Blaine A. Chronik

A cadaveric study of the anterolateral ligament: re-introducing the lateral capsular ligament.

PurposeThe purpose of this study was to verify and characterize the anatomical properties of the anterolateral capsule, with the aim of establishing a more accurate anatomical description of the anterolateral ligament (ALL). Furthermore, microscopic analysis of the tissue was performed to determine whether the ALL can morphologically be classified as ligamentous tissue, as well as reveal any potential functional characteristics.MethodsThree different modalities were used to validate the existence of the ALL: magnetic resonance imagining (MRI), anatomical dissection, and histological analysis. Ten fresh-frozen cadaveric knee specimens underwent MRI, followed by anatomical dissection which allowed comparison of MRI to gross anatomy. Nine additional fresh-frozen cadaveric knees (19 total) were dissected for a further anatomical description. Four specimens underwent H&E staining to look at morphological characteristics, and one specimen was analysed using immunohistochemistry to locate peripheral nervous innervation.ResultsThe ALL was found in all ten knees undergoing MRI and all nineteen knees undergoing anatomical dissection, with MRI being able to predict its corresponding anatomical dissection. The ALL was found to have bone-to-bone attachment points from the lateral femoral epicondyle to the lateral tibia, in addition to a prominent meniscal attachment. Histological sectioning showed ALL morphology to be characteristic of ligamentous tissue, having dense, regularly organized collagenous bundles. Immunohistochemistry revealed a large network of peripheral nervous innervation, indicating a potential proprioceptive role.ConclusionFrom this study, the ALL is an independent structure in the anterolateral compartment of the knee and may serve a proprioceptive role in knee mechanics.

Design and fabrication of a three-axis edge ROU head and neck gradient coil

The design, fabrication, and testing of a complete three‐axis gradient coil capable of imaging the human neck is described. The analytic method of constrained current minimum inductance (CCMI) was used to position the uniform region of the gradient coil adjacent to and extending beyond the physical edge of the coil. The average gradient efficiency of the three balanced axes is 0.37 mT/m/A and the average inductance is 827 μH. With maximum amplifier current of 200A and receive signal sweep width of ±125 kHz, the average minimum FOV using this gradient set is 7.9 cm. The completed coil has an inner diameter of 32 cm, an outer diameter of 42 cm, and a length (including cabling connections) of 80 cm. The entire coil was built in‐house. The structure is actively water cooled. Heating measurements were made to characterize the thermal response of the coil under various operating conditions and it was determined that a continuous current of 100A could be passed through all three axes simultaneously without increasing the internal coil temperature by more than 23°C. Eddy current measurements were made for all axes. With digital compensation, the gradient eddy current components could be adequately compensated. A large Bo eddy current field is produced by the Gz axis that could be corrected through the use of an auxiliary Bo compensation coil. Preliminary imaging results are shown in both phantoms and human subjects. Magn Reson Med 44:955–963, 2000.

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.

Delta relaxation enhanced MR: improving activation-specificity of molecular probes through R1 dispersion imaging.

MR molecular imaging enables high‐resolution, in vivo study of molecular processes frequently utilizing gadolinium‐based probes that specifically bind to a particular biological molecule or tissue. While some MR probes are inactive when unbound and produce enhancement only after binding, the majority are less specific and cause enhancement in either state. Accumulation processes are then required to increase probe concentration in regions of the target molecule/tissue. Herein, a method is described for creating specificity for traditionally nonspecific probes. This method utilizes MR field‐cycling methods to produce MRI contrast related to the dependence of R1 upon magnetic field. It is shown that the partial derivative of R1 with respect to magnetic field strength, R1′, can be used as an unambiguous measure of probe binding. T1‐weighted images and R1′ images were produced for samples of albumin and buffer both enhanced with the albumin‐binding agent Vasovist. For T1 images, samples with low concentrations of Vasovist in an albumin solution could not be differentiated from samples with higher concentrations of Vasovist in buffer. Conversely, the R1′ images showed high specificity to albumin. Albumin samples with a 10‐μM concentration of Vasovist were enhanced over buffer samples containing up to 16 times more Vasovist. Magn Reson Med, 2009.

Design and fabrication of a three-axis multilayer gradient coil for magnetic resonance microscopy of mice.

There is great interest in the non-destructive capabilities of magnetic resonance microscopy for studying murine models of both disease and normal function; however, these studies place extreme demands on the MR hardware, most notably the gradient field system. We designed, using constrained current minimum inductance methods. and fabricated a complete, unshielded three-axis gradient coil set that utilizes interleaved, multilayer axes to achieve maximum gradient strengths of over 2000 mT m−1 in rise times of less than 50 μs with an inner coil diameter of 5 cm. The coil was wire-wound using a rectangular wire that minimizes the deposited power for a given gradient efficiency. Water cooling was also incorporated into the coil to assist in thermal management. The duty cycle for the most extreme cases of single shot echo planar imaging (EPI) is limited by the thermal response and expressions for maximum rates of image collection are given for burst and continuous modes of operation. The final coil is capable of the collection of single shot EPI images with 6 mm field of view and 94 μm isotropic voxels at imaging rates exceeding 50 s−1.

