Rapid polarizing field cycling in magnetic resonance imaging

Encoding scheme design for gradient-free, nonlinear projection imaging using Bloch-Siegert RF spatial encoding in a low-field, open MRI system

Eliminating conventional pulsed B0-gradient coils for magnetic resonance imaging (MRI) can significantly reduce the cost of and increase access to these devices. Phase shifts induced by the Bloch-Siegert shift effect have been proposed as a means for gradient-free, RF spatial encoding for low-field MR imaging. However, nonlinear phasor patterns like those generated from loop coils have not been systematically studied in the context of 2D spatial encoding. This work presents an optimization algorithm to select an efficient encoding trajectory among the nonlinear patterns achievable with a given hardware setup. Performance of encoding trajectories or projections was evaluated through simulated and experimental image reconstructions. Results show that the encodings schemes designed by this algorithm provide more efficient spatial encoding than comparison encoding sets, and the method produces images with the predicted spatial resolution and minimal artifacts. Overall, the work demonstrates the feasibility of performing 2D gradient-free, low-field imaging using the Bloch-Siegert shift which is an important step towards creating low-cost, point-of-care MR systems.

Magnetic resonance imaging using a nonuniform Bo (NuBo) field-cycling magnet

Magnetic resonance imaging (MRI) is a powerful noninvasive diagnostic tool with superior soft tissue contrast. However, access to MRI is limited since current systems depend on homogeneous, high field strength main magnets (B0-fields), with strong switchable gradients which are expensive to install and maintain. In this work we propose a new approach to MRI where imaging is performed in an inhomogeneous field using radiofrequency spatial encoding, thereby eliminating the need for uniform B0-fields and conventional cylindrical gradient coils. The proposed technology uses an innovative data acquisition and reconstruction approach by integrating developments in field cycling, parallel imaging and non-Fourier based algebraic reconstruction. The scanner uses field cycling to image in an inhomogeneous B0-field; in this way magnetization is maximized during the high field polarization phase, and B0 inhomogeneity effects are minimized by using a low field during image acquisition. In addition to presenting the concept, this work provides experimental verification of a long-lived spin echo signal, spatially varying resolution, as well as both simulated and experimental 2D images. Our initial design creates an open MR system that can be installed in a patient examination table for body imaging (e.g., breast or liver) or built into a wall for weighted-spine imaging. The proposed system introduces a new class of inexpensive, open, silent MRIs that could be housed in doctor's offices much like ultrasound is today, making MRI more widely accessible.

Prepolarized MRI of hard tissues and solid-state matter

Prepolarized MRI (PMRI) is a long-established technique conceived to counteract the loss in signal-to-noise ratio (SNR) inherent to low-field MRI systems. When it comes to hard biological tissues and solid-state matter, PMRI is severely restricted by their ultra-short characteristic relaxation times. Here we demonstrate that efficient hard-tissue prepolarization is within reach with a special-purpose 0.26 T scanner designed for ex vivo dental MRI and equipped with suitable high-power electronics. We have characterized the performance of a 0.5 T prepolarizer module, which can be switched on and off in 200 μs. To this end, we have used resin, dental and bone samples, all with T1 times of the order of 20 ms at our field strength. The measured SNR enhancement is in good agreement with a simple theoretical model, and deviations in extreme regimes can be attributed to mechanical vibrations due to the magnetic interaction between the prepolarization and main magnets.

Dual k-space and image-space post-processing for field-cycling MRI under low magnetic field stability and homogeneity conditions

We present a method to correct artifacts typically present in images acquired in field-cycled MRI experiments under poor magnetic field spatial-homogeneity and time-stability conditions. The proposed method was tested in both simulated and experimental data. The experiments were performed using a fast field-cycling MRI relaxometer of own design, based on a current-driven variable-geometry electromagnet. Current instability-induced artifacts in the images were mitigated through a phase correction array resulted from entropy and background minimization. Image distortions due to magnetic field inhomogeneity were compensated through two different approaches, involving a previous determination of the magnetic field homogeneity-map, or an experimental protocol where two images are acquired with inverted readout gradient polarity. Results show that images acquired at extreme conditions can be successfully improved, thus strengthening the possibilities for both low-cost MRI devices and faster field-switched MRI systems.

