Superconducting magnets can be passively shimmed to have the static B0
field with 1–2 ppm uniformity over typical regions of interest (22
). The remaining static field inhomogeneities are usually comprised of low order spatial variations that can be reduced through the use of similarly low order spherical harmonic active shim coils (31
). Thus the main sources of static inhomogeneities in the B0 field arise from objects with varying magnetic susceptibilities in the body or sample. Consequently, air-tissue and tissue-tissue boundaries in the body are unavoidable and typically uncorrectable sources of field gradients within the main field. These perturbations occur across all susceptibility boundaries (some with diminished magnitude due to gross geometry) and contain higher order spatial variations in the field that cannot be corrected by the lower order active shim coils (6
Previously, other passive techniques have been introduced to reduce or eliminate the effects of these susceptibility boundaries. These methods include the use of pads of external diamagnetic fluids, such as water or perfluorocarbon (13
). Water or perfluorocarbon pads can be used to surround the relevant tissue of interest because their magnetic susceptibilities are naturally close to those of human tissues. In effect, this moves the field gradients from the mismatched susceptibility boundaries out of the field of view or away from the region of interest. Perfluorocarbon has no intrinsic water signal and water can be doped with paramagnetic ions to have no MRI signal so as not to saturate the image. The disadvantage of this technique is the incompatibility with embedded RF coils, bulkiness, and lack of patient comfort. Of course, PG foam can only address field perturbations due to external air cavities; it cannot improve the magnetic field disturbance due to sinus cavities or lungs.
Others have used the field gradients arising from oral and aural shims of the strongly diamagnetic pyrolytic crystals in an attempt to directly compensate for inhomogeneities caused by air-tissue interfaces in the sinus and aural cavities (8
). However, it is difficult to compensate for the dipole-like field effects from the air cavities using the similarly dipole-like effects of the graphite shims at a distance away from the original source. This technique does have the advantage of improving the field in areas where the air cavity cannot be directly accessed or filled, but similar to an active shim, it cannot completely correct the higher order spatial variations. The active current-carrying coil methods have been shown to improve field homogeneity in the human prefrontal cortex (11
), but has not been tested or modified for use in other body geometries and applications.
The uniformity of the main field is especially important in applications such as fat suppression. In MRI fat appears with a bright signal and can cover up relevant features in the region of interest. Fat suppression techniques in MRI are often particularly relevant around areas with natural susceptibility boundaries, such as the breast, head, back, and other extremities. Traditional fat suppression techniques include spectrally selective RF fat saturation pulses, short T1 inversion recovery (STIR) imaging, 2 point Dixon, and 3 point Dixon/IDEAL techniques.
Spectrally selective fat saturation methods typically use the 3.5 ppm chemical shift between fat and water to exclusively excite and saturate the fat signal prior to MRI (32
), or to excite only the water (spectral spatial). Because the chemical shift between fat and water is small, a uniform static field on the order of 1 ppm is required to correctly perform chemical shift selective RF pulses for fat suppression. Thus, these methods typically fail near areas with strong susceptibility mismatches (33
), with either unreliable fat suppression or erroneous suppression of water (tissue) signal.
STIR methods use an inversion pulse to suppress fat by its short T1
. While fat suppression can be uniform with STIR using a non-frequency selective pulse, any species with T1
similar to fat, including contrast-enhanced tissue, is also suppressed (34
). STIR typically has poor SNR performance (35
) due to saturation from the inversion pulse. Frequency selective STIR pulse sequences do not suffer this SNR degradation, but again these methods require pristine 1 ppm field homogeneity.
A class of pulse sequence, often called 3-point Dixon methods, include IDEAL (Iterative Decomposition of water/fat using Echo Asymmetry and Least-squares estimation) methods (36
), which generally use repetitive phase-shifted acquisitions (scans) and special reconstruction algorithms to account for field inhomogeneities to resolve chemical shift from field inhomogeneities. However, drawbacks to these methods include increased time necessary for acquisition and reconstruction (38
). Specialized reconstruction techniques and software are also often used. Hence, it would be advantageous to improve field homogeneity so that these 3-point Dixon methods are not required
The use of the tissue susceptibility matched PG foam is especially relevant to applications such as breast MRI, for example, in which the air/tissue susceptibility boundary is accessible to the foam and requires an extremely uniform static field for proper fat suppression using the faster spectrally selective techniques. PG foam cushions that would correct for a more uniform field to enable more robust frequency selective fat suppression could be pre-molded in various sizes for patient fit and comfort. Also, in breast MRI, the patient is typically imaged with a surface coil, in which SNR decreases as the distance away from the surface coil increases. Because PG foam is non-conductive, it is also compatible with embedded RF coils allowing maximal SNR from a given coil, something that could be unsafe with perfluorocarbon or doped water matching materials. In addition, in clinical practice, breast MRIs are often performed in conjunction with biopsy procedures. With the use of PG foam, the clinician can directly perform MR guided biopsies through the PG foam and around the surface coil without moving the patient. Also, anatomical features and positions can vary for each patient between scans depending on the placement of the breast. PG foam can be molded to the correct shape for a patient allowing for reliable and repeatable positioning prior to each scan.
PG foam also has applications in the general MR suite. In a typical MRI scan, the patient rests on a foam cushion on the exam bed and foam pieces may be used to secure the patient from moving during the scan. PG foam is lightweight and relatively inexpensive, so it can naturally replace other foams currently used in the MR suite, including the patient table bed. In addition, foam ear plugs are ubiquitous for patient safety inside the magnet bore and might be replaced with PG foam ear plugs, which may reduce susceptibility artifacts near the temporal lobe due to the air in the aural cavities.
Translating the PG foam from phantom applications to in vivo
imaging raises two issues concerning compressibility and air gaps. First, using the foam as padding can result in compression for certain applications, such as under the head or back or inside the ear cavity. The change in volume will cause a change in density and PG volume fraction, and thus a susceptibility change. Our theory and phantom results show that a 10% error in volume fraction produces susceptibility matching to ~1 ppm, which would suggest that at least 10% compression would be tolerable for human imaging. The density and softness of the underlying foams used for the manufacturing of PG foams can be independently varied, so PG foams of varying compressibility and feel can be tailored towards specific applications, depending on whether pressure padding is required or the level of compression expected. Second, unlike the initial phantom experiments, the foam padding may not conform itself perfectly to the imaged body part, even if the foam is pre-molded, and thus air gaps are possible, which may be a source of dipole-like B0
inhomogeneities. Solutions for simple ellipsoid patterns, such as spheres or cylinders have been published previously (2
) and more complicated patterns can be predicted using a Fourier based method to calculate field perturbations from a heterogeneous magnetic susceptibility distribution (31
). Because PG foam can be compressible and is relatively conforming, air gaps of less than a few millimeters are feasible, and our initial estimates of simple gap geometries suggest that the field homogeneities will be with a tolerable ~1 ppm.
In conclusion, we have described a tissue susceptibility matching pyrolytic graphite foam that will improve the homogeneity of the B0 static magnetic field within the patient. PG foam reduces local inhomogeneities due to field gradients by moving the susceptibility boundary away from the field of view or region of interest and it produces no MRI signal. It is inexpensive, lightweight, non-conductive, does not heat, and safe to use with patients. It is also compatible with embedded RF coils. Future work will include developing foams for use in specific applications, such as in breast MRI, cervical spine, other extremity imaging applications, and spectroscopy.