Keywords bedside imaging - extremity MR imaging - low-field MRI - magnetic resonance imaging
- portable MRI
Introduction
Magnetic resonance imaging (MRI) is a widely accepted noninvasive imaging modality
for imaging internal structures with great precision, intricacy, and quality.[1 ] It uses a large superconducting bore magnet, radiofrequency (RF) pulses, a liquid
cryogen cooling system, and a set of coils along with other acquisition and processing
software and hardware made to work in synchrony for the production of images for diagnosis.[2 ] The magnetic field strength is measured in either gauss (G) or tesla (T), where
1 T = 10,000 G. But in the case of MRI magnetic field strength, tesla is more appropriate
and acceptable. Based on this magnetic field strength the scanners can be classified
as low (< 0.5 T), medium (0.5–1.0 T), and high (> 1.0 T).[3 ] Exploiting the differences in magnetic field strengths and other related factors
that form an image, MRI, as a modality has ventured into various subtypes and fields
of advancement that are enabling diagnosis at early stages on a rapid and large scale
for a massive variety of disorders and illnesses.[1 ] Right from the basic anatomical MRI based on T1 and T2 contrast differences in tissues
that portrays excellent morphological details to the ultrafast imaging that provides
functional details—fMRI. Similar examples would be diffusion tensor imaging, arterial
spin labeling, ultrahigh field neuromelanin-sensitive MRI, and quantitative susceptibility
mapping that operates on the magnetic susceptibility to produce phase contrast.[4 ] As we have been witnessing, MRI has come a long way in improvement and advancement
since the early 1980s when it was established. Yet, like several other man-made inventions
MRI too has its downside, the acquisition of MRI data are an inherently slow process
due to the high sampling requirements and thus cannot be used as an emergency modality
where quick diagnosis and immediate action are the key. Also, the long data acquisition
time leads to low patient throughput, discomfort, motion artifacts, and costly examinations.
These drawbacks have motivated the development of methods for faster MRI.[5 ] The large and specific infrastructural demand for the installation of the MRI system
makes it difficult for bedside diagnosis in cases of immobile or critically ill patients.
The development of a portable, low-cost MRI device for body imaging could expand access
and feasibility of MRI and enable point-of-care (POC) diagnostics thus enhancing its
use in the medical arena.[6 ]
Importance in the Portability of an Imaging Modality
Importance in the Portability of an Imaging Modality
Mobile or portable radiography refers to the one that is performed outside the comforts
of the radiology department with an instrument that can be moved and maneuvered single-handedly.
This is performed in times when patient transfer could lead to worse complications
and danger to patients' health. It could be in places within the hospital like the
intensive care units (ICUs), coronary care units, cardiac surgery units, neonatal
or pediatric care units, accident and emergency department resuscitation units (casualty),
and postanesthesia care units (theater recovery rooms), etc. It could also be in places
outside the hospital like the nursing care center, military field hospitals, sports
arena, hospice, etc.[7 ] Quick diagnosis becomes highly essential in cases where immediate action and treatment
is required to save lives, and this has been made easy and possible due to the portability
of radiological imaging modalities.
Portable Ultrasonography
Ultrasound (USG) machines have truly come a long way and are available in various
sizes and specifications and accessible to patients at their bedside. These can be
dragged single-handedly by their wheels or are even available as completely compact
hand-held devices. An added advantage of it is the use of sound waves and not ionizing
radiations apart from its efficiency and cost-effectiveness.[8 ] MRI can definitely take a lesson or two from the efficiency of the portable USG
machines especially in terms of compactness, cost, and accessibility.
Portable X-Ray
Portable or mobile X-ray units are in extensive use for various reasons especially
for chest radiographs or postsurgical examinations.[9 ] Yet, its source of ionizing radiations that could have harmful effects on the populations
makes it questionable to use thus making it necessary for ionizing radiation-free
diagnostic equipment such as a portable MRI (pMRI) unit.
