Phase-sensitive Imaging Research Articles

Split Mach–Zehnder interferometer for surface plasmon resonance based phase modulation

Split Mach–Zehnder interferometer is proposed for demonstration of phase modulation of p- and s- polarized beams in a surface plasmon resonance based phase sensitive interference imaging platform. Significant phase change has been observed in Al coated prism based SPR configuration for p-polarized beam, whereas practically no phase change occurs for s-polarized beam. Qualitative analysis of SPR modulated interference fringes also validates the observed effects. A proposed Split Mach–Zehnder set up is shown for simultaneous sensing of two different samples.

Highly phase-stable 200 kHz swept-source optical coherence tomography based on KTN electro-optic deflector

The rapid advance in swept-source optical coherence tomography (SS-OCT) technology has enabled exciting new applications in elastography, angiography, and vibrometry, where both high temporal resolution and phase stability are highly sought-after. In this paper, we present a 200 kHz SS-OCT system centered at 1321 nm by using an electro-optically tuned swept source. The proposed system's performance was fully characterized, and it possesses superior phase stability (0.0012% scanning variability and <1 ns timing jitter) that is promising for many phase-sensitive imaging applications. Biological experiments were demonstrated within ex vivo human tracheobronchial ciliated epithelium where both the ciliary motion and ciliary beat frequency were successfully extracted.

Magnetic resonance thermometry of flowing blood.

Blood temperature is a key determinant of tissue temperature and can be altered under normal physiological states, such as exercise, in diseases such as stroke or iatrogenically in therapies which modulate tissue temperature, such as therapeutic hypothermia. Currently available methods for the measurement of arterial and venous temperatures are invasive and, for small animal models, are impractical. Here, we present a methodology for the measurement of intravascular and tissue temperature by magnetic resonance imaging (MRI) using the lanthanide agent TmDOTMA- (DOTMA, tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid; Tm, thulium). The approach makes use of phase-sensitive imaging measurements, combined with spectrally selective excitation, to monitor the temperature-dependent shift in the resonance of proton nuclei associated with water and with methyl groups of TmDOTMA- . Measurements were first made in a flow phantom modelling diastolic blood flow in the mouse aorta or inferior vena cava (IVC) and imaged using 7-T preclinical MRI with a custom-built surface coil. Flowing and static fluid temperatures agreed to within 0.12°C for these experiments. Proof-of-concept experiments were also performed on three healthy adult mice, demonstrating temperature measurements in the aorta, IVC and kidney following a bolus injection of contrast agent. A small (0.7-1°C), but statistically significant, higher kidney temperature compared with the aorta (p=0.002-0.007) and IVC (p=0.003-0.03) was shown in all animals. These findings demonstrate the feasibility of the technique for in vivo applications and illustrate how the technique could be used to explore the relationship between blood and tissue temperature for a wide range of applications.

Detectability comparison between a high energy x-ray phase sensitive and mammography systems in imaging phantoms with varying glandular-adipose ratios

The objective of this study was to demonstrate the potential benefits of using high energy x-rays in comparison with the conventional mammography imaging systems for phase sensitive imaging of breast tissues with varying glandular-adipose ratios.This study employed two modular phantoms simulating the glandular (G) and adipose (A) breast tissue composition in 50 G–50 A and 70 G–30 A percentage densities. Each phantom had a thickness of 5 cm with a contrast detail test pattern embedded in the middle. For both phantoms, the phase contrast images were acquired using a micro-focus x-ray source operated at 120 kVp and 4.5 mAs, with a magnification factor (M) of 2.5 and a detector with a 50 µm pixel pitch. The mean glandular dose delivered to the 50 G–50 A and 70 G–30 A phantom sets were 1.33 and 1.3 mGy, respectively. A phase retrieval algorithm based on the phase attenuation duality that required only a single phase contrast image was applied. Conventional low energy mammography images were acquired using GE Senographe DS and Hologic Selenia systems utilizing their automatic exposure control (AEC) settings. In addition, the automatic contrast mode (CNT) was also used for the acquisition with the GE system. The AEC mode applied higher dose settings for the 70 G–30 A phantom set. As compared to the phase contrast images, the dose levels for the AEC mode acquired images were similar while the dose levels for the CNT mode were almost double.The observer study, contrast-to-noise ratio and figure of merit comparisons indicated a large improvement with the phase retrieved images in comparison to the AEC mode images acquired with the clinical systems for both density levels. As the glandular composition increased, the detectability of smaller discs decreased with the clinical systems, particularly with the GE system, even at higher dose settings. As compared to the CNT mode (double dose) images, the observer study also indicated that the phase retrieved images provided similar or improved detection for all disc sizes except for the disk diameters of 2 mm and 1 mm for the 50 G–50 A phantom and 3 mm and 0.5 mm for the 70 G–30 A phantom.This study demonstrated the potential of utilizing a high energy phase sensitive x-ray imaging system to improve lesion detection and reduce radiation dose when imaging breast tissues with varying glandular compositions.