Approaches in cooling of resistive coil-based low-field Magnetic Resonance Imaging (MRI) systems for application in low resource settings

Magnetic Resonance Imaging (MRI), a non-invasive method for the diagnosis of diverse health conditions has experienced growing popularity over other imaging modalities like ultrasound and Computer Tomography. Initially, proof-of-concept and earlier MRI systems were based on resistive and permanent magnet technology. However, superconducting magnets have long held monopoly of the market for MRI systems with their high-field (HF) strength capability, although they present high construction, installation, and siting requirements. Such stringent prerequisites restrict their availability and use in low-middle income countries. Resistive coil-based magnet, albeit low-field (LF) in capacity, represent a plausible boost for the availability and use of MRI systems in resource constrained settings. These systems are characterized by low costs coupled with substantial image quality for diagnosis of some conditions such as hydrocephalus common is such regions. However, the nature of resistive coils causes them to heat up during operation, thus necessitating a dedicated cooling system to improve image quality and enhance system longevity. This paper explores a range of cooling methods as have been applied to resistive magnets, citing their pros and cons and areas for improvement.

An Unmatched Radio Frequency Chain for Low-Field Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) is a safe and versatile diagnostic tool for intracranial imaging, however it is also one of the most expensive and specialized making it scarce in low- to middle-income countries (LMIC). The affordability and portability of low-field MRI offers the potential for increased access to brain imaging for diseases like Hydrocephalus in LMIC. In this tutorial style work, we show the design of a low powered and low cost radio frequency chain of electronics to be paired with a previously reported prepolarized low-field MRI for childhood hydrocephalus imaging in sub-Saharan Africa where the incidence of this condition is high. Since the Larmor frequency for this system is as low as 180 kHz, we are able to minimize the impedance of the transmit coil to 5 ohms rather than match to 50 ohms as is traditionally the case. This reduces transmit power consumption by a factor of 10. We also show the use of inexpensive and commonly available animal enclosure fencing ("chicken wire") as a shield material at this frequency and compare to more traditional shield designs. These preliminary results show that highly portable and affordable low-field MRI systems could provide image resolution and signal-to-noise sufficient for planning hydrocephalus treatment in areas of the world with substantial resource limitations. Employment of these technologies in sub-Saharan Africa offers a cost-effective, sustainable approach to neurological diagnosis and treatment planning in this disease burdened region.

Multifield and inverse-contrast switching of magnetocaloric high contrast ratio MRI labels

PURPOSE

Demonstrating multifield and inverse contrast switching of magnetocaloric high contrast ratio MRI labels that either have increasing or decreasing moment versus temperature slopes depending on the material at physiological temperatures and different MRI magnetic field strengths.

METHODS

Two iron-rhodium samples of different purity (99% and 99.9%) and a lanthanum-iron-silicon sample were obtained from commercial vendors. Temperature and magnetic field-dependent magnetic moment measurements of the samples were performed on a vibrating sample magnetometer. Temperature-dependent MRI of different iron-rhodium and lanthanum-iron-silicon samples were performed on 3 different MRI scanners at 1 Tesla (T), 4.7T, and 7T.

RESULTS

Sharp, first-order magnetic phase transition of each iron-rhodium sample at a physiologically relevant temperature (~37°C) but at different MRI magnetic fields (1T, 4.7T, and 7T, depending on the sample) showed clear image contrast changes in temperature-dependent MRI. Iron-rhodium and lanthanum-iron-silicon samples with sharp, first-order magnetic phase transitions at the same MRI field of 1T and physiological temperature of 37°C, but with positive and negative slope of magnetization versus temperature, respectively, showed clear inverse contrast image changes. Temperature-dependent MRI on individual microparticle samples of lanthanum-iron-silicon also showed sharp image contrast changes.

CONCLUSION

Magnetocaloric materials of different purity and composition were demonstrated to act as diverse high contrast ratio switchable MRI contrast agents. Thus, we show that a range of magnetocaloric materials can be optimized for unique image contrast response under MRI-appropriate conditions at physiological temperatures and be controllably switched in situ.