Portable Computed Tomography Scanners
Portable Computed Tomography Scanners
This equipment can be operated at the normal power supply of a hospital ward or room
and does not require a specialized high-power supply. It makes it possible to scan
anatomy “stat” in cases like stroke, trauma, head injury, hemorrhage, etc.[10 ] The disadvantage being the radiation dose to staff and patients in a ward or surrounding
the portable scanner due to the utilization of high-energy ionizing radiation while
also lacking the soft tissue contrast imaging efficiency when compared with MRI.[8 ] These properties of low demand of power supply, immediate diagnosis with high contrast
resolution in critical illnesses, and without conferring the ill-effects of radiation
dose to the people around seen in pMRI systems are a plus point.
Portable MRI Systems
Even after 45 years of discovering the MRI scanner, portable systems were not developed.[8 ] Mostly owing to its complexity, bulk, and high-end equipment requirement. Though
MRI had excellent image quality and also uses nonionizing radiation, it did take time
and effort in innovating a low-field, low-cost pMRI scanner. This is made possible
by using a permanent magnet, which increases the feasibility and accessibility to
a wide range of the population. The low field also diminishes the fringe-field shielding
challenges.[11 ]
Equipment Construction and Working Principle
Equipment Construction and Working Principle
The Bore-Magnet
Low-field pMRI systems that operate at a magnetic field strength of 0.25 and 0.5 T,
are made either using a permanent magnet based on rare earth material, neodymium-iron-boron
(NdFeB) covered by fiberglass,[12 ] or one that combines a permanent magnet with an additional electromagnet.[13 ] The cylindrical permanent magnet discs further consist of many varied sized smaller
pieces of magnetized materials each of approximately 1 × 1 × 14” dimensions that store
energy. The geometries of which are optimized to produce the strongest and most homogeneous
field that is maintained without any power supply and has a cryogen-free operation.[8 ]
[13 ] The entire assembly sits on a set of aluminum rollers that are covered with high-friction
urethane.[14 ]
The Gradient (Image Encoding) System
The gradient system or image encoding usually requires a 3-axis gradient system, having
one on each axis. The pMRI scanner avoids this by building in a “permanent” gradient
into the magnet and utilizing that field for either readout or slice selection.[8 ] Another advanced way of image encoding described elegantly by Cooley et al,[14 ] is based on rotating spatial encoding magnetic fields where the bore magnet and
linear gradient coils are replaced with a rotating permanent magnet featuring an inhomogeneous
field pattern that is exploited for spatial encoding. This technique not only helps
in reducing the extra magnetic material required thus aiding in the lightweight machinery
but it also replaces the function of heavy switchable gradient coils with significant
power requirements thus reducing the noise and cost. This synchronized magnet rotation
of one degree at a rate of 10°/s is incorporated into the pulse sequence, and so is
controlled with precision by the MRI console.[14 ]
RF Subsystem
The RF pulse transmits and receives demand of portable and low-cost MRI identical
to the conventional systems. Using a low-field system increases the power efficiency
of excitation, and therefore only around 5 kW (kilowatt) of RF receiver coil strength
is needed for a head scanner and even lesser for an extremity scanner but higher strength
coils would be required for a multipurpose whole-body pMRI scanner.[8 ] A fully integrated dual-channel complementary metal–oxide–semiconductor receiver
chip fixed on the surface coils described by Sporrer[15 ] holds a brighter promise in the further advancement of the portable scanner. The
integrated circuit minimizes the size of the receiver modules by completely removing
all shielded RF cables. Also, the immediate digital signal conversion paves way for
efficient data transmission leading to greater signal strength.
Portable MRI Scanner: A Boon
Using low-field strength accentuates patient safety and comfort, in terms of reduced
projectile risks, implant compliance, and the possibility to image closer to implants
due to smaller magnetic susceptibility artifacts, reduced specific absorption rate
(SAR) limitations, and reduced acoustic noise because of lowered forces on the gradient
coil windings, it also aids in easy extremity positioning due to its open structure.