Feasibility study of phase-sensitive imaging based on multiple reference optical coherence tomography

Multiple reference optical coherence tomography (MR-OCT) is a recently developed, low-cost and compact time-domain OCT solution for primary care and consumer level applications. A combination of a voice coil actuator and a partial mirror (PM) extends the scan range for imaging depths of approximately 1 mm in biological samples. Our previous research on MR-OCT is based only on intensity information obtained from the depth-resolved interference signal. In this Letter, we extract the phase information from the MR-OCT signal and, hence, provide an additional contrast modality. The phase sensitivity of the system is measured to be approximately 0.2 and 1.5 rad for the first and twelfth orders of reflection when using a mirror as the sample. This Letter describes first results of phase-sensitive data measured on a phantom obtained with MR-OCT. Data from a chick embryo chorioallantoic membrane (CAM) is used to demonstrate the feasibility of MR-OCT for in vivo phase-sensitive imaging.

Synchrotron-based phase-sensitive imaging ofleaves grown from magneto-primed seeds of soybean.

Experiments were conducted to study the effects of static magnetic fields (SMFs) on the venation network of soybean leaves using the synchrotron-based X-ray micro-imaging technique. The seeds of soybean (Glycine max, variety JS-335) were pretreated with different SMFs from 50 to 300 mT in steps of 50 mT for 1 h. The phase-contrast images obtained showed that, as the strength of the SMF increased, the area, width of the midrib, area of the midrib and minor vein of the middle leaflets of third trifoliate leaves also increased up to the SMF strength of 200 mT (1 h) and decreased thereafter. Quantification of the major conducting vein also showed the differences in the major and minor vein structures of the soybean leaves as compared with control leaves. Further, the phase-retrieval technique has been applied to make the segmentation process easy and to quantify the major and minor veins in the venation network. The width and area of midrib enhancement by pre-treatment with SMF implies an enhancement in the uptake of water, which in turn causes an increased rate of photosynthesis and stomatal conductance.

Phase-Sensitive Single-Pixel THz Imaging Using Intensity-Only Measurements

We present a submillimeter-wave phase-sensitive imaging system by using single-pixel, intensity-only measurements. We implement this unique modality by utilizing the photoelectric effect on a high-resistivity silicon wafer illuminated by visible light from a commercial liquid crystal display projector. Randomly distributed mask patterns ( $16\times 16$ pixel) are generated using two levels of light intensity projected on a high-resistivity silicon wafer. The spatial modulation of the amplitude and phase of the incident terahertz is achieved via the photoelectric effect, resulting in the random modulation of the object beams phase fronts accordingly. Subsequently, intensity-only measurements are collected by using a single-pixel sensor for each of the random mask patterns, and the “Phaselift” algorithm is applied to reconstruct the magnitude and phase of the $16\times 16$ pixel image. We also demonstrate that the phase image of the scene shows superior contrast to the intensity image and offers thickness discernibility of a paper object down to 190 $\mu$ m at 690 GHz.