A fast field-cycling MRI relaxometer for physical contrasts design and pre-clinical studies in small animals

We present a fast field-cycling NMR relaxometer with added magnetic resonance imaging capabilities. The instrument operates at a maximum proton Larmor frequency of 5 MHz for a sample volume of 35 mL. The magnetic field homogeneity across the sample is 1400 ppm. The main field is generated with a notch-coil electromagnet of own design, fed with a current whose stability is 220 ppm. We show that images of reasonable quality can still be produced under such conditions. The machine is being designed for concept testing of the involved instrumentation and specific contrast agents aimed for field-cycling magnetic resonance imaging applications. The general performance of the prototype was tested through localized relaxometry experiments, T₁-dispersion weighted images, temperature maps and T₁-weighted images at different magnetic field intensities. We introduce the concept of positive and negative contrast depending on the use of pre-polarized or non-polarized sequences. The system is being improved for pre-clinical studies in small animals.

Pulsed Excitation in Magnetic Particle Imaging

Magnetic particle imaging (MPI) is a promising new tracer-based imaging modality. The steady-state, nonlinear magnetization physics most fundamental to MPI typically predicts improving resolution with increasing tracer magnetic core size. For larger tracers, and given typical excitation slew rates, this steady-state prediction is compromised by dynamic processes that induce a significant secondary blur and prevent us from achieving high resolution using larger tracers. Here, we propose a new method of excitation and signal encoding in MPI we call pulsed MPI to overcome this phenomenon. Pulsed MPI allows us to directly encode the steady-state magnetic physics into the time-domain signal. This in turn gives rise to a simple reconstruction algorithm to obtain images free of secondary relaxation-induced blur. Here, we provide a detailed description of our approach in 1D, discuss how it compares with alternative approaches, and show experimental data demonstrating better than 500- [Formula: see text] resolution (at 7 T/m) with large tracers. Finally, we show experimental images from a 2D implementation.

A whole-body Fast Field-Cycling scanner for clinical molecular imaging studies

Fast Field-Cycling (FFC) is a well-established Nuclear Magnetic Resonance (NMR) technique that exploits varying magnetic fields to quantify molecular motion over a wide range of time scales, providing rich structural information from nanometres to micrometres, non-invasively. Previous work demonstrated great potential for FFC-NMR biomarkers in medical applications; our research group has now ported this technology to medical imaging by designing a whole-body FFC Magnetic Resonance Imaging (FFC-MRI) scanner capable of performing accurate measurements non-invasively over the entire body, using signals from water and fat protons. This is a unique tool to explore new biomarkers related to disease-induced tissue remodelling. Our approach required making radical changes in the design, construction and control of MRI hardware so that the magnetic field is switched within 12.5 ms to reach any field strength from 50 μT to 0.2 T, providing clinically useful images within minutes. Pilot studies demonstrated endogenous field-dependant contrast in biological tissues in good agreement with reference data from other imaging modalities, confirming that our system can perform multiscale structural imaging of biological tissues, from nanometres to micrometres. It is now possible to confirm ex vivo results obtained from previous clinical studies, offering applications in diagnosis, staging and monitoring treatment for cancer, stroke, osteoarthritis and oedema.

Design of a sustainable prepolarizing magnetic resonance imaging system for infant hydrocephalus

OBJECTIVES

The need for affordable and appropriate medical technologies for developing countries continues to rise as challenges such as inadequate energy supply, limited technical expertise, and poor infrastructure persist. Low-field magnetic resonance imaging (LF MRI) is a technology that can be tailored to meet specific imaging needs within such countries. Its low power requirements and the possibility of operating in minimally shielded or unshielded environments make it especially attractive. Although the technology has been widely demonstrated over several decades, it is yet to be shown that it can be diagnostic and improve patient outcomes in clinical applications. We here demonstrate the robustness of prepolarizing MRI (PMRI) technology for assembly and deployment in developing countries for the specific application to infant hydrocephalus. Hydrocephalus treatment planning and management requires only modest spatial resolution, such that the brain can be distinguished from fluid-tissue contrast detail within the brain parenchyma is not essential.

MATERIALS AND METHODS

We constructed an internally shielded PMRI system based on the Lee-Whiting coil system with a 22-cm diameter of spherical volume.

RESULTS

In an unshielded room, projection phantom images were acquired at 113 kHz with in-plane resolution of 3 mm × 3 mm, by introducing gradient fields of sufficient magnitude to dominate the 5000 ppm inhomogeneity of the readout field.

DISCUSSION

The low cost, straightforward assembly, deployment potential, and maintenance requirements demonstrate the suitability of our PMRI system for developing countries. Further improvement in image spatial resolution and contrast of LF MRI will broaden its potential clinical utility beyond hydrocephalus.