These systems alleviate footprint, which could take MRI to the POC. It is known to
have reduced siting requirements in terms of space (just ∼9 m2 ), weight (> 1.5 tons) shielding, power, and cooling.[16 ]
[17 ] These are also much more sustainable, wherein relatively inexpensive repair and
replacement of hardware modules is required in comparison to a superconducting magnet-based
system.[13 ] Cardiac implantable electronic devices (CIEDs), like pacemakers, and implantable
cardioverter-defibrillator are often contraindicated in MRI thus prohibiting imaging
of patients with these implants thereby hampering their diagnosis.[18 ] However, owing to the low-field strengths of the portable scanners these patients
with CIEDs can be imaged appropriately. The open nature of these scanners also enables
the imaging of claustrophobic patients who would otherwise dread staying still in
the deep gantry bore thus hampering diagnosis and treatment; also, oversized patients
can now be more comfortable during imaging.[3 ]
Specific Clinical Applications
Specific Clinical Applications
Neurological Diagnosis
Hydrocephalus : Low-field MRI could have an important diagnostic value in the diagnosis of hydrocephalus
which is characterized by an excessive volume of cerebrospinal fluid (CSF) in the
ventricles of the brain which in turn leads to increased intracranial pressure.[13 ] A study by Krishnan et al[19 ] elaborates on the MRI sequences for pediatric and fetal hydrocephalus, to be able
to investigate all possible etiopathogenesis through the CSF pathway and assess the
efficacy of treatment in a noninvasive standardized manner.
Intracranial hemorrhage (ICH) : Patients that present with brain injuries or are suspected of stroke are treated
immediately using thrombolytics, but these patients have to first be imaged proficiently
to rule out any hemorrhage prior to the treatment.[20 ] Computed tomography (CT) scan has been used as of now, but owing to the higher sensitivity
of MRI in characterizing the ICH thus aiding in better clinical management and prognosis,
and also due to the negative effects of ionizing radiation used in CT, the pMRI holds
better promise, which has been proven in a recent study by Mazurek et al,[21 ] where they obtained neuroimaging results that enabled easy bedside detection and
characterization of ICH at par with conventional techniques (CT or 1.5–3 T MRI) in
reduced time.
Ischemic stroke (IS) : Stroke is the leading cause of death, among which IS is the most common finding,
requiring stat treatment.[22 ] A pMRI that operates at very low magnetic field strength (0.064 T) used at the bedside
of critically ill patients in the ICU has a novel neuroimaging solution in the subacute
phase, that can acquire T1-weighted (T1W), T2W, fluid-attenuated inversion recovery
(FLAIR), and diffusion-weighted imaging (DWI) sequences accurately and safely without
having to transfer the patient, thus saving time in the diagnosis and hence treatment
and also avoiding the further risk of complications in the patient.[23 ]
Intracranial midline shift : The low-field pMRI images acquired for bedside assessment of midline shift in patients
with IS and ICH as well as cerebral mass effect prove to be consistent with the measurements
obtained on conventional MRI and CT studies in a clinical trial conducted by Sheth
et al.[24 ]
Pediatric or Neonatal Imaging
Using low-field pMRI scanners in pediatrics for the diagnosis of various pathologies
is highly advantageous due to the vastly reduced acoustic noise, the open nature that
allows direct parental participation, and the much lower SAR.[13 ] A study by Deoni et al[25 ] demonstrated the ability to acquire structural neuroimaging data in infants, children,
and young adolescents. It has been proven that volumetric measurements and developmental
patterns derived from higher field strength (3 T) systems are replicable using low-field
portable scanners.
Isolation Patients Imaging
In cases where the disease is contagious and the patients are kept in isolation, for
example, during the pandemic of coronavirus disease 2019 (COVID-19), where the transfer
of the patient for imaging purposes is not safe, pMRI usage has been approved by the
Food and Drug Administration.[26 ] Patients that suffer from any acute neuroinjury, can be successfully imaged. The
low field nature also makes it possible to perform the scan without moving the ferromagnetic
materials from the room including the vital signs monitor, compressed oxygen tanks,
etc., thus avoiding patient discomfort.[27 ]
Dental Imaging
“DentMRI - Gen I” a prototype low-field scanner built to produce high-quality combined
images of soft and hard biological tissues—such as teeth at approximately equal to
260 mT using two pulse sequences: Pointwise Encoding Time Reduction with Radial Acquisition
(PETRA) and Double Radial Non-Stop Spin Echo (DRaNSSE) and reconstructed using both
algebraic reconstruction techniques and traditional Fourier transformation, has been
well penned down by Algarín et al,[28 ] in their recent study in 2020. The magnetic susceptibility effects, which lead to
artifacts in the presence of metallic fillings or implants, are greatly reduced due
to the low field scans. Though not fully portable, it holds promise for a bright future
in portable dental scanning.
Extremity Imaging
The previously described Halbach design of a 50-mT pMRI scanner has been tested on
both phantoms and in vivo scans of the knee of a healthy volunteer. Images were acquired
at a spatial resolution of approximately 3 × 2 × 2 mm and an signal-to-noise ratio
(SNR) of approximately 20:1 within 12 minutes to show the applicability of the system
to extremity imaging. The system has a high degree of portability with a magnet weight
of approximately just 75 kg. The study proved that this design can be expanded in
the future for use in the breast or spinal imaging as well.[29 ] Another study by Nakagomi et al[30 ] portrays the development of a small (∼80 kg) car-mounted MRI system for human elbows
using a 0.2-T permanent magnet. It has enabled easy positioning due to its open system
and has been shown to render clinically relevant images, thus making it easily deployable
for extremity imaging in remote areas, sports arenas, or emergency departments at
ease. In yet another study, an MRI of the shoulder was performed on a 0.2-T extremity
scanner utilizing a dedicated shoulder coil which proved to be an accurate predictor
of rotator cuff tears, full or partial thickness tears with a sensitivity higher than
85% in each case which is more than sufficient to aid in clinical management.[31 ] Finally, all doubts about extremity scanners came to rest after the thorough evaluation
done by Guallart-Naval et al,[16 ] where they tested a 70-mT extremity MRI scanner mounted on a wheeled structure.
This unit weighed about approximately equal to 250 kg and could be used in various
environments such as hospitals, at a house, outdoors, etc., and provided good image
quality with valuable anatomical information in clinically acceptable times.
Chest Imaging
Recently, when everyone was hit hard by the devastating pandemic of COVID-19, longitudinal
imaging series was warranted for a better understanding of underlying pathomechanisms
of pulmonary damage, for which CT is of limited usability due to repeated exposure
to X-rays. Also, higher field strength systems are prone to susceptibility effects
due to direct tissue-air interface in lung imaging.[32 ] Hence, low-field MRI was used and it enabled a precise visualization of persistent
pulmonary changes including ground-glass opacities, which were consistent with CT
performed on the same day. Another study combined the high-performance hardware and
software of a high-field strength MRI at a lower-field strength (0.55 T) in a COVID-19
patient that produced images with quality comparable to CT, showing potential of low-field
systems that offer promise to reduce distortion for lung MRI.[33 ] Functional alterations associated with persistent symptoms after COVID-19 have also
been successfully detected.[34 ] Thus, proving the potential of low-field systems for repetitive lung examinations
in monitoring the reconvalescence after pulmonary infections and pneumonia.[35 ]
Vascular or Contrast-Enhanced Studies
It was highly doubtful whether more sophisticated magnetic resonance angiography (MRA)
or contrast studies could be performed in low-field MRI scanners; however, this doubt
was clarified by Masumoto et al[36 ] in their study in Japan, where they successfully developed an intra-arterial contrast-enhanced
two-dimensional magnetic resonance dynamic subtraction angiography sequence using
Magnevist (gadopentetate dimeglumine) on a 0.3-T low-field MRI scanner, thus showing
the potential of these scanners in the visualization of vascular anatomy and hemodynamics.
