envolveu os anos de 2000 a 2010 (inclusive) e as bases de dados pesquisadas: SciELO, PEDro, Cochrane e Pubmed. RESULTADO: Encontrou-se que as primeiras pesquisas sobre função respiratória enfocavam alterações encontradas em mudanças significativas da postura corporal, geralmente em indivíduos saudáveis. O aprimoramento científico permitiu a incorporação tecnológica aos métodos de avaliação da função respiratória. Nos estudos sobre postura sentada, observa-se que as alterações são significativas em indivíduos com doenças pulmonares, cardíacas e idosos, entre outros, e que, mesmo em indivíduos saudáveis, as alterações nos testes de função podem ultrapassar a variação dos valores considerados normais para uma dada posição. CONCLUSÃO: São necessários maiores estudos para determinar o momento em que essas alterações podem ser significativas em indivíduos saudáveis e quais as alternativas possíveis para minimizar esses efeitos.

An open-access, very-low-field MRI system for posture-dependent 3He human lung imaging

We describe the design and operation of an open-access, very-low-field, magnetic resonance imaging (MRI) system for in vivo hyperpolarized 3He imaging of the human lungs. This system permits the study of lung function in both horizontal and upright postures, a capability with important implications in pulmonary physiology and clinical medicine, including asthma and obesity. The imager uses a bi-planar B(0) coil design that produces an optimized 65 G (6.5 mT) magnetic field for 3He MRI at 210 kHz. Three sets of bi-planar coils produce the x, y, and z magnetic field gradients while providing a 79-cm inter-coil gap for the imaging subject. We use solenoidal Q-spoiled RF coils for operation at low frequencies, and are able to exploit insignificant sample loading to allow for pre-tuning/matching schemes and for accurate pre-calibration of flip angles. We obtain sufficient SNR to acquire 2D 3He images with up to 2.8mm resolution, and present initial 2D and 3D 3He images of human lungs in both supine and upright orientations. 1H MRI can also be performed for diagnostic and calibration reasons.