Earth's field NMR detection of oil under arctic ice-water suppression

Earth's field NMR has been developed to detect oil trapped under or in Arctic sea-ice. A large challenge, addressed here, is the suppression of the water signal that dominates the oil signal. Selective suppression of water is based on relaxation time T₁ because of the negligible chemical shifts in the weak earth's magnetic field, making all proton signals overlap spectroscopically. The first approach is inversion-null recovery, modified for use with pre-polarization. The requirements for efficient inversion over a wide range of B₁ and subsequent adiabatic reorientation of the magnetization to align with the static field are stressed. The second method acquires FIDs at two durations of pre-polarization and cancels the water component of the signal after the data are acquired. While less elegant, this technique imposes no stringent requirements. Similar water suppression is found in simulations for the two methods. Oil detection in the presence of water is demonstrated experimentally with both techniques.

Fast Room Temperature Very Low Field-Magnetic Resonance Imaging System Compatible with MagnetoEncephaloGraphy Environment

In recent years, ultra-low field (ULF)-MRI is being given more and more attention, due to the possibility of integrating ULF-MRI and Magnetoencephalography (MEG) in the same device. Despite the signal-to-noise ratio (SNR) reduction, there are several advantages to operating at ULF, including increased tissue contrast, reduced cost and weight of the scanners, the potential to image patients that are not compatible with clinical scanners, and the opportunity to integrate different imaging modalities. The majority of ULF-MRI systems are based, until now, on magnetic field pulsed techniques for increasing SNR, using SQUID based detectors with Larmor frequencies in the kHz range. Although promising results were recently obtained with such systems, it is an open question whether similar SNR and reduced acquisition time can be achieved with simpler devices. In this work a room-temperature, MEG-compatible very-low field (VLF)-MRI device working in the range of several hundred kHz without sample pre-polarization is presented. This preserves many advantages of ULF-MRI, but for equivalent imaging conditions and SNR we achieve reduced imaging time based on preliminary results using phantoms and ex-vivo rabbits heads.

T 1 relaxation measurement of ex-vivo breast cancer tissues at ultralow magnetic fields

We investigated T 1 relaxations of ex-vivo cancer tissues at low magnetic fields in order to check the possibility of achieving a T 1 contrast higher than those obtained at high fields. The T 1 relaxations of fifteen pairs (normal and cancerous) of breast tissue samples were measured at three magnetic fields, 37, 62, and 122 μT, using our superconducting quantum interference device-based ultralow field nuclear magnetic resonance setup, optimally developed for ex-vivo tissue studies. A signal reconstruction based on Bayesian statistics for noise reduction was exploited to overcome the low signal-to-noise ratio. The ductal and lobular-type tissues did not exhibit meaningful T 1 contrast values between normal and cancerous tissues at the three different fields. On the other hand, an enhanced T 1 contrast was obtained for the mucinous cancer tissue.

Conductive shield for ultra-low-field magnetic resonance imaging: Theory and measurements of eddy currents

Eddy currents induced by applied magnetic-field pulses have been a common issue in ultra-low-field magnetic resonance imaging. In particular, a relatively large prepolarizing field-applied before each signal acquisition sequence to increase the signal-induces currents in the walls of the surrounding conductive shielded room. The magnetic-field transient generated by the eddy currents may cause severe image distortions and signal loss, especially with the large prepolarizing coils designed for in vivo imaging. We derive a theory of eddy currents in thin conducting structures and enclosures to provide intuitive understanding and efficient computations. We present detailed measurements of the eddy-current patterns and their time evolution in a previous-generation shielded room. The analysis led to the design and construction of a new shielded room with symmetrically placed 1.6-mm-thick aluminum sheets that were weakly coupled electrically. The currents flowing around the entire room were heavily damped, resulting in a decay time constant of about 6 ms for both the measured and computed field transients. The measured eddy-current vector maps were in excellent agreement with predictions based on the theory, suggesting that both the experimental methods and the theory were successful and could be applied to a wide variety of thin conducting structures.