BMIT facility at the Canadian Light Source: Advances in X-ray phase-sensitive imaging

The BioMedical Imaging and Therapy (BMIT) facility [1,2] located at the Canadian Light Source, provides synchrotron-specific imaging and radiation therapy capabilities. There are two separate beamlines used for experiments: the bending magnet (05B1-1) and the insertion device (05ID-2) beamline.The bending magnet beamline provides access to monochromatic beam spanning a spectral range of 15–40keV, and the beam is 240mm wide in the POE-2 experimental hutch. Users can also perform experiments with polychromatic (pink) beam.The insertion device beamline was officially opened for general user program in 2015. The source for the ID beamline is a multi-pole, superconducting 4.3T wiggler. The high field gives a critical energy over 20keV. The optics hutches prepare a beam that is 220mm wide in the last experimental hutch SOE-1. The monochromatic spectral range spans 25–150+keV. Several different X-ray detectors are available for both beamlines, with resolutions ranging from 2μm to 200μm.BMIT provides a number of imaging techniques including standard absorption X-ray imaging, K-edge subtraction imaging (KES), in-line phase contrast imaging (also known as propagation based imaging, PBI) and Diffraction Enhanced Imaging/Analyzer Based Imaging (DEI/ABI), all in either projection or CT mode. PBI and DEI/ABI are particularly important tools for BMIT users since these techniques enable visualization of soft tissue and allow for low dose imaging.

Correction of aberrations in the human eye using computational methods.

Phase-sensitive imaging and computational correction of patient-specific optical aberrations enable high-resolution imaging of the human retina to aid diagnosis and treatment of eye diseases.

Strategies to improve phase-stability of ultrafast swept source optical coherence tomography for single shot imaging of transient mechanical waves at 16 kHz frame rate.

We present single-shot phase-sensitive imaging of propagating mechanical waves within tissue, enabled by an ultrafast optical coherence tomography (OCT) system powered by a 1.628 MHz Fourier domain mode-locked (FDML) swept laser source. We propose a practical strategy for phase-sensitive measurement by comparing the phases between adjacent OCT B-scans, where the B-scan contains a number of A-scans equaling an integer number of FDML buffers. With this approach, we show that micro-strain fields can be mapped with ∼3.0 nm sensitivity at ∼16 000 fps. The system's capabilities are demonstrated on porcine cornea by imaging mechanical wave propagation launched by a pulsed UV laser beam, promising non-contact, real-time, and high-resolution optical coherence elastography.

Applications using high-Tc superconducting terahertz emitters.

Using recently-developed THz emitters constructed from single crystals of the high-Tc superconductor Bi2Sr2CaCu2O8+δ, we performed three prototype tests of the devices to demonstrate their unique characteristic properties for various practical applications. The first is a compact and simple transmission type of THz imaging system using a Stirling cryocooler. The second is a high-resolution Michelson interferometer used as a phase-sensitive reflection-type imaging system. The third is a system with precise temperature control to measure the liquid absorption coefficient. The detailed characteristics of these systems are discussed.

Low dose high energy x-ray in-line phase sensitive imaging prototype: Investigation of optimal geometric conditions and design parameters.

The objective of this study was to investigate the optimization of a high energy in-line phase sensitive x-ray imaging prototype under different geometric and operating conditions for mammography application. A phase retrieval algorithm based on phase attenuation duality (PAD) was applied to the phase contrast images acquired by the prototype. Imaging performance was investigated at four magnification values of 1.67, 2, 2.5 and 3 using an acrylic edge, an American College of Radiology (ACR) mammography phantom and contrast detail (CD) phantom with tube potentials of 100, 120 and 140 kVp. The ACR and CD images were acquired at the same mean glandular dose (MGD) of 1.29 mGy with a computed radiography (CR) detector of 43.75 μm pixel pitch at a fixed source to image distance (SID) of 170 cm. The x-ray tube focal spot size was kept constant as 7 μm while a 2.5 mm thick aluminum (Al) filter was used for beam hardening. The performance of phase contrast and phase retrieved images were compared with computer simulations based on the relative phase contrast factor (RPF) at high x-ray energies. The imaging results showed that the x-ray tube operated at 100 kVp under the magnification of 2.5 exhibits superior imaging performance which is in accordance to the computer simulations. As compared to the phase contrast images, the phase retrieved images of the ACR and CD phantoms demonstrated improved imaging contrast and target discrimination. We compared the CD phantom images acquired in conventional contact mode with and without the anti-scatter grid using the same prototype at 1.295 mGy and 2.59 mGy using 40 kVp, a 25 μm rhodium (Rh) filter. At the same radiation dose, the phase sensitive images provided improved detection capabilities for both the large and small discs, while compared to the double dose image acquired in conventional mode, the observer study also indicated that the phase sensitive images provided improved detection capabilities for the large discs. This study therefore validates the potential of using high energy phase contrast x-ray imaging to improve lesion detection and reduce radiation dose for clinical applications such as mammography.