A system for MRI‐guided transperineal delivery of needles to the prostate for focal therapy

PURPOSE To demonstrate the capabilities of a new magnetic resonance imaging (MRI)-guided system for delivering needles to the prostate for focal therapy. Included is a presentation of the design of the system and its user interface, evaluation of MR-compatibility, and quantitative evaluation of guidance accuracy and repeatability within the bore of a clinical MRI scanner. METHODS The use of MRI for visualization of tumors, intraoperative visualization of interventional tools, and thermometry for controlled ablation of lesions is becoming increasingly prevalent. In this work, the authors present a prototype system for guiding needles to prostate tumors within the bore of an MRI scanner for use in focal laser thermal ablation of prostate tumors. The system consists of a manually actuated trajectory alignment device that allows a physician to precisely align a set of needle guides with an intended target in the prostate within the bore of a clinical closed-bore MRI scanner. Needle insertion is then performed transperineally, with the patient in the bore of the MRI, and custom software provides monitoring of thermal ablative procedures. RESULTS The system is shown to have a minimal effect on image distortion, and only a 6% decrease in image signal-to-noise ratio. Through needle insertion tests in tissue-mimicking phantoms, the systems potential for reliably guiding needles to intra-MR targets within 2.64 mm has been demonstrated. Use of the system to deliver focal laser ablation therapy to two patients showed that it can be used to deliver needles with minimal disruption of workflow, and in less time than when insertions are performed freehand or with a fixed grid template. CONCLUSIONS A system for delivering needles to a patients prostate for focal therapy within the bore of an MRI scanner has been developed. Results from needle insertion tests in phantoms suggest that the system has the potential to provide accurate delivery of focal therapy to prostate tumors of the smallest clinically significant size. Initial tests in two patients showed that needle deflection was larger than in phantoms, but methods of manually compensating for this effect were employed and needles were delivered to treatment sites with sufficient accuracy to deliver effective treatment. In addition, the treatment was delivered in less time than with a fixed grid template or freehand insertions. Despite this success, methods of reducing needle deflection are needed in order to fully utilize the potential of this system, and further reduce total procedure time.

Simple linear formulation for magnetostimulation specific to MRI gradient coils

A simple linear formulation for magnetostimulation thresholds specific to MRI gradient coils is derived based on established hyperbolic electrostimulation strength vs. duration relations. Thresholds are derived in terms of the gradient excursion required to cause stimulation, and it is demonstrated that the threshold curve is a linear function of the gradient switching time. A parameter β is introduced as being fundamental in the evaluation of gradient coil stimulation. β is a map of the induced electric field per unit gradient slew rate, and can be calculated directly from the gradient coil wire pattern. Consideration of β alone is sufficient to compare stimulation thresholds between different gradient coil designs, as well as to evaluate the expected dependency of stimulation threshold on position within the gradient coil. The linear gradient threshold curve is characterized by two parameters: SRmin and ΔGmin. SRmin is the slope of the threshold curve and represents the minimum slew rate required to cause stimulation in the limit of infinite gradient strength. ΔGmin is the vertical axis intercept of the curve and represents the minimum gradient excursion required to cause stimulation in the limit of infinite slew rate. Both SRmin and ΔGmin are functions of both β and the standard tissue parameters Er (rheobase) and τc (chronaxie time). The ease with which both the gradient system performance and the stimulation thresholds can be plotted on the same axes is noted and is used to introduce the concept of a piece‐wise linear operational limit curve for a gradient system. Magn Reson Med 45:916–919, 2001.

Consideration of magnetically-induced and conservative electric fields within a loaded gradient coil

We present a method to calculate the electric (E)‐fields within and surrounding a human body in a gradient coil, including E‐fields induced by the changing magnetic fields and “conservative” E‐fields originating with the scalar electrical potential in the coil windings. In agreement with previous numerical calculations, it is shown that magnetically‐induced E‐fields within the human body show no real concentration near the surface of the body, where nerve stimulation most often occurs. Both the magnetically‐induced and conservative E‐fields are shown to be considerably stronger just outside the human body than inside it, and under some circumstances the conservative E‐fields just outside the body can be much larger than the magnetically‐induced E‐fields there. The order of gradient winding and the presence of conductive RF shield can greatly affect the conservative E‐field distribution in these cases. Though the E‐fields against the outer surface of the body are not commonly considered, understanding gradient E‐fields may be important for reasons other than peripheral nerve stimulation (PNS), such as potential interaction with electrical equipment. Magn Reson Med, 2006.

The discrepancy between human peripheral nerve chronaxie times as measured using magnetic and electric field stimuli: the relevance to MRI gradient coil safety.

Peripheral nerve stimulation (PNS) resulting from electric fields induced from the rapidly changing magnetic fields of gradient coils is a concern in MRI. Nerves exposed to either electric fields or changing magnetic fields would be expected to display consistent threshold characteristics, motivating the direct application of electric field exposure criteria from the literature to guide the development of gradient magnetic field exposure criteria for MRI. The consistency of electric and magnetic field exposures was tested by comparing chronaxie times for electric and magnetic PNS curves for 22 healthy human subjects. Electric and magnetic stimulation thresholds were measured for exposure of the forearm using both surface electrodes and a figure-eight magnetic coil, respectively. The average chronaxie times for the electric and magnetic field conditions were 109 +/- 11 micros and 651 +/- 53 micros (+/-SE), respectively. We do not propose that these results call into question the basic mechanism, namely that rapidly switched gradient magnetic fields induce electric fields in human tissues, resulting in PNS. However, this result does motivate us to suggest that special care must be taken when using electric field exposure data from the literature to set gradient coil PNS safety standards in MRI.