Current-density imaging using ultra-low-field MRI with zero-field encoding

Electric current density can be measured noninvasively with magnetic resonance imaging (MRI). Determining all three components of the current density, however, requires physical rotation of the sample or current injection from several directions when done with conventional methods. However, the emerging technology of ultra-low-field (ULF) MRI, in which the signal encoding and acquisition is conducted at a microtesla-range magnetic field, offers new possibilities. The low applied magnetic fields can even be switched off completely within the pulse sequence, increasing the flexibility of the available sequences. In this article, we present a ULF-MRI sequence designed for obtaining all three components of a current-density pattern without the need of sample rotations. The sequence consists of three steps: prepolarization of the sample, signal encoding in the current-density-associated magnetic field without applying any MRI fields, and spatial encoding in a microtesla-range field using any standard ULF-MRI sequence. The performance of the method is evaluated by numerical simulations. The method may find applications, e.g., in noninvasive conductivity imaging of tissue.

Hybrid ultra-low-field MRI and magnetoencephalography system based on a commercial whole-head neuromagnetometer

Ultra-low-field MRI uses microtesla fields for signal encoding and sensitive superconducting quantum interference devices for signal detection. Similarly, modern magnetoencephalography (MEG) systems use arrays comprising hundreds of superconducting quantum interference device channels to measure the magnetic field generated by neuronal activity. In this article, hybrid MEG-MRI instrumentation based on a commercial whole-head MEG device is described. The combination of ultra-low-field MRI and MEG in a single device is expected to significantly reduce coregistration errors between the two modalities, to simplify MEG analysis, and to improve MEG localization accuracy. The sensor solutions, MRI coils (including a superconducting polarizing coil), an optimized pulse sequence, and a reconstruction method suitable for hybrid MEG-MRI measurements are described. The performance of the device is demonstrated by presenting ultra-low-field-MR images and MEG recordings that are compared with data obtained with a 3T scanner and a commercial MEG device.

Narrowband magnetic particle imaging

The magnetic particle imaging (MPI) method directly images the magnetization of super-paramagnetic iron oxide (SPIO) nanoparticles, which are contrast agents commonly used in magnetic resonance imaging (MRI). MPI, as originally envisioned, requires a high-bandwidth receiver coil and preamplifier, which are difficult to optimally noise match. This paper introduces Narrowband MPI, which dramatically reduces bandwidth requirements and increases the signal-to-noise ratio for a fixed specific absorption rate. We employ a two-tone excitation (called intermodulation) that can be tailored for a high-Q, narrowband receiver coil. We then demonstrate a new MPI instrument capable of full 3-D tomographic imaging of SPIO particles by imaging acrylic and tissue phantoms.

Noise performance of a precision pulsed electromagnet power supply for magnetic resonance imaging

Prepolarized magnetic resonance imaging (PMRI) uses two pulsed electromagnets to achieve high-field image quality with the benefits of low-field data acquisition. The principal challenge with all resistive MRI systems is the implementation of a highly precise magnet current supply. The noise current through the magnet is fundamentally limited by the current transducer used to provide feedback and the voltage reference used to generate the demand signal. Field instability in the main field magnet can both corrupt the received data and degrade the robustness of Carr¿Purcell¿Meiboom¿Gill (CPMG) echo trains, which are paramount to efficient imaging in PMRI. In this work, we present the magnet control system that achieved sufficient field stability for PMRI at $0.5/0.13$ T, identify the dominant sources of noise in the control system, examine the imaging artifacts that can occur if the field stability is insufficient, and identify how the design can be improved for better field stability, should it be required for future implementations of PMRI.

Three-dimensional prepolarized magnetic resonance imaging using rapid acquisition with relaxation enhancement

Prepolarized MRI (PMRI) with pulsed electromagnets has the potential to produce diagnostic quality 0.5- to 1.0-T images with significantly reduced cost, susceptibility artifacts, specific absorption rate, and gradient noise. In PMRI, the main magnetic field cycles between a high field (B(p)) to polarize the sample and a homogeneous, low field (B(0)) for data acquisition. This architecture combines the higher SNR of the polarizing field with the imaging benefits of the lower field. However, PMRI can only achieve high SNR efficiency for volumetric imaging with 3D rapid imaging techniques, such as rapid acquisition with relaxation enhancement (RARE) (FSE, TSE), because slice-interleaved acquisition and longitudinal magnetization storage are both inefficient in PMRI. This paper demonstrates the use of three techniques necessary to achieve efficient, artifact-free RARE in PMRI: quadratic nulling of concomitant gradient fields, electromotive force cancelation during field ramping, and phase compensation of CPMG echo trains. This paper also demonstrates the use of 3D RARE in PMRI to achieve standard T(1) and fat-suppressed T(2) contrast in phantoms and in vivo wrists. These images show strong potential for future clinical application of PMRI to extremity musculoskeletal imaging and peripheral angiography.