Dose fractionation in synchrotron radiation x-ray phase micro-tomography

Phase sensitive x-ray imaging expands the applicability of standard attenuation based techniques by offering several orders of magnitude of increase in sensitivity. Due to the short wavelength, x-ray phase is not directly measurable, but has to be put in evidence by the use of phase contrast techniques. The phase can then be reconstructed from one or several phase contrast images. In this study, we consider synchrotron x-ray phase micro-computed tomography (μCT) based on free space propagation for heterogeneous and strongly absorbing objects. This technique generally relies on acquiring several scans of the sample at different detector distances. It is also generally believed that multi-distance phase μCT needs a higher dose input than single distance phase μCT. The purpose of this work is to study the impact of different means of dose fractionation on the reconstructed image quality. We define different acquistion schemes in multi-distance in-line phase μCT. Previously, the exposure time at each sample-to-detector distance was usually kept the same. Here, we let not only the number of distances vary but also the fraction of exposure time at each distance, the total exposure time being kept constant. Phase retrieval is performed with the mixed approach algorithm. The reconstructed μCT images are compared in terms of accuracy, precision and resolution. In addition, we also compare the result of dose fractionated multi distance phase μCT to single distance phase μCT using the same total radiation dose. In the multi-distance approach, we find that using different exposure times on each distance improves the image quality in the reconstructed image. Further, we show that, despite having the same total dose delivery, the multi distance imaging method gives better image quality than the single distance method, at the cost of an additional overhead from camera displacements and reference images. We show that by optimizing the acquistion parameters in terms of number of distances and exposure time at each distance, the resulting image quality can be improved. This means that for a desired image quality, a lower radiation dose can be used. This is important especially in high resolution imaging where the radiation dose used for imaging can be very large, potentially damaging the sample. Based on the acquired data, we define an optimal protocol for use in together with the heterogeneous object mixed approach.

Ultra-subwavelength phase-sensitive Fano-imaging of localized photonic modes

Photonic and plasmonic devices rely on nanoscale control of the local density of optical states (LDOS) in dielectric and metallic environments. The tremendous progress in designing and tailoring the electric LDOS of nano-resonators requires an investigation tool that is able to access the detailed features of the optical localized resonant modes with deep-subwavelength spatial resolution. This scenario has motivated the development of different nanoscale imaging techniques. Here, we prove that a technique involving the combination of scanning near-field optical microscopy with resonant scattering spectroscopy enables imaging the electric LDOS in nano-resonators with outstanding spatial resolution (λ/19) by means of a pure optical method based on light scattering. Using this technique, we investigate the properties of photonic crystal nanocavities, demonstrating that the resonant modes appear as characteristic Fano line shapes, which arise from interference. Therefore, by monitoring the spatial variation of the Fano line shape, we locally measure the phase modulation of the resonant modes without the need of external heterodyne detection. This novel, deep-subwavelength imaging method allows us to access both the intensity and the phase modulation of localized electric fields. Finally, this technique could be implemented on any type of platform, being particularly appealing for those based on non-optically active material, such as silicon, glass, polymers, or metals. A purely optical method for realizing ultra-subwavelength phase-sensitive imaging of localized photonic modes has been developed. The technique combines scanning near-field optical microscopy with resonance scattering spectroscopy and involves monitoring the spatial variation of the Fano resonance line shape in photonic-crystal nanocavities. Using the technique, Niccolo Caselli and co-workers in Italy, the USA, the UK, and the Netherlands realized a spatial resolution of λ/19 (where λ is the wavelength) for the electric local density of optical states (LDOS). The technique can be applied to resonators made from any kind of material and over a wide spectral range. Furthermore, it does not suffer from bleaching or tip-induced perturbation. This demonstration opens up new strategies for investigating the electric LDOS and phase distribution of modes in a wide range of nanophotonic and nanoplasmonic resonators.

Characterization of Silicon Solar Cells and Module Materials Using Terahertz Radiation

In this paper, we present our investigations of silicon solar cells and module structures by means of terahertz (THz) imaging and Fourier Transform Infrared (FTIR) spectroscopy. Several types of structures were investigated: anisotropically etched surfaces of monocrystalline silicon wafers, complete commercial solar cells, interconnections of the cells, encapsulation materials, such as polydimethylsiloxane (PDMS) and glass. We demonstrated, how etching time and depth variations of the pyramid-shaped texture influence far-IR local reflectance spectra of the samples; how phase-sensitive imaging and time-domain data can be processed for full 3D characterization of cell and module structures, including layer thicknesses; positions of the surfaces and defects. In addition, applicability of sub-THz radiation to the investigation of module structures including glass is discussed.

Holographic metalens for switchable focusing of surface plasmons.

Surface plasmons polaritons (SPPs) are light-like waves confined to the interface between a metal and a dielectric. Excitation and control of these modes requires components such as couplers and lenses. We present the design of a new lens based on holographic principles. The key feature is the ability to switchably control SPP focusing by changing either the incident wavelength or polarization. Using phase-sensitive near-field imaging of the surface plasmon wavefronts, we have observed their switchable focusing and steering as the wavelength or polarization is changed.

Enhancing the quantification of tissue sodium content by MRI: time-efficient sodium B1 mapping at clinical field strengths.

Tissue sodium content (TSC) is a sensitive measure of pathological changes and can be detected non-invasively by MRI. For the absolute quantification of TSC, B1 inhomogeneities must be corrected, which is not well established beyond research applications. An in-depth analysis of B1 mapping methods which are suitable for application in TSC quantification is presented. On the basis of these results, a method for simultaneous B1 mapping and imaging is proposed in order to enhance accuracy and to reduce measurement time at clinical field strengths. The B1 mapping techniques used were phase-sensitive (PS), Bloch-Siegert shift (BSS), double-angle (DAM) and actual flip-angle imaging (AFI) methods. Experimental and theoretical comparisons demonstrated that the PS technique yields the most accurate field profiles and exhibits the highest signal-to-noise ratio (SNR). Simultaneous B1 mapping and imaging was performed for the PS method, employing both degrees of freedom of the MR signal: the B1 field is encoded into signal phase and the amplitude provides the concentration information. In comparison with the more established DAM, a 13% higher SNR was obtained and field effects could be corrected more accurately without the need for additional measurement time. The protocol developed was applied to measure TSC in the healthy human head at an isotropic resolution of 4 mm. TSC was determined to be 35 ± 1 mM in white matter and 134 ± 3 mM in vitreous humor. By employing the proposed simultaneous characterization of the B1 field and acquisition of the spin density-weighted sodium signal, the accuracy of the non-invasive measurement of TSC is enhanced and the measurement time is reduced. This should allow (23)Na MRI to be better incorporated into clinical studies and routine.

Age, Gender and Normalization Covariates for Spinal Cord Gray Matter and Total Cross-Sectional Areas at Cervical and Thoracic Levels: A 2D Phase Sensitive Inversion Recovery Imaging Study

The source of inter-subject variability and the influence of age and gender on morphometric characteristics of the spinal cord, such as the total cross-sectional area (TCA), the gray matter (GM) and white matter (WM) areas, currently remain under investigation. Understanding the effect of covariates such as age, gender, brain volumes, and skull- and vertebra-derived metrics on cervical and thoracic spinal cord TCA and GM areas in healthy subjects would be fundamental for exploring compartment specific changes in neurological diseases affecting the spinal cord. Using Magnetic Resonance Imaging at 3T we investigated 32 healthy subjects using a 2D phase sensitive inversion recovery sequence and we measured TCA, GM and WM areas at 4 cervical and thoracic levels of the spinal cord. We assessed age and gender relationships of cord measures and explored associations between cord measures and a) brain volumes and b) skull- and vertebra-derived metrics. Age and gender had a significant effect on TCA, WM and GM areas (with women and elderly having smaller values than men and younger people respectively), but not on the GM area/TCA ratio. The total intracranial volume and C3 vertebra dimensions showed the highest correlations with cord measures. When used in multi-regression models, they reduced cord areas group variability by approximately a third. Age and gender influences on cord measures and normalization strategies here presented might be of use in the study of compartment specific changes in various neurological diseases affecting the spinal cord.

High incidence of asymptomatic cerebral microbleeds in patients with hemorrhagic onset-type moyamoya disease: a phase-sensitive MRI study and meta-analysis

Moyamoya disease is a relatively rare cerebrovascular occlusive disorder. Several studies have reported cerebral microbleeds (CMBs) in moyamoya disease patients using T2*-weighted imaging (T2*WI) and/or susceptibility-weighted imaging (SWI). To investigate the incidence, distribution patterns, and influencing factors of asymptomatic CMBs in patients with moyamoya disease. Phase-sensitive imaging (PSI) was used to investigate 27 consecutive moyamoya disease patients with a 3-T magnetic resonance imaging system, then a meta-analysis of 245 patients (asymptomatic moyamoya disease, n = 23; ischemic moyamoya disease, n = 161; hemorrhagic moyamoya disease, n = 61) from four previous individual studies and our PSI study was performed. The meta-analysis was performed in accordance with the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines. Based on the clinical and radiological data, we divided the studies into different model groups to calculate the incidence of CMBs and discuss the distribution patterns of CMBs. Thirty-five asymptomatic CMBs were demonstrated in 14 moyamoya disease patients (51.9%) in our PSI study. Of these, 45.7% were located in the periventricular white matter. In the meta-analysis, the pooled incidence of asymptomatic CMBs in moyamoya disease was 46% (95% confidence interval [CI], 28.2-63.8%) on SWI or PSI and 29.6% (95% CI, 17.4-41.7%) on T2*WI. Statistical analysis showed that PSI or SWI offered better detection of CMBs in moyamoya disease than T2*WI, and 3-T T2*WI offered better detection than 1.5-T T2*WI. Furthermore, hemorrhagic onset-type moyamoya disease correlated with a high incidence of asymptomatic CMBs. PSI or SWI can detect CMBs better than T2*WI, and 3-T T2*WI. Hemorrhagic onset-type moyamoya disease seems to correlate with a high incidence of asymptomatic CMBs. The meta-analysis indicates that asymptomatic CMBs may be an important factor for hemorrhagic stroke risk. Long-term evaluation of CMBs using PSI or SWI may contribute to the management of moyamoya disease.

3D high spectral and spatial resolution imaging of ex vivo mouse brain.

Widely used MRI methods show brain morphology both in vivo and ex vivo at very high resolution. Many of these methods (e.g., T2*-weighted imaging, phase-sensitive imaging, or susceptibility-weighted imaging) are sensitive to local magnetic susceptibility gradients produced by subtle variations in tissue composition. However, the spectral resolution of commonly used methods is limited to maintain reasonable run-time combined with very high spatial resolution. Here, the authors report on data acquisition at increased spectral resolution, with 3-dimensional high spectral and spatial resolution MRI, in order to analyze subtle variations in water proton resonance frequency and lineshape that reflect local anatomy. The resulting information compliments previous studies based on T2* and resonance frequency. The proton free induction decay was sampled at high resolution and Fourier transformed to produce a high-resolution water spectrum for each image voxel in a 3D volume. Data were acquired using a multigradient echo pulse sequence (i.e., echo-planar spectroscopic imaging) with a spatial resolution of 50 × 50 × 70 μm(3) and spectral resolution of 3.5 Hz. Data were analyzed in the spectral domain, and images were produced from the various Fourier components of the water resonance. This allowed precise measurement of local variations in water resonance frequency and lineshape, at the expense of significantly increased run time (16-24 h). High contrast T2*-weighted images were produced from the peak of the water resonance (peak height image), revealing a high degree of anatomical detail, specifically in the hippocampus and cerebellum. In images produced from Fourier components of the water resonance at -7.0 Hz from the peak, the contrast between deep white matter tracts and the surrounding tissue is the reverse of the contrast in water peak height images. This indicates the presence of a shoulder in the water resonance that is not present at +7.0 Hz and may be specific to white matter anatomy. Moreover, a frequency shift of 6.76 ± 0.55 Hz was measured between the molecular and granular layers of the cerebellum. This shift is demonstrated in corresponding spectra; water peaks from voxels in the molecular and granular layers are consistently 2 bins apart (7.0 Hz, as dictated by the spectral resolution) from one another. High spectral and spatial resolution MR imaging has the potential to accurately measure the changes in the water resonance in small voxels. This information can guide optimization and interpretation of more commonly used, more rapid imaging methods that depend on image contrast produced by local susceptibility gradients. In addition, with improved sampling methods, high spectral and spatial resolution data could be acquired in reasonable run times, and used for in vivo scans to increase sensitivity to variations in local susceptibility.