Also using these next-gen advanced 0.55-T low-field systems, it is possible to obtain
time-of-flight MRA depicting vessels with comparable image quality to 1.5 T scanners.[37 ] Few other studies have been able to show pleasing evidences with respect to contrast-based
studies.[38 ]
[39 ]
Interventional Procedures
The open structure of low-field scanners aids to guide interventions while the subject
is within the scanner.[13 ] Also, the possibility of obtaining quality dynamic contrast images with high temporal
resolution portrays well the potential of these scanners as a monitoring tool during
MR-guided endovascular procedures.[36 ] MR-guided cryotherapy for malignant liver tumors performed using a horizontal-magnetic
open system in a study by Dohi et al[40 ] and cardiac catheterization performed successfully in 7 patients with no heating
caused by the guidewire or other adverse effects[41 ] proves the feasibility and accuracy of low-field open scanners in performing such
sophisticated interventions in patients safely after incorporating some high-performance
hardware or software used in higher field strength systems. Thus, curbing the disadvantage
of high radiation doses to the patients while using other radiographic modalities
like fluoroscopy, C-arm equipment, etc.[41 ]
Discussion
This review significantly proves great potential in revolutionizing bedside imaging
with the advancement of an all-around, low-field pMRI scanner. For even smaller sizes
and lighter weights dedicated brain or extremity scanners can be used. The portable
80 mT scanner specifically for brain imaging described by Cooley et al[6 ] is extremely lightweight, weighing only approximately 100 to 125 kg that works on
a standard wall outlet power supply with no additional cooling required with an easy
mounting facility for a quick commute. The drawback noticed was due to the usage of
two permanent readout gradients instead of the rotating permanent magnet system which
led to a nonlinear mapping of voxels in the image that manifest as geometric distortion.[6 ]
Diagnostic-grade T1W, T2W, T2 FLAIR, and DWI sequences were obtained in a time of
less than 35 minutes using the currently available portable brain-dedicated MRI scanner
at the Yale New Haven Hospital, Connecticut, United States, in a study by Sheth et
al.[27 ] In this study all POC MRI findings synced with conventional radiology reports. Various
diagnoses were correctly made in patients suffering from common neuroinjuries like
ICH, diffuse cerebral edema, cerebral infarction, and small and large ISs, thus proving
the effectiveness of portable systems in emergency and bedside neuroimaging. Drawbacks
related to the decreased SNR that hamper overall image quality are expected due to
the lower field strengths; however, these could be overcome by improvement in the
other technical aspects, like modified pulse sequences, coils, or exploiting some
deep learning techniques.[17 ] Apart from SNR, image distortion caused by the increased level of inhomogeneities
in the main magnetic field especially while using gradient sequences, is an area where
more research regarding better processing techniques to improve image quality is essential.[42 ]
While we talk about low-field scanners, Sarracanie et al[43 ] demonstrated short acquisition times with high SNR per unit time using the ultra-low
field (6.5 mT) MRI, dating back to 2015. This was accomplished using the sparse sampling
strategies (50% of k-space is sampled) and fully refocused three-dimensional (3D)
balanced steady-state free precession sequences in an optimized electromagnet “20 cm
field of view” mobile brain scanner with neither prepolarization nor cryogenics. The
unique mechanism in this scanner was the use of a 30-turn 3D Archimedean spiral wire
placement in lieu of conventional coils, thus ensuring that the magnetic field B1 produced by the spiral pattern is orthogonal to the main magnetic field B0 . The hemispheric spiral design results in a very homogeneous magnetic field making
it suitable for both receiving and transmitting RF pulses and thus obtaining good
quality images required for basic neurological diagnosis. Thereby laying the basis
of the foundation for the development of portable scanners using ultra-low field strength
machinery which could lead to a further reduction in cost and acquisition time.[43 ] Since the last few years extensive fieldwork has been going on concerning the working,
construction, and implementation of low-field, pMRI scanners; however, currently,
the only available portable low-field MRI scanner is Hyperfine Swoop ([Fig. 1 ]) in Connecticut, United States.[44 ] These scanners are slowly changing the age-old perception that lower field strengths
caused deterioration of the image quality and are proving to hold great promise for
the future.[17 ] Also, with the advent of artificial intelligence and remarkable developments in
deep learning techniques, noise due to low static field strengths can be removed more
effectively than before thus increasing the efficiency and potential of these low-field
scanners.[37 ]
Fig. 1 Hyperfine Swoop portable low-field magnetic resonance imaging (MRI) system.
Conclusion
Back in the 1980s, the introduction of MRI scanners was using low-field strengths;
however, due to a lack of image quality and other disadvantages higher strength scanners
came into being displaying greater image quality and sophisticated added specifications.[37 ] With higher fields there came a need for higher shielding and more complex infrastructure
and limitations that lacked portability. Hence, the renaissance of low-field scanners,
which have overcome the past inadequacies using modern, improved technological advances,
is surely a boon to the health care system. It can aid in bedside, outdoor, low-resource,
or remote area imaging for cost-effective, quick, and efficient diagnosis. This would
in turn lead to better clinical management and thus decreased mortality rates and
a better life expectancy.
