Magnetic Resonance Imaging

Magnetic field effects on the vestibular system: calculation of the pressure on the cupula due to ionic current-induced Lorentz force

by Omar Mian

Antunes A, Glover PM, Li Y, Mian OS, Day BL
Physics in Medicine and Biology (In Press)

Large static magnetic fields may be employed in magnetic resonance imaging (MRI). At high magnetic field strengths... more Large static magnetic fields may be employed in magnetic resonance imaging (MRI). At high magnetic field strengths (usually from about 3 tesla and above) it is possible for humans to perceive a number of effects. One such effect is mild vertigo. Recently, Roberts et al (Current Biology 21:1635-1640 2011) proposed a Lorentz-force mechanism resulting from the ionic currents occurring naturally in the endolymph of the vestibular system. In the present work a more detailed calculation of the forces and resulting pressures in the vestibular system is carried out using a numerical model. Firstly, realistic 3D finite element conductivity and fluid maps of the utricle and a single semi-circular canal containing the current sources (dark cells) and sinks (hair cells) of the utricle and ampulla were constructed. Secondly, the electrical current densities in the fluid are calculated. Thirdly, the developed Lorentz force is used directly in the Navier-Stokes equation and the trans-cupular pressure is computed. Since the driving force field is relatively large in comparison with the advective acceleration, we demonstrate that it is possible to perform an approximation in the Navier-Stokes equations that reduces the problem to solving a simpler Poisson equation. This simplification allows rapid and easy calculation for many different directions of applied magnetic field. At 7 tesla a maximum cupula pressure difference of 1.6 mPa was calculated for the combined ampullar (0.7 uA) and utricular (3.31 uA) distributed current sources, assuming a hair-cell resting current of 100 pA per unit. These pressure values are up to an order of magnitude lower than those proposed by Roberts et al using a simplistic model and calculation, and are in good agreement with the estimated pressure values for nystagmus velocities in caloric experiments. This modeling work supports the hypothesis that the Lorentz force mechanism is a significant contributor to the perception of magnetic field induced vertigo.

The relationship between patellofemoral and tibiofemoral morphology and gait biomechanics following arthroscopic partial medial meniscectomy

by Alasdair Dempsey

Dempsey AR*, Wang Y*, Thorlund JB, Mills PM, Wrigley T, Bennell K, Metcalf B, Hanna F, Cicuttini FM, Lloyd DG. The relationship between patellofemoral and tibiofemoral morphology and gait biomechanics following arthroscopic partial medial meniscectomy Knee Surgery, Sports Traumatology, Arthroscopy (Accepted 22 May 2012) (Joint First Author)

Purpose: To examine the relationship between tibiofemoral and patellofemoral joint articular cartilage and subchondral... more Purpose: To examine the relationship between tibiofemoral and patellofemoral joint articular cartilage and subchondral bone in the medial and gait biomechanics following partial medial meniscectomy.
Methods: For this cross-sectional study, 122 patients aged 30-55 years, without evidence of knee osteoarthritis at arthroscopic partial medial meniscectomy, underwent gait analysis and MRI on the operated knee once for each sub-cohort of 3 months, 2 years, or 4 years post-surgery. Cartilage volume, cartilage defects, and bone size were assessed from the MRI using validated methods.  The 1st peak in the knee adduction moment, knee adduction moment impulse, 1st peak in the knee flexion moment, knee extension range of motion, and the heel strike transient from the vertical ground reaction force trace were identified from the gait data.
Results: Increased knee stance phase range of motion was associated with decreased patella cartilage volume (B = -17.9 (95% CI-35.4, -0.4) p =0.045) while knee adduction moment impulse was associated with increased medial tibial plateau area (B = 7.7 (95% 0.9, 13.3) p = 0.025).  A number of other variables approached significance.
Conclusions: Knee joint biomechanics exhibited by persons who had undergone arthroscopic partial meniscectomy gait may go some way to explaining the morphological degeneration observed at the patellofemoral and tibiofemoral compartments of the knee as patients progress from surgery.
Level of Evidence: III

Personal exposure to static and time-varying magnetic fields during MRI system test procedures

by Frank de Vocht

J Magn Reson Imaging. 2009 Nov;30(5):1223-8.

PURPOSE: To assess personal time-weighted average (TWA) static magnetic field and time-varying magnetic field exposure... more PURPOSE: To assess personal time-weighted average (TWA) static magnetic field and time-varying magnetic field exposure for several system testing tasks routinely conducted by engineers at an MRI manufacturing plant. MATERIALS AND METHODS: Personal exposure was measured using a personal dosimeter that measured B and dB/dt in all three orthogonal directions with a 1 Hz frequency. TWA exposure was calculated and random effects and linear mixed effects models were used to assess exposure levels and variability. RESULTS: Whereas full-body peak (2T) and TWA (200 mT/8 h) exposure limits for the static magnetic field originally proposed by ICNIRP were not exceeded, time-varying magnetic field peaks (dB/dt) did exceed the proposed threshold value (767.9 mT/s) suggested by IEEE on several occasions at the 3.0 Tesla (T) whole-body cylindrical system. CONCLUSION: TWA exposure levels are well below the proposed limit values, but peak exposure limits are exceeded during certain procedures performed on 1.0T to 3.0T systems.

MRI compatibility of robot actuation techniques--a comparative study

by Gregory Fischer

Sliding mode Control of piezoelectric valve regulated pneumatic actuator for MRI-compatible robotic intervention

by Gregory Fischer

Computed tomography and magnetic resonance imaging studies of Latimeria chalumnae

by Richard Cloutier

Authors: Schultze, H.-P. & R. Cloutier
Year: 1991
Reference: Environmental Biology of Fishes, 32: 159-181.

Recent radiologic imaging techniques (CT[Computed Tomography] and MRI[Magnetic Resonance Imaging]) were used to... more Recent radiologic imaging techniques (CT[Computed Tomography] and MRI[Magnetic Resonance Imaging]) were used to investigate the cranial anatomy of the coelacanth Latimeria chalumnae. The non-invasive CT and MRI techniques were performed successfully on a 1.45 m female specimen. This specimen had been frozen a year earlier for future research; the CT was conducted on the frozen animal, whereas the MRI method was performed immediately after thawing. The CT technique provides information about differential density of the organism (especially informative with respect to hard tissues, bone and cartilage), whereas three different types of MRI (proton resonance T,, T, and ‘flash’) distinguish cartilage, muscles, and different connective tissues. A total of 381 CT cross sections (2 mm thick with 1 mm of overlap) through the head region were used in a computerized three-dimensional reconstruction program to address questions concerning cranial morphology. The results obtained from these radiologic imaging techniques confirmed most of the basic anatomy known from traditional dissections. However, the morphology of complex structures, such as the cartilaginous processes of the neurocranium, and the integration of the branchial arches and palate can only now be described more accurately

Family history of alcohol dependence and gray matter abnormalities in non-alcoholic adults

by Zsuzsika Sjoerds

published in 'World Journal of Biological Psychiatry', 2012

Objectives. Alcohol-use disorders in adolescents are associated with gray matter (GM) abnormalities suggesting... more Objectives. Alcohol-use disorders in adolescents are associated with gray matter (GM) abnormalities suggesting neurotoxicity by alcohol. However, recently similar GM abnormalities were found in non-drinking children with a family history (FH) of alcohol dependence (AD). The question thus rises whether these abnormalities represent a transient delay in brain maturation or a persistent risk factor for developing neuropsychiatric disorders, rather than a (neurotoxic) consequence of AD. This study investigated whether a FH of AD in non-drinking adults is associated with abnormal GM-volumes similar to those observed in drinking and non-drinking adolescents with a FH of AD. Methods. GM-images were analyzed using Voxel-Based Morphometry in non-alcoholics with (FH+; N = 36) and without (FH-; N = 107) familial AD. Additionally we controlled for possible confounders: diagnosis of depression/anxiety, childhood trauma and familial depression/anxiety. Results. Smaller GM-volumes were shown in the right parahippocampal gyrus in FH+ compared with FH-. Results were unaffected by confounders. Conclusions. We demonstrated an effect of familial AD in non-alcoholic adults on GM volume in the parahippocampal gyrus, similar to drinking and non-drinking FH+ adolescents. These findings suggest that GM abnormalities in the parahippocampal gyrus represent a persistent biological susceptibility for AD or related psychopathology and not neurotoxicity of alcohol or delayed brain maturation.

Spatially resolved pore size distributions by NMR

by Beau Webber

J.H. Strange and J.B.W. Webber. Meas. Sci. Technol. 8, No 5, 555-561, May 1997 .

We have developed a novel method of determining median pore size and pore size distributions as a function of spatial... more We have developed a novel method of determining median pore size and pore size distributions as a function of spatial position inside a porous sample.
Pore sizes have been measured with one-, two- and three-dimensional spatial resolutions, using nuclear magnetic resonance (NMR) cryoporometry in conjunction with magnetic resonance imaging techniques. NMR cryoporometry is a method of measuring pore sizes and pore size distributions in the range of less than 40 °A to over 2000 °A pore diameter, by the technique of freezing a liquid in the pores and measuring the melting temperature by NMR. Since the melting point is depressed for crystals of small size, the melting point depression gives a measurement of pore size.

Multidimensionally resolved pore size distributions

by Beau Webber

J.H. Strange and J.B.W. Webber. Appl. Magn. Reson. 12, 231-245, 1997. DOI: 10.1007/BF03162190

A novel method of determining median pore size and pore size distributions asa function of spatial position inside a... more A novel method of determining median pore size and pore size distributions asa function of spatial position inside a porous sample is described. Pore sizes have been measured with 1-, 2- and 3-dimensional spatial resolution, using NMR cryoporometry in conjunction with magnetic resonance imaging techniques. The method is suitable for pore diameters in the range of 30 A to over 2000 A pore diameter, and is based on the technique of freezing a liquid in the pores and measuring the melting temperature by nuclear magnetie resonance. Since the melting point is depressed for crystals of small size, the melting point depression gives a measurement of pore size.

Propagation of Probabilistic Tractography of the Optic Radiation for Neuronavigation in Epilepsy Surgery

by Pankaj Daga

Published in ISMRM 2011.

Near real time brain shift estimation for interventional MRI suite

by Pankaj Daga

Current state of the art neurosurgical systems do not account for brain shift that occurs during a typical resection... more Current state of the art neurosurgical systems do not account for brain shift that occurs during a typical resection procedure when aligning the pre-operative and intra-operative images. We propose a near real time registration scheme to align pre-operative anatomical MRI datasets to intra-operative MRI datasets that is suitable for use in a neurosurgical setting. The global registration is performed using a block-matching algorithm combined with a robust transformation estimator that is able to cope with resected brain regions. The local registration is done using a re-factored version of the free form deformation algorithm. The registration framework is optimized for use on graphics processing units and was applied to pre and intra-operative datasets and achieved average computation time of under a minute. We assessed the accuracy of our algorithm qualitatively and quantitatively and our results show improved brain shift estimation. This work could be of significant utility in image-guided interventions and facilitate effective surgical treatments.

Improved Neuronavigation through Integration of Intraoperative Anatomical and Diffusion Images in an Interventional MRI Suite

by Pankaj Daga

Integration of information from complementary imaging modalities in medical image registration schemes potentially... more Integration of information from complementary imaging modalities in medical image registration schemes potentially improves the registration accuracy. MRI is now being used for guidance of various neurosurgical procedures like anterior temporal lobe resection in patients with refractory temporal lobe epilepsy. Accurate localisation of critical white matter tracts, such as the optic radiation, during neurosurgery is critical in ensuring good patient outcome. Current commercial interventional MR scanners are capable of performing anatomical and diffusion weighted imaging. We propose a near real-time multivariate registration scheme that uses both anatomical and diffusion images from the pre and intraoperative imaging sessions. The registration framework is optimized for use on graphical processing units and we perform a full multivariate non-rigid registration in under three minutes making the proposed framework suitable for use under the stringent time constraints of neurosurgical procedures. We assess the accuracy of our algorithm using a numerical phantom and demonstrate accurate localisation of the optic radiation in clinical datasets. This work could be of significant utility in image guided interventions and facilitate effective surgical treatments.

Integrating structural and diffusion MR information for optic radiation localisation in focal epilepsy patients

by Pankaj Daga

Current state of the art neurosurgical systems do not exploit the complementary information provided by structural and... more Current state of the art neurosurgical systems do not exploit the complementary information provided by structural and diffusion MRI when aligning pre-operative and intra-operative images. We propose a multivariate registration scheme where structural and fractional anisotropy data are combined in a single similarity measure. We formulate the normalised mutual information expression for the multichannel scheme and compute its analytical derivative. The method was validated using both a numerical phantom and clinical data using pre and post-operative images from patients who had undergone surgery for treatment of refractory focal epilepsy and shows correlation between visual field deficit and predicted damage to the optic radiation. This work could be of significant utility in image-guided interventions and facilitate effective surgical treatments.

MRI Image Overlay: Application to Arthrography Needle Insertion

by Gregory Fischer

Fischer G.S., Deguet A., Csoma C., Taylor R.H., Fayad L., Carrino J.A., Zinreich S.J., Fichtinger G. MRI Image Overlay: Application to Arthrography Needle Insertion. Comput Aided Surg. 2007 Jan;12(1):2-14. PMID: 17364654.

Magnetic Resonance Imaging (MRI) offers great potential for planning, guiding, monitoring and controlling... more Magnetic Resonance Imaging (MRI) offers great potential for planning, guiding, monitoring and controlling interventions. MR arthrography (MRAr) is the imaging gold standard for assessing small ligament and fibrocartilage injury in joints. In contemporary practice, MRAr consists of two consecutive sessions: (1) an interventional session where a needle is driven to the joint space and MR contrast is injected under fluoroscopy or CT guidance; and (2) a diagnostic MRI imaging session to visualize the distribution of contrast inside the joint space and evaluate the condition of the joint. Our approach to MRAr is to eliminate the separate radiologically guided needle insertion and contrast injection procedure by performing those tasks on conventional high-field closed MRI scanners. We propose a 2D augmented reality image overlay device to guide needle insertion procedures. This approach makes diagnostic high-field magnets available for interventions without a complex and expensive engineering entourage. In preclinical trials, needle insertions have been performed in the joints of porcine and human cadavers using MR image overlay guidance; in all cases, insertions successfully reached the joint space on the first attempt.

MRI-compatible pneumatic robot for transperineal prostate needle placement

by Gregory Fischer

Fischer GS, Iordachita I, Csoma C, Tokuda J, DiMaio SP, Tempany CM, Hata N, Fichtinger G, MRI-Compatible Pneumatic Robot for Transperineal Prostate Needle Placement, IEEE / ASME Transactions on Mechatronics – Focused section on MRI Compatible Mechatronic Systems, Vol 13, No 3, pp 295-305, June 2008.

Magnetic resonance imaging (MRI) can provide high-quality 3-D visualization of prostate and surrounding tissue, thus... more Magnetic resonance imaging (MRI) can provide high-quality 3-D visualization of prostate and surrounding tissue, thus granting potential to be a superior medical imaging modality for guiding and monitoring prostatic interventions. However, the benefits cannot be readily harnessed for interventional procedures due to difficulties that surround the use of high-field (1.5T or greater) MRI. The inability to use conventional mechatronics and the confined physical space makes it extremely challenging to access the patient. We have designed a robotic assistant system that overcomes these difficulties and promises safe and reliable intraprostatic needle placement inside closed high-field MRI scanners. MRI compatibility of the robot has been evaluated under 3T MRI using standard prostate imaging sequences and average SNR loss is limited to 5%. Needle alignment accuracy of the robot under servo pneumatic control is better than 0.94 mm rms per axis. The complete system workflow has been evaluated in phantom studies with accurate visualization and targeting of five out of five 1 cm targets. The paper explains the robot mechanism and controller design, the system integration, and presents results of preliminary evaluation of the system.

Closed-Loop Actuated Surgical System Utilizing Real-Time In-Situ MRI Guidance

by Gregory Fischer

G. Cole, K. Harrington, H. Su, A. Camilo, J. Pilitsis, G. S. Fischer, “Closed-Loop Actuated Surgical System Utilizing In-Situ Real-Time MRI Guidance”, 12th International Symposium on Experimental Robotics (ISER2010), New Delhi & Agra, India, 2010

High-field MRI-Compatible needle placement robots for prostate interventions: pneumatic and piezoelectric approaches

by Gregory Fischer

Patellofemoral and tibiofemoral articular cartilage and subchondral bone health following arthroscopic partial medial meniscectomy

by Alasdair Dempsey

Dempsey AR *, Wang Y*, Lloyd DG, Mills PM, Wrigley T, Bennell K, Metcalf B, Hanna F, Cicuttini FM

* Joint First Author

Published ahead of print 24/9/2011 DOI 10.1007/s00167-011-1681-z

Accepted by 12/9/2011 Knee Surgury, Sports Traumatology, Arthroscopy,

Purpose: To examine articular cartilage and subchondral bone changes in tibiofemoral and patellofemoral joints... more Purpose: To examine articular cartilage and subchondral bone changes in tibiofemoral and patellofemoral joints following partial medial meniscectomy.

Methods: For this cross-sectional study, 158 patients aged 30-55 years, without evidence of knee osteoarthritis at arthroscopic partial medial meniscectomy (APMM), and 38 controls were recruited. MRI was performed once on the operated knee for each subcohort of 3 months, 2 years, or 4 years post-surgery, and the randomly assigned knee of the controls. Cartilage volume, cartilage defects, and bone size were assessed using validated methods.

Results: Compared with controls, APMM patients had more prevalent cartilage defects in medial tibiofemoral (OR=3.17, 95%CI 1.24-8.11) and patellofemoral (OR=13.76, 95%CI 1.52-124.80) compartments, and increased medial tibial plateau bone area (B=143.8, 95%CI 57.4-230.2). Time from APMM was positively associated with cartilage defect prevalence in medial tibiofemoral (OR=1.02, 95%CI 1.00-1.03) and patellofemoral (OR=1.04, 95%CI 1.01-1.07) compartments, and medial tibial plateau area (B=2.5, 95%CI 0.8-4.3), but negatively associated with lateral tibial cartilage volume (B=-4.9, 95%CI -8.4 to -1.5). The association of APMM and time from APMM with patellar cartilage defects was independent of tibial cartilage volume.

Conclusions: Partial medial meniscectomy is associated with adverse effects on articular cartilage and subchondral bone, which are associated with subsequent osteoarthritis, in both tibiofemoral and patellofemoral compartments.

Magnetic resonance imaging (MRI) scanning for research: the experiences of healthy volunteers and patients with remitted depressive illness

by Victoria Tischler

We report the findings from a study exploring the experiences of individuals undergoing MRI scanning for more We report the findings from a study exploring the experiences of individuals undergoing MRI scanning for
research. Semi-structured interviews took place before and after scanning with 17 participants; 12 were
healthy volunteers and five were patients with a diagnosis of remitted depression. Themes of apprehension
and curiosity prior to scanning were common in both groups. Patients were often confused about the
procedure. Negative feelings were an issue at the outset, characterised by shock related to the physical
surroundings, after which positive feelings, for example relaxation, were often experienced, and in the case
of patients, learning more about their brain. Written information about imaging was deemed satisfactory;
however the ability to ‘experience’ aspects of scanning beforehand was suggested. Scanning may be viewed
as a process beginning prior to the procedure itself and involving positive and negative emotions. Increased
information, reassurance and a more interactive intervention to reduce anxiety may be beneficial and may
improve individuals’ experience of this widely used procedure.

Regionally Localized Thinning of the Cerebral Cortex In Schizophrenia

by Matthew Broome