Prepolarized magnetic resonance imaging around metal orthopedic implants

A prepolarized MRI (PMRI) scanner was used to image near metal implants in agar gel phantoms and in in vivo human wrists. Comparison images were made on 1.5- and 0.5-T conventional whole-body systems. The PMRI experiments were performed in a smaller bore system tailored to extremity imaging with a prepolarization magnetic field of 0.4 T and a readout magnetic field of 27-54 mT (1.1-2.2 MHz). Scan parameters were chosen with equal readout gradient strength over a given field of view and matrix size to allow unbiased evaluation of the benefits of lower readout frequency. Results exhibit substantial reduction in metal susceptibility artifacts under PMRI versus conventional scanners. A new artifact quantification technique is also presented, and phantom results confirm that susceptibility artifacts improve as expected with decreasing readout magnetic field using PMRI. This proof-of-concept study demonstrates that prepolarized techniques have the potential to provide diagnostic cross-sectional images for postoperative evaluation of patients with metal implants.

Magnetic resonance imaging with T1 dispersion contrast

Prepolarized MRI uses pulsed magnetic fields to produce MR images by polarizing the sample at one field strength (approximately 0.5 T) before imaging at a much lower field (approximately 50 mT). Contrast reflecting the T(1) of the sample at an intermediate field strength is achieved by polarizing the sample and then allowing the magnetization to decay at a chosen "evolution" field before imaging. For tissues whose T(1) varies with field strength (T(1) dispersion), the difference between two images collected with different evolution fields yields an image with contrast reflecting the slope of the T(1) dispersion curve between those fields. Tissues with high protein content, such as muscle, exhibit rapid changes in their T(1) dispersion curves at 49 and 65 mT due to cross-relaxation with nitrogen nuclei in protein backbones. Tissues without protein, such as fat, have fairly constant T(1) over this range; subtracting images with two different evolution fields eliminates signal from flat T(1) dispersion species. T(1) dispersion protein-content images of the human wrist and foot are presented, showing clear differentiation between muscle and fat. This technique may prove useful for delineating regions of muscle tissue in the extremities of patients with diseases affecting muscle viability, such as diabetic neuropathy, and for visualizing the protein content of tissues in vivo.

Design of field-cycled magnetic resonance systems for small animal imaging

This paper presents a design study for a field-cycled magnetic resonance imaging (MRI) system directed at small animal imaging applications. A field-cycled MRI system is different from a conventional MRI system in that it uses two separate and dynamically controllable magnetic fields. A strong magnetic field is used to polarize the object, and a relatively weak magnetic field is used during signal acquisition. The potential benefits of field-cycled MRI are described. The theoretical dependences of field-cycled MRI performance on system design are introduced and investigated. Electromagnetic, mechanical and thermal performances of the system were considered in this design study. A system design for imaging 10 cm diameter objects is presented as an example, capable of producing high-duty-cycle polarizing magnetic fields of 0.5 T and readout magnetic fields corresponding to a proton Larmor frequency of 5 MHz. The specifications of the final design are presented along with its expected electromagnetic and thermal performance.

Simulation of scattering and attenuation of 511 keV photons in a combined PET/field-cycled MRI system

Mixing the imaging modalities of positron emission tomography (PET) and magnetic resonance imaging (MRI) will offer the best soft tissue contrast (MRI) with information about metabolic function (PET). The high magnetic field environment of an MRI system makes the detection of annihilation photons difficult, as the response of standard photo-multiplier tubes is compromised. An approach using field-cycled MRI is discussed here, as field-cycled MRI makes it possible to have long periods of time available for nuclear imaging when there is no magnetic field present. This work focuses upon the effect of the field-cycled MRI upon the nuclear image due to the added material providing additional attenuation of the PET signal, and additional nuclei for scatter. These effects are studied using a Monte Carlo simulation based upon the GEANT libraries. Attenuation effects are shown to be significant, approximately 6% for the RF shield and coil and approximately 24% for the gradients. No significant effect is seen in image quality due to the scattering of the gammas. With these levels of attenuation it is concluded that open gradient coils and shim coils are required around the imaging volume.