Development of an MR-Guided Focused Ultrasound (MRgFUS) Lesioning Approach for the Fornix in the Rat Brain

OBJECTIVE

High-intensity magnetic resonance-guided focused ultrasound (MRgFUS) is a non-invasive therapy to lesion brain tissue, used clinically in patients and pre-clinically in several animal models. Challenges with focused ablation in rodent brains can include skull and near-field heating and accurately targeting small and deep brain structures. We overcame these challenges by creating a novel method consisting of a craniectomy skull preparation, a high-frequency transducer (3 MHz) with a small ultrasound focal spot, a transducer positioning system with an added manual adjustment of ∼0.1 mm targeting accuracy, and MR acoustic radiation force imaging for confirmation of focal spot placement.

METHODS

The study consisted of two main parts. First, two skull preparation approaches were compared. A skull thinning approach (n = 7 lesions) was compared to a craniectomy approach (n = 22 lesions), which confirmed a craniectomy was necessary to decrease skull and near-field heating. Second, the two transducer positioning systems were compared with the fornix chosen as a subcortical ablation target. We evaluated the accuracy of targeting using histologic methods from a high-frequency transducer with a small ultrasound focal spot and MR acoustic radiation force imaging.

RESULTS

Comparing a motorized adjustment system (∼1 mm precision, n = 17 lesions) to the motorized system with an added micromanipulator (∼0.1 mm precision, n = 14 lesions), we saw an increase in the accuracy of targeting the fornix by 133%.

CONCLUSIONS

The described work allows for repeatable and accurate targeting of small and deep structures in the rodent brain, such as the fornix, enabling the investigation of neurological disorders in chronic disease models.

Evaluation of acoustic-thermal simulations of in vivo magnetic resonance guided focused ultrasound ablative therapy

PURPOSE

To evaluate numerical simulations of focused ultrasound (FUS) with a rabbit model, comparing simulated heating characteristics with magnetic resonance temperature imaging (MRTI) data collected during in vivo treatment.

METHODS

A rabbit model was treated with FUS sonications in the biceps femoris with 3D MRTI collected. Acoustic and thermal properties of the rabbit muscle were determined experimentally. Numerical models of the rabbits were created, and tissue-type-specific properties were assigned. FUS simulations were performed using both the hybrid angular spectrum (HAS) method and k-Wave. Simulated power deposition patterns were converted to temperature maps using a Pennes' bioheat equation-based thermal solver. Agreement of pressure between the simulation techniques and temperature between the simulation and experimental heating was evaluated. Contributions of scattering and absorption attenuation were considered.

RESULTS

Simulated peak pressures derived using the HAS method exceeded the simulated peak pressures from k-Wave by 1.6 ± 2.7%. The location and FWHM of the peak pressure calculated from HAS and k-Wave showed good agreement. When muscle acoustic absorption value in the simulations was adjusted to approximately 54% of the measured attenuation, the average root-mean-squared error between simulated and experimental spatial-average temperature profiles was 0.046 ± 0.019 °C/W. Mean distance between simulated and experimental COTMs was 3.25 ± 1.37 mm. Transverse FWHMs of simulated sonications were smaller than in in vivo sonications. Longitudinal FWHMs were similar.

CONCLUSIONS

Presented results demonstrate agreement between HAS and k-Wave simulations and that FUS simulations can accurately predict focal position and heating for in vivo applications in soft tissue.

A Non-Contrast Multi-Parametric MRI Biomarker for Assessment of MR-Guided Focused Ultrasound Thermal Therapies

OBJECTIVE

We present the development of a non-contrast multi-parametric magnetic resonance (MPMR) imaging biomarker to assess treatment outcomes for magnetic resonance-guided focused ultrasound (MRgFUS) ablations of localized tumors. Images obtained immediately following MRgFUS ablation were inputs for voxel-wise supervised learning classifiers, trained using registered histology as a label for thermal necrosis.

METHODS

VX2 tumors in New Zealand white rabbits quadriceps were thermally ablated using an MRgFUS system under 3 T MRI guidance. Animals were re-imaged three days post-ablation and euthanized. Histological necrosis labels were created by 3D registration between MR images and digitized H&E segmentations of thermal necrosis to enable voxel-wise classification of necrosis. Supervised MPMR classifier inputs included maximum temperature rise, cumulative thermal dose (CTD), post-FUS differences in T2-weighted images, and apparent diffusion coefficient, or ADC, maps. A logistic regression, support vector machine, and random forest classifier were trained in red a leave-one-out strategy in test data from four subjects.

RESULTS

In the validation dataset, the MPMR classifiers achieved higher recall and Dice than a clinically adopted 240 cumulative equivalent minutes at 43 °C (CEM 43) threshold (0.43) in all subjects. The average Dice scores of overlap with the registered histological label for the logistic regression (0.63) and support vector machine (0.63) MPMR classifiers were within 6% of the acute contrast-enhanced non-perfused volume (0.67).

CONCLUSIONS

Voxel-wise registration of MPMR data to histological outcomes facilitated supervised learning of an accurate non-contrast MR biomarker for MRgFUS ablations in a rabbit VX2 tumor model.

Development of an MR-guided focused ultrasound (MRgFUS) lesioning approach for small and deep structures in the rat brain

Objective

High-intensity magnetic resonance-guided focused ultrasound (MRgFUS) is a noninvasive therapy to lesion brain tissue, used clinically in patients and preclinically in several animal models. Challenges with focused ablation in rodent brains can include skull and near-field heating and accurately targeting small and deep brain structures. We overcame these challenges by creating a novel method consisting of a craniectomy skull preparation, a high-frequency transducer (3 MHz) with a small ultrasound focal spot, a transducer positioning system with an added manual adjustment of ∼0.1 mm targeting accuracy, and MR acoustic radiation force imaging for confirmation of focal spot placement.

Methods

The study consisted of two main parts. First, two skull preparation approaches were compared. A skull thinning approach (n=7 lesions) was compared to a craniectomy approach (n=22 lesions), which confirmed a craniectomy was necessary to decrease skull and near-field heating. Second, the two transducer positioning systems were compared with the fornix chosen as a subcortical ablation target. We evaluated the accuracy of targeting using a high-frequency transducer with a small ultrasound focal spot and MR acoustic radiation force imaging.

Results

Comparing a motorized adjustment system (∼1 mm precision, n=17 lesions) to the motorized system with an added micromanipulator (∼0.1 mm precision, n=14 lesions), we saw an increase in the accuracy of targeting the fornix by 133%. The described work allows for repeatable and accurate targeting of small and deep structures in the rodent brain, such as the fornix, enabling the investigation of neurological disorders in chronic disease models.

Validation of a drift-corrected 3D MR temperature imaging sequence for breast MR-guided focused ultrasound treatments

Real-time temperature monitoring is critical to the success of thermally ablative therapies. This work validates a 3D thermometry sequence with k-space field drift correction designed for use in magnetic resonance-guided focused ultrasound treatments for breast cancer. Fiberoptic probes were embedded in tissue-mimicking phantoms, and temperature change measurements from the probes were compared with the magnetic resonance temperature imaging measurements following heating with focused ultrasound. Precision and accuracy of measurements were also evaluated in free-breathing healthy volunteers (N = 3) under a non-heating condition. MR temperature measurements agreed closely with those of fiberoptic probes, with a 95% confidence interval of measurement difference from -2.0 °C to 1.4 °C. Field drift-corrected measurements in vivo had a precision of 1.1 ± 0.7 °C and were accurate within 1.3 ± 0.9 °C across the three volunteers. The field drift correction method improved precision and accuracy by an average of 46 and 42%, respectively, when compared to the uncorrected data. This temperature imaging sequence can provide accurate measurements of temperature change in aqueous tissues in the breast and support the use of this sequence in clinical investigations of focused ultrasound treatments for breast cancer.

Comparison of knowledge acquisition and retention following traditional didactic vs. flipped classroom education utilizing a standardized national curriculum: a randomized controlled trial

OBJECTIVE

Measure the effectiveness of and preference for a standardized, national curriculum utilizing flipped classrooms (FC) in neonatal-perinatal medicine (NPM) fellowships.

STUDY DESIGN

Multicentered equivalence, cluster randomized controlled trial of NPM fellowship programs randomized to receive standardized physiology education as in-class lectures (traditional didactic, TD arm) or as pre-class online videos followed by in-class discussions (FC arm). Four multiple-choice question quizzes and three surveys were administered to measure knowledge acquisition, retention, and educational preferences.

RESULTS

530 fellows from 61 NPM fellowships participated. Quiz performance was comparable between groups at all time points (p = NS, TD vs FC at 4 time points). Post intervention, more fellows in both groups preferred group discussions (pre/post FC 42% vs. 58%, P = 0.002; pre/post TD 43% vs. 60%, P = < 0.001). FC fellows were more likely to rate classroom effectiveness positively (FC/TD, 70% vs. 36%, P < 0.001).

CONCLUSIONS

FCs promote knowledge acquisition and retention equivalent to TD and FC modalities are preferred by fellows.

Flipping the Teachers: Impact of a Standardized Physiology Curriculum on Neonatology Medical Educators

OBJECTIVE

 Academic physicians must teach elements in an Accreditation Council for Graduate Medical Education (ACGME)-mandated curriculum while balancing career development and clinical workload. Exploring educator perceptions on the learning environment and comparing two instructional methods (traditional didactics [TD] vs. flipped classroom [FC]) in one pediatric subspecialty may elucidate current challenges, barriers, and strategies to optimize learning and educator satisfaction.

STUDY DESIGN

 A randomized trial comparing effectiveness and learner preference for FC versus TD physiology teaching was conducted in ACGME-accredited neonatal-perinatal medicine (NPM) fellowship programs in 2018 to 2019. Educator preferences were elicited through online surveys pre- and postintervention. Free-text comments were provided for questions exploring strengths, challenges, and opportunities in fellowship education. Statistical analysis included comparisons of demographics and pre-post-intervention educator responses between groups. Thematic analysis of text responses was conducted to identify common subthemes.

RESULTS

 From 61 participating programs, 114 FC educators and 130 TD educators completed surveys. At baseline, all educators experienced professional satisfaction from teaching fellows, but noted challenges with time available to create and/or deliver educational content, limited content expertise amongst faculty, colleagues' limited enthusiasm toward educating fellows, and lack of perceived value of education by institutions given limited protected time or credit toward promotion. Postintervention, educators in both groups noted a preference to teach physiology using FC due to interactivity, learner enthusiasm, and learner-centeredness. FC educators had a 17% increase in preference to teach using FC (p = 0.001). Challenges with FC included ensuring adequate trainee preparation, protecting educational time, and providing educators with opportunities to develop facilitation skills.

CONCLUSION

 Overall, NPM educators in a trial evaluating a standardized, peer-reviewed curriculum report professional satisfaction from teaching, but described logistical challenges with developing/delivering content. Educators preferred instruction using FC, but identified challenges with learner preparedness and ensuring adequate educator time and skill. Future efforts should be dedicated to addressing these barriers.

KEY POINTS

· Many challenges exist for educators teaching neonatal-perinatal medicine fellows, including time, support, and recognition.. · Many educators preferred using flipped classroom methodology with a standardized curriculum due to interactivity and learner-centeredness.. · Benefits of a standardized, peer-reviewed curriculum include reduced preparation time, adaptability of content, and learning environment enhancement..

A k-space-based method to measure and correct for temporal B0 field variations in MR temperature imaging

PURPOSE

Present a method to use change in phase in repeated Cartesian k-space measurements to monitor the change in magnetic field for dynamic MR temperature imaging.

METHODS

The method is applied to focused ultrasound heating experiments in a gelatin phantom and an ex vivo salt pork sample, without and with simulated respiratory motion.

RESULTS

In each experiment, phase variations due to B0 field drift and respiration were readily apparent in the measured phase difference. With correction, the SD of the temperature over time was reduced from 0.18°C to 0.14°C (no breathing) and from 0.81°C to 0.22°C (with breathing) for the gelatin phantom, and from 0.68°C to 0.13°C (no breathing) and from 1.06°C to 0.17°C (with breathing) for the pork sample. The accuracy in nonheated regions, assessed as the RMS error deviation from 0°C, improved from 1.70°C to 1.11°C (no breathing) and from 4.73°C to 1.47°C (with breathing) for the gelatin phantom, and from 5.95°C to 0.88°C (no breathing) and from 13.40°C to 1.73°C (with breathing) for the pork sample. The correction did not affect the temperature measurement accuracy in the heated regions.

CONCLUSION

This work demonstrates that phase changes resulting from variations in B0 due to drift and respiration, commonly seen in MR thermometry applications, can be measured directly from 3D Cartesian acquisition methods. The correction of temporal field variations using the presented technique improved temperature accuracy, reduced variability in nonheated regions, and did not reduce accuracy in heated regions.

DNA methylation in former extremely low birth weight newborns: association with cardiovascular and endocrine function

BACKGROUND

There is increased risk of cardiovascular, metabolic, and hypertensive disorders in later life in the preterm population. We studied school-age children who had been born extremely premature who had undergone endocrine, cardiovascular, and anthropometric evaluations.

METHODS

School age measurements of salivary cortisol, adrenal androgens, blood pressure, and anthropometric markers were correlated with DNA methylation of 11-betahydroxysteroid dehydrogenase type 2 (11BHSD2), leptin, and the LINE1 repetitive DNA element.

RESULTS

We observed a modest correlation between log AUC for salivary cortisol and methylation of leptin in preterm infants and a negative correlation between methylation of region 1 of the glucocorticoid receptor (GR in term-born infants. There was an association between LINE1 methylation and cortisol response to awakening and a negative correlation between LINE1 and systolic blood pressure at 6-7 years. Methylation of the GR promoter region showed a positive association with systolic blood pressure at 6-7 years of age.

CONCLUSIONS

These results show that extremely preterm birth, followed by complex patterns of endocrine, cardiovascular, and metabolic exposures during early postnatal life, is associated with lasting changes in DNA methylation patterns in genes involved in hypothalamic pituitary adrenal axis function, adrenal hormonal regulation, and cardiometabolic risk.

IMPACT

Preterm infants have significant environmental and physiological exposures during early life that may have lasting impact on later function. Alterations in hypothalamic pituitary adrenal axis (HPA) function have been associated with these exposures. We examined the associated changes in DNA methylation of important genes involved in HPA function, metabolism, and global DNA methylation. The changes we saw in DNA methylation may help to explain associated cardiovascular, metabolic, and growth disturbance in these children in later life.

391 An Audit of Nurse-Led Follow-Up of Patients with Colorectal Cancer in a District General Hospital

Introduction

Rates of recurrence of colorectal cancer after potentially curative surgery vary between 30–65%. Regular post-operative surveillance of these patients is essential in detecting recurrence and improves overall survival. NICE guidelines recommend follow-up with measurement of serum carcinoembryogenic antigen (CEA) and with CT scans of the chest abdomen and pelvis. In our unit, patients have been followed up by Cancer Nurse Specialists (CSNs) since 2016. The aim of this audit is to determine the compliance with surveillance guidelines in our nurse-led follow-up programme.

Method

This is a retrospective audit of randomly selected post-operative patients with colorectal cancer who were followed up by CSNs. All patients with appointments with CSNs in 2 randomly selected months (1st January- 28th February 2020) were included. Data was collected using electronic patient records.

Results

54 patients were identified. For low-risk patients, 100% (34/34) had their CT surveillance at 1 year, and, for those 30 months post- surgery, 100% (30/30) had their 30 months CT scan. 94% (32/34) had recommended CEA measurements.

In high-risk patients, 85% (17/20) had their 1-year CT scan, 100% of eligible patients (19/19) had their 2-year CT, or a plan in place for one. Of patients eligible for a 3-year CT scan, 46% (6/13) had one. 95% (19/20) had recommended CEA measurements.

Conclusions

Nurse-led follow-up overall leads to adequate follow-up, better compliance with guidelines and improved continuity of care.

Part 6: Essentials of Neonatal-Perinatal Medicine fellowship: program administration

A successful Neonatal-Perinatal Medicine fellowship (NPM-F) program requires presence and insight of national and institutional supervisory organizations as well as effective program-specific leaders: program director (PD), associate program director (APD), program coordinator (PC), and core faculty. It is becoming more common for PDs and APDs to have advanced training in medical education and conduct medical education research. While NPM-F program leaders benefit from a strong national NPM educator community, they face challenges of increased regulatory burden and unclear national guidelines with variable local interpretation for protected time. National and local organizations can support program leaders and promote their academic success while reducing burnout and turnover by providing leadership training, academic mentoring, and adequate protected time for research and program-specific tasks.

Improving image quality in transcranial magnetic resonance guided focused ultrasound using a conductive screen

Transcranial Magnetic Resonance guided Focused Ultrasound (TcMRgFUS) has been proven to be an effective treatment for some neurological disorders such as essential and Parkinson's tremor. However, magnetic resonance guidance at 3 Tesla (3T) frequencies and using the large hemispherical transducers required for TcMRgFUS results in artifactual low-signal bands that pass through key regions of the brain. The purpose of this work was to investigate the use of a circular conductive Radio Frequency (RF) screen, that is bent to have a 12 cm radius in one direction and positioned near the top or back of the head, to reduce or remove these artifactual low-signal bands in TcMRgFUS. The impact of using an RF screen to remove these low signal bands was studied in both imaging experiments and electromagnetic simulations. Hydrophone measurements of the acoustic transparency of the bronze 2 mm diameter square mesh screen used in the imaging studies were compared with temperature measurements with and without the screen in heating studies in the TcMRgFUS system. The imaging and simulation studies both show that for the different screen configurations studied in this work, RF screen removes the low-signal bands and increases both homogeneity and signal-to-noise ratio (SNR) throughout the region of the brain. Hydrophone and heating studies indicate that even a 2 mm wire mesh provides minimal attenuation to the ultrasound beam. Simulation results also suggest that a 1 cm mesh will provide adequate artifact suppression with even less ultrasound attenuation. An RF screen that disrupts the natural waveguide nature of the transducer in the 3T MR environment can change the electromagnetic field profile to reduce unwanted artifacts and provide an imaging region which has more homogeneity and higher SNR throughout the brain.

600 Bilateral Upper Limb Compartment Syndrome After Massive Postpartum Haemorrhage

Case Summary

Upper limb compartment syndrome (ULCS) is an uncommon limb-threatening emergency occurring as a result of increased interstitial pressure within osteofascial compartments. The overwhelming majority of causes occur in one limb, with frequent causes including fractures, reperfusion injury and burns. We report the unusual case of bilateral upper limb CS in a previously well 35-year-old female secondary to massive resuscitation following postpartum haemorrhage. The patient received 7 litres of blood products and 4 litres of crystalloid over a period of 8 hours. One hour postoperatively both hands became painful and swollen. Symptoms worsened over the following hour: both forearms were tense to palpate with demonstrable pain on passive extension of bilateral wrists and digits of the left hand, and capillary refill to all digits was brisk. Creatinine kinase was measured at 4,600U/L and bilateral forearm and left-hand fasciotomies were performed. The presence of bulging muscles in both forearms and the left hand confirmed the diagnosis of compartment syndrome, and perfusion was restored to all digits after release of the compartments. The patient later underwent split thickness skin grafting to both forearms and left-hand fasciotomy wounds. After 5 weeks of hand therapy, she regained good function of both hands. Bilateral ULCS is extremely uncommon but has been reported secondary to massive resuscitation, particularly in trauma patients, and we suspect that this was the cause in our patient.

596 Redeployment of Surgical Trainees to Intensive Care During the COVID-19 Pandemic: Evaluation of the Impact on Training and Wellbeing

Aim

The aim of this study was to evaluate the impact of redeployment of surgical trainees to intensive care units (ICUs) during the COVID-19 pandemic in terms of transferrable technical and nontechnical skills and wellbeing.

Method

This was a survey study consisting of a 23-point questionnaire. The study involved senior house officer level surgical trainees that had been redeployed to the (ICU) across all hospitals in London during the COVID-19 pandemic. The survey was sent to 90 surgical trainees. Thirty-two trainees responded to the questionnaire and were included in the study results.

Results

All respondents spent between 4 and 8 weeks working in ICU. Prior to redeployment, 78% of participants had previous experience of ICU or an affiliated specialty, and >90% had attended at least 1 educational course with relevance to ICU. There were statistically significant increases in confidence performing central venous cannulation and peripheral arterial catheterisation (p < 0.05). With regards to clinical skills, respondents reported feeling more confident managing ventilated patients, patients on noninvasive ventilation, dialysis, and circulatory failure patients after working in ICU. Respondents (97%) felt that the experience would be beneficial to their future careers but 53% felt the redeployment had a negative impact on their mental health.

Conclusions

Redeployment of surgical trainees to ICU led to increased confidence in a number of technical and nontechnical skills. However, proactive interventions are needed for training surgeons with regard to their psychological wellbeing in these extraordinary circumstances and to improve workforce planning for future pandemics.

599 Intramammary Melanoma Micrometastasis Within A Silicone Implanted Breast

Case Summary

Melanoma of the abdominal wall is not uncommon, and sentinel lymph nodes are usually located in the lymphatic drainage basins of the axilla or inguinal region. Less frequently, interval sentinel lymph nodes can be found along the lymphatic vessels between the primary cancer and nearest basin.

We present the case of a 53-year-old female with silicone breast implants who underwent scar excision and sentinel lymph node biopsy for a 1mm Breslow thickness superficial spreading melanoma of the abdomen. Two lymph nodes were excised; both lying in the subcutaneous fat at the lateral aspect of the right breast capsule. Lymph node histology revealed a subcapsular melanoma deposit along with silicone lymphadenopathy in the sentinel node, and silicone lymphadenopathy in the second node. Whole body positron emission tomography (PET/CT) and magnetic resonance imaging (MRI) of the brain showed no evidence of metastases or implant rupture. Subsequent MRI breast revealed likely intracapsular implant rupture. The patient was offered removal of implants and remains under follow-up.

The unusual location of the sentinel node in our patient highlights the possibility that previous breast augmentation may have altered the pattern of lymphatic drainage to the axilla. In addition, to our knowledge, this is the first reported case of silicone and melanoma deposits in a single sentinel node.

598 Burns Management at A UK Major Trauma Centre During The COVID-19 Lockdown

Aim

The COVID-19 pandemic forced huge changes in healthcare delivery in the UK within a matter of weeks. We report our experience of managing burns presenting to a UK Major Trauma Centre during the COVID-19 lockdown period.

Method

Retrospective patient data was collected on aetiology, severity and management of burns during the first six weeks of the government-enforced lockdown. Data was compared with patients presenting with burns during the same period in 2019 (control) and statistical analysis was performed.

Results

Fifteen patients were treated during the COVID-19 lockdown and fourteen in the control group. Adults treated during the COVID-19 lockdown had a greater total body surface area (TBSA) (9% vs. 2.9%, p 0.035) and were more likely to suffer full thickness burns (40% vs. 0%). In the COVID-19 group all patients that did not require resuscitation, seven of whom met criteria for referral to burns services, were managed locally by the plastic surgery department with good outcomes.

Conclusions

Despite the increase in severity of adult burns seen during the COVID-19 lockdown period, local plastic surgery units have been able to adapt their practice and successfully manage more complex burns. This adaptability will be key as we move through the pandemic.

A T1-based correction method for proton resonance frequency shift thermometry in breast tissue

PURPOSE

Develop and evaluate the effectiveness of a T1-based correction method for errors in proton resonant frequency shift thermometry due to non-local field effects caused by heating in fatty breast tissues.

METHODS

Computational models of human breast tissue were created by segmenting MRI data from a healthy human volunteer. MR-guided focused ultrasound (MRgFUS) heating and MR thermometry measurements were simulated in several locations in the heterogeneous segmented breast models. A T1-based correction method for PRF thermometry errors was applied and the maximum positive and negative errors and the root mean squared error (RMSE) in a region around each heating location was evaluated with and without correction. The method uses T1 measurements to estimate the temperature change in fatty tissues and correct for their influence. Experimental data from a heating study in cadaver breast tissue were analyzed, and the expected PRFS error computed.

RESULTS

The simulated MR thermometry had maximum single voxel errors ranging between 10% and 18% when no correction was applied. Applying the correction led to a considerable improvement, lowering the maximum error range to 2%-5%. The 5th to 95th percentile interval of the temperature error distribution was also lowered with correction, from approximately 3.5 to 1°C. This correction worked even when T1 times were uniformly raised or lowered by 5%-10%. The experimental data showed predicted errors of 15%.

CONCLUSIONS

This simulation study demonstrates that the T1-based correction method reduces MR thermometry errors due to non-local effects from heating in fatty tissues, potentially improving the accuracy of thermometry measurements during MRgFUS treatments. The presented correction method is reliant on having a patient-specific 3D model of the breast, and may be limited by the accuracy of the fat temperatures which in turn may be limited by noise or bias present in the T1 measurements.

Design and evaluation of an open-source, conformable skin-cooling system for body magnetic resonance guided focused ultrasound treatments

Purpose:

Magnetic resonance guided focused ultrasound (MRgFUS) treatment of tumors uses inter-sonication delays to allow heat to dissipate from the skin and other near-field tissues. Despite inter-sonication delays, treatment of tumors close to the skin risks skin burns. This work has designed and evaluated an open-source, conformable, skin-cooling system for body MRgFUS treatments to reduce skin burns and enable ablation closer to the skin.

Methods:

A MR-compatible skin cooling system is described that features a conformable skin-cooling pad assembly with feedback control allowing continuous flow and pressure maintenance during the procedure. System performance was evaluated with hydrophone, phantom and in vivo porcine studies. Sonications were performed 10 and 5 mm from the skin surface under both control and forced convective skin-cooling conditions. 3D MR temperature imaging was acquired in real time and the accumulated thermal dose volume was measured. Gross analysis of the skin post-sonication was further performed. Device conformability was demonstrated at several body locations.

Results:

Hydrophone studies demonstrated no beam aberration, but a 5–12% reduction of the peak pressure due to the presence of the skin-cooling pad assembly in the acoustic near field. Phantom evaluation demonstrated there is no MR temperature imaging precision reduction or any other artifacts present due to the coolant flow during MRgFUS sonication. The porcine studies demonstrated skin burns were reduced in size or eliminated when compared to the control condition.

Conclusion:

An open-source design of an MRgFUS active skin cooling system demonstrates device conformability with a reduction of skin burns while ablating superficial tissues.

AAPM Task Group 241: A medical physicist's guide to MRI-guided focused ultrasound body systems

Magnetic resonance-guided focused ultrasound (MRgFUS) is a completely non-invasive technology that has been approved by FDA to treat several diseases. This report, prepared by the American Association of Physicist in Medicine (AAPM) Task Group 241, provides background on MRgFUS technology with a focus on clinical body MRgFUS systems. The report addresses the issues of interest to the medical physics community, specific to the body MRgFUS system configuration, and provides recommendations on how to successfully implement and maintain a clinical MRgFUS program. The following sections describe the key features of typical MRgFUS systems and clinical workflow and provide key points and best practices for the medical physicist. Commonly used terms, metrics and physics are defined and sources of uncertainty that affect MRgFUS procedures are described. Finally, safety and quality assurance procedures are explained, the recommended role of the medical physicist in MRgFUS procedures is described, and regulatory requirements for planning clinical trials are detailed. Although this report is limited in scope to clinical body MRgFUS systems that are approved or currently undergoing clinical trials in the United States, much of the material presented is also applicable to systems designed for other applications.

A Breast-Specific MR Guided Focused Ultrasound Platform and Treatment Protocol: First-in-Human Technical Evaluation

OBJECTIVE

This paper presents and evaluates a breast-specific magnetic resonance guided focused ultrasound (MRgFUS) system. A first-in-human evaluation demonstrates the novel hardware, a sophisticated tumor targeting algorithm and a volumetric magnetic resonance imaging (MRI) protocol.

METHODS

At the time of submission, N = 10 patients with non-palpable T0 stage breast cancer have been treated with the breast MRgFUS system. The described tumor targeting algorithm is evaluated both with a phantom test and in vivo during the breast MRgFUS treatments. Treatments were planned and monitored using volumetric MR-acoustic radiation force imaging (MR-ARFI) and temperature imaging (MRTI).

RESULTS

Successful technical treatments were achieved in 80 % of the patients. All patients underwent the treatment with no sedation and 60 % of participants had analgesic support. The total MR treatment time ranged from 73 to 114 minutes. Mean error between desired and achieved targeting in a phantom was 2.9 ±1.8 mm while 6.2 ±1.9 mm was achieved in patient studies, assessed either with MRTI or MR-ARFI measurements. MRTI and MR-ARFI were successful in 60 % and 70 % of patients, respectively.

CONCLUSION

The targeting accuracy allows the accurate placement of the focal spot using electronic steering capabilities of the transducer. The use of both volumetric MRTI and MR-ARFI provides complementary treatment planning and monitoring information during the treatment, allowing the treatment of all breast anatomies, including homogeneously fatty breasts.

The COVID-19 Pandemic: The effect on open lower limb fractures in a London major trauma centre - a plastic surgery perspective

BACKGROUND

COVID-19 has created huge pressures on healthcare systems. The ongoing provision of major trauma services during this time has proved challenging. We report our experience of managing open lower limb fractures (oLLFs) during the pandemic in a London major trauma centre (MTC).

METHODS

This was a prospective study of all open lower limb fractures presenting to our unit over the initial 48 days of UK government lockdown - 24th March till 10th May 2020. Results were compared to the same time period in 2019 retrospectively. Epidemiological data, mechanism, Gustilo-Anderson (G-A) severity grading, time to initial debridement and definitive coverage were analysed.

RESULTS

There was a 64% reduction in emergency department (ED) attendances (25,264 vs 9042). There was an 18% reduction in oLLFs (22 vs 18). Approximately three-quarters of injuries were in males across both cohorts (77% vs 78%) and tended to occur in younger patients (median age, 37 vs 35). Road-traffic-accidents (RTAs) were the most common injury mechanism in both 2019 and lockdown, but a rise in jumpers from height was seen in the latter. A similar pattern of G-A severities were seen, however only 3 injuries during lockdown required major soft tissue reconstruction. There was no significant difference in times taken for initial debridement (p = 0.72786) or definitive wound coverage (p = 0.16152). A greater proportion of independent operating was seen during lockdown between orthopaedics and plastic surgery.

CONCLUSIONS

Despite government lockdown measures, oLLFs still placed significant burden on our MTC. Notwithstanding significant staffing alterations and theatre pressures, we have been able to ensure these lower limb emergencies remain a surgical priority and have managed to utilise resources appropriately.

Magnetic resonance shear wave elastography using transient acoustic radiation force excitations and sinusoidal displacement encoding

A magnetic resonance (MR) shear wave elastography technique that uses transient acoustic radiation force impulses from a focused ultrasound (FUS) transducer and a sinusoidal-shaped MR displacement encoding strategy is presented. Using this encoding strategy, an analytic expression for calculating the shear wave speed in a heterogeneous medium was derived. Green's function-based simulations were used to evaluate the feasibility of calculating shear wave speed maps using the analytic expression. Accuracy of simulation technique was confirmed experimentally in a homogeneous gelatin phantom. The elastography measurement was compared to harmonic MR elastography in a homogeneous phantom experiment and the measured shear wave speed values differed by less than 14%. This new transient elastography approach was able to map the position and shape of inclusions sized from 8.5 to 14 mm in an inclusion phantom experiment. These preliminary results demonstrate the feasibility of using a straightforward analytic expression to generate shear wave speed maps from MR images where sinusoidal-shaped motion encoding gradients are used to encode the displacement-time history of a transiently propagating wave-packet. This new measurement technique may be particularly well suited for performing elastography before, during, and after MR-guided FUS therapies since the same device used for therapy is also used as an excitation source for elastography.

Experimental validation of acoustic and thermal modeling in heterogeneous phantoms using the hybrid angular spectrum method

PURPOSE

The aim was to quantitatively validate the hybrid angular spectrum (HAS) algorithm, a rapid wave propagation technique for heterogeneous media, with both pressure and temperature measurements.

METHODS

Heterogeneous tissue-mimicking phantoms were used to evaluate the accuracy of the HAS acoustic modeling algorithm in predicting pressure and thermal patterns. Acoustic properties of the phantom components were measured by a through-transmission technique while thermal properties were measured with a commercial probe. Numerical models of each heterogeneous phantom were segmented from 3D MR images. Cylindrical phantoms 30-mm thick were placed in the pre-focal field of a focused ultrasound beam and 2D pressure measurements obtained with a scanning hydrophone. Peak pressure, full width at half maximum, and normalized root mean squared difference (RMSDn) between the measured and simulated patterns were compared. MR-guided sonications were performed on 150-mm phantoms to obtain MR temperature measurements. Using HAS-predicted power density patterns, temperature simulations were performed. Experimental and simulated temperature patterns were directly compared using peak and mean temperature plots, RMSDn metrics, and accuracy of heating localization.

RESULTS

The average difference between simulated and hydrophone-measured peak pressures was 9.0% with an RMSDn of 11.4%. Comparison of the experimental MRI-derived and simulated temperature patterns showed RMSDn values of 10.2% and 11.1% and distance differences between the centers of thermal mass of 2.0 and 2.2 mm.

CONCLUSIONS

These results show that the computationally rapid hybrid angular spectrum method can predict pressure and temperature patterns in heterogeneous models, including uncertainties in property values and other parameters, to within approximately 10%.

Development and characterization of a tissue mimicking psyllium husk gelatin phantom for ultrasound and magnetic resonance imaging

Purpose: To develop and characterize a tissue-mimicking phantom that enables the direct comparison of magnetic resonance (MR) and ultrasound (US) imaging techniques useful for monitoring high-intensity focused ultrasound (HIFU) treatments. With no additions, gelatin phantoms produce little if any scattering required for US imaging. This study characterizes the MR and US image characteristics as a function of psyllium husk concentration, which was added to increase US scattering.Methods: Gelatin phantoms were constructed with varying concentrations of psyllium husk. The effects of psyllium husk concentration on US B-mode and MR imaging were evaluated at nine different concentrations. T1, T2, and T2* MR maps were acquired. Acoustic properties (attenuation and speed of sound) were measured at frequencies of 0.6, 1.0, 1.8, and 3.0 MHz using a through-transmission technique. Phantom elastic properties were evaluated for both time and temperature dependence.Results: Ultrasound image echogenicity increased with increasing psyllium husk concentration while quality of gradient-recalled echo MR images decreased with increasing concentration. For all phantoms, the measured speed of sound ranged between 1567-1569 m/s and the attenuation ranged between 0.42-0.44 dB/(cm·MHz). Measured T1 ranged from 974-1051 ms. The T2 and T2* values ranged from 97-108 ms and 48-88 ms, respectively, with both showing a decreasing trend with increased psyllium husk concentration. Phantom stiffness, measured using US shear-wave speed measurements, increased with age and decreased with increasing temperature.Conclusions: The presented dual-use tissue-mimicking phantom is easy to manufacture and can be used to compare and evaluate US-guided and MR-guided HIFU imaging protocols.

Learning Multiparametric Biomarkers for Assessing MR-Guided Focused Ultrasound Treatment of Malignant Tumors

Noninvasive MR-guided focused ultrasound (MRgFUS) treatments are promising alternatives to the surgical removal of malignant tumors. A significant challenge is assessing the viability of treated tissue during and immediately after MRgFUS procedures. Current clinical assessment uses the nonperfused volume (NPV) biomarker immediately after treatment from contrast-enhanced MRI. The NPV has variable accuracy, and the use of contrast agent prevents continuing MRgFUS treatment if tumor coverage is inadequate. This work presents a novel, noncontrast, learned multiparametric MR biomarker that can be used during treatment for intratreatment assessment, validated in a VX2 rabbit tumor model. A deep convolutional neural network was trained on noncontrast multiparametric MR images using the NPV biomarker from follow-up MR imaging (3-5 days after MRgFUS treatment) as the accurate label of nonviable tissue. A novel volume-conserving registration algorithm yielded a voxel-wise correlation between treatment and follow-up NPV, providing a rigorous validation of the biomarker. The learned noncontrast multiparametric MR biomarker predicted the follow-up NPV with an average DICE coefficient of 0.71, substantially outperforming the current clinical standard (DICE coefficient = 0.53). Noncontrast multiparametric MR imaging integrated with a deep convolutional neural network provides a more accurate prediction of MRgFUS treatment outcome than current contrast-based techniques.

Improved detection of sinusoidal obstructive syndrome using pediatric-AYA diagnostic criteria and severity grading

New diagnostic criteria and severity grading for sinusoidal obstructive syndrome (SOS) among pediatric and adolescent young adult (AYA) patients have been recently endorsed by international consensus. The extent to which these have been adopted in the US remains unclear. We sought to assess the potential impact via retrospective application of these criteria among patients treated at a large academic center in the United States. This is a single center retrospective study of pediatric-AYA patients who underwent hematopoietic cell transplantation (HCT) between July 2009 and 2019. The incidence of SOS was assessed using historic Baltimore and Seattle diagnostic criteria and compared with more recent guidelines (pEBMT) as proposed by the Paediatric Diseases Working Party of the European Society for Blood and Marrow Transplantation. Among 226 patients, application of the pEBMT diagnostic criteria was associated with a higher incidence (15.9%) and earlier time to diagnosis of SOS (by 2.5-3 days) compared with the modified Seattle (12.3%), and Baltimore (6.6%) criteria, respectively. The pEBMT criteria were sensitive and highly specific. Refractory thrombocytopenia was present in 75% of patients at diagnosis. Approximately 61% of patients with SOS were anicteric at diagnosis, though the majority (94.4%) developed hyperbilirubinemia as SOS progressed over a median time of 4 (1-57) days. Application of pEBMT criteria may have resulted in earlier indication for definitive treatment by 3 days. Timely diagnosis and administration of definitive treatment of SOS has been associated with improved outcomes. Prospective studies may better characterize the risk factors and natural course of SOS using pEBMT criteria.

Technical Note: Effect of transducer position and ground plane configuration on image quality in MR-guided focused ultrasound therapies

PURPOSE

To evaluate the effect of a focused ultrasound transducer position and ground plane configuration on magnetic resonance image quality.

METHODS

The effect of transducer position with respect to the MRI B0 field and the radiofrequency receive coils was evaluated in a breast-specific MRgFUS system with an integrated RF phased-array coil. Image signal-to-noise ratio was evaluated at different transducer locations. The effect of ultrasound transducer ground plane configuration was evaluated using a replica transducer with 12 ground plane configurations. All evaluations were performed at 3 Tesla.

RESULTS

Both transducer position and ground plane configuration were found to have a considerable effect on overall image SNR. A 67% increase in SNR was achieved by positioning the transducer face perpendicular to the B0 field. A 25% increase in SNR was achieved by segmenting the replica transducer ground plane from one continuous plane to nine individual segments.

CONCLUSIONS

Advances in focused ultrasound hardware allow for integrated radiofrequency MRI coils as well as adjustable transducer positioning. The placement of the ultrasound transducer with respect to both the magnetic field and RF coils can have a considerable effect on image SNR and the resulting MR images that are used for MR-guided focused ultrasound treatment planning, monitoring and assessment.

Safety of immediate skin-to-skin contact after vaginal birth in vigorous late preterm neonates - A pilot study

BACKGROUND

To evaluate the safety of immediate skin-to-skin contact (SSC) in vigorous late preterm neonates, where observation under radiant warmer is standard of care, in a prospective, randomized, controlled, and equivalence pilot study.

METHODS

Singletons born vaginally at 35-36 6/7 weeks gestation were randomized to initiate immediate SSC or standard of care with continuous pulse oximeter monitoring for the first hour of life.

RESULTS

Forty-seven dyads were randomized to SSC (n = 21) or radiant warmer (n = 26). Vitals were recorded at designated time intervals to assess tolerance of postnatal transitioning. We found no significant difference in the number of SSC interruptions, pulse oximeter readings, initial glucose level, and rates of hypoglycemia, hypothermia, or NICU admission between the two groups.

CONCLUSIONS

Vigorous late preterm neonates transitioned to immediate SSC without additional risks compared to control counterparts. Large, multicenter, and randomized-control studies need to be conducted to establish standardized guidelines for this practice.

Inducing remission in paediatric Crohn's disease using nutritional therapies - A systematic review

BACKGROUND

Exclusive enteral nutrition (EEN) is known to be a safe and effective treatment option for managing active Crohn's disease (CD) in children, although no uniform protocol exists. The aim of this systematic review was to evaluate and compare the clinical effectiveness of aspects of EEN protocols to ascertain whether an optimum regimen can be identified.

METHODS

A systematic search of the Cochrane Library, PubMed, MEDLINE, EMBASE, CINAHL and AMED was conducted for studies published between 1998 and 2018 that examined paediatric patients being treated with an enteral nutrition protocol to induce remission. Studies that included patients receiving concurrent medication for active disease were excluded. Quality assessment was performed using separate tools for randomised controlled trials, cohort studies and for studies without a control group.

RESULTS

Sixteen studies met the inclusion criteria. Of these, six found insufficient evidence to support use of a specific formula. One study examined the route of EEN, finding no difference between oral or nasogastric tube administration with respect to inducing remission. Three examined the use of partial enteral nutrition to induce remission, although conflicting results were seen. No studies explored the effect of length of treatment or energy prescription on remission rates CONCLUSIONS: An optimum enteral nutrition protocol for inducing remission cannot be identified. Further focused research is required by well designed, adequately powered prospective clinical trials to examine aspects of enteral feeding protocols that remain uncertain, including the use of partial enteral nutrition as a potential alternative to EEN.

Development and validation of the African Women Awareness of CANcer (AWACAN) tool for breast and cervical cancer

BACKGROUND

Measuring factors influencing time to presentation is important in developing and evaluating interventions to promote timely cancer diagnosis, yet there is a lack of validated, culturally relevant measurement tools. This study aimed to develop and validate the African Women Awareness of CANcer (AWACAN) tool to measure awareness of breast and cervical cancer in Sub-Saharan Africa (SSA).

METHODS

Development of the AWACAN tool followed 4 steps: 1) Item generation based on existing measures and relevant literature. 2) Refinement of items via assessment of content and face validity using cancer experts' ratings and think aloud interviews with community participants in Uganda and South Africa. 3) Administration of the tool to community participants, university staff and cancer experts for assessment of validity using test-retest reliability (using Intra-Class Correlation (ICC) and adjusted Kappa coefficients), construct validity (comparing expert and community participant responses using t-tests) and internal reliability (using the Kuder-Richarson (KR-20) coefficient). 4) Translation of the final AWACAN tool into isiXhosa and Acholi.

RESULTS

ICC scores indicated good test-retest reliability (≥ 0.7) for all breast cancer knowledge domains and cervical cancer risk factor and lay belief domains. Experts had higher knowledge of breast cancer risk factors (p < 0.001), and cervical cancer risk factors (p = 0.003) and symptoms (p = 0.001) than community participants, but similar knowledge of breast cancer symptoms (p = 0.066). Internal reliability for breast cancer risk factors, lay beliefs and symptom and cervical cancer symptom subscales was good with KR-20 values > 0.7, and lower (0.6) for the cervical cancer risk subscale.

CONCLUSION

The final AWACAN tool includes items on socio-demographic details; breast and cervical cancer symptom awareness, risk factor awareness, lay beliefs, anticipated help-seeking behaviour; and barriers to seeking care. The tools showed evidence of content, face, construct and internal validity and test-retrest reliability and are available for use in SSA in three languages.

A tissue preparation to characterize uterine fibroid tissue properties for thermal therapies

PURPOSE

Treating uterine fibroids with less invasive therapies such as magnetic resonance-guided focused ultrasound (MRgFUS) is an attractive alternative to surgery. Treatment planning can improve MRgFUS procedures and reduce treatment times, but the tissue properties that currently inform treatment planning tools are not adequate. This study aims to develop an ex vivo uterine fibroid model that can emulate the in vivo environment allowing for characterization of the uterus and fibroid MR, acoustic, and thermal tissue properties while maintaining viability for the necessary postsurgical histopathological assessments.

METHODS

Women undergoing a hysterectomy due to fibroid-related symptoms were invited to undergo a preoperative pelvic MRI and to permit postoperative testing of their uterine specimen. Patients that declined or could not be scheduled for a pre-operative MRI were still able to allow post-operative testing of their excised tissue. Following surgical removal of the uterus, nonmorcellated tissues were reperfused with a Krebs-Henseleit buffer solution. An MR-compatible perfusion system was designed to maintain tissue viability inside the MR suite during scanning. MR imaging protocols utilized preoperatively were repeated on whole sample, reperfused ex vivo uterus specimens. Thermal properties including thermal diffusivity and thermal conductivity of the uterus and fibroids were determined using an invasive needle sensor device in 50% of the specimens. Acoustic property measurements (density, speed of sound and attenuation) were obtained for approximately 20% of the tissue samples using both through-transmission and radiation force balance techniques. Differences between fibroid and uterus and in vivo and ex vivo measurements were evaluated with a two-tailed Student t test.

RESULTS

Fourteen patients participated in the study and measurements were obtained from 22 unique fibroids. Of the 16 fibroids available for preoperative MRI testing, 69% demonstrated classic hypo-intensity relative to the myometrium, with the remainder presenting with iso- (25%) or hyper-intensity (6%). While thermal diffusivity was not significantly different between fibroid and myometrium tissues (0.217 ± 0.047 and 0.204 ± 0.039 mm² /s, respectively), the acoustic attenuation in fibroid tissue was significantly higher than myometrium (0.092 ± 0.021 and 0.052 ± 0.023 Np/cm/MHz, respectively). When comparing in vivo with ex vivo MRI T1 and T2 measurements in fibroids and myometrium tissue, the only difference was found in the fibroid T2 property (P < 0.05). Finally, the developed perfusion protocol successfully maintained tissue viability in ex vivo tissues as evaluated through histological analysis.

CONCLUSIONS

This study developed an MR-compatible extracorporeal perfusion technique that effectively maintains tissue viability, allowing for the direct measurement of patient-specific MR, thermal, and acoustic property values for both fibroid and myometrium tissues. These measured tissue property values will enable further development and validation of treatment planning models that can be utilized during MRgFUS uterine fibroid treatments.

Efficient shear wave elastography using transient acoustic radiation force excitations and MR displacement encoding

PURPOSE

To present a novel MR shear wave elastography (MR-SWE) method that efficiently measures the speed of propagating wave packets generated using acoustic radiation force (ARF) impulses.

METHODS

ARF impulses from a focused ultrasound (FUS) transducer were applied sequentially to a preselected set of positions and motion encoded MRI was used to acquire volumetric images of the propagating shear wavefront emanating from each point. The wavefront position at multiple propagation times was encoded in the MR phase image using a train of motion encoding gradient lobes. Generating a transient propagating wavefront at multiple spatial positions and sampling each at multiple time-points allowed for shear wave speed maps to be efficiently created. MR-SWE was evaluated in tissue mimicking phantoms and ex vivo bovine liver tissue before and after ablation.

RESULTS

MR-SWE maps, covering an in-plane area of ~5 × 5 cm, were acquired in 12 s for a single slice and 144 s for a volumetric scan. MR-SWE detected inclusions of differing stiffness in a phantom experiment. In bovine liver, mean shear wave speed significantly increased from 1.65 ± 0.18 m/s in normal to 2.52 ± 0.18 m/s in ablated region (n = 581 pixels; P-value < 0.001).

CONCLUSION

MR-SWE is an elastography technique that enables precise targeting and excitation of the desired tissue of interest. MR-SWE may be particularly well suited for treatment planning and endpoint assessment of MR-guided FUS procedures because the same device used for therapy can be used as an excitation source for tissue stiffness quantification.

Simultaneous proton resonance frequency shift thermometry and T1 measurements using a single reference variable flip angle T1 method

PURPOSE

Implement simultaneous proton resonance frequency (PRF) shift and T₁ measurements with equivalent temporal resolution using a single reference variable flip angle method. This novel method allows for simultaneous thermometry in both aqueous and fatty tissue.

METHODS

This method acquires a single reference image at the lower flip angle and all dynamic images at the higher angle. T₁ is calculated using a single reference variable flip angle method, which accounts for the reference image temperature remaining constant. Monte Carlo simulations determined the optimal dynamic flip angle for combined PRF and T₁ measurements. This method was evaluated in MR-guided focused ultrasound heating experiments using a gelatin phantom and human cadaver breasts. In vivo measurement precision was demonstrated in healthy female volunteers under nonheating conditions.

RESULTS

Temperature rise during MR-guided focused ultrasound heating was measured in aqueous tissue with both PRF and T₁ . Both measures show good qualitative agreement in both space and time in aqueous tissue. The T₁ change due to temperature increase was measured in fat, demonstrating the expected temporal response. The dynamic flip angle that produces optimal SNR for PRF measurements is lower than the optimal angle for T₁ measurements, necessitating the selection of a compromise angle.

CONCLUSION

The single reference variable flip angle method provides a reliable way to simultaneously measure PRF temperature and T₁ change and overcomes PRF's inability to simultaneously monitor temperature in aqueous and adipose tissues. Future work will calibrate T₁ change to temperature, enabling real-time temperature in fat and increasing patient safety and treatment efficacy during thermal interventional treatments.

Validation of hybrid angular spectrum acoustic and thermal modelling in phantoms

In focused ultrasound (FUS) thermal ablation of diseased tissue, acoustic beam and thermal simulations enable treatment planning and optimization. In this study, a treatment-planning methodology that uses the hybrid angular spectrum (HAS) method and the Pennes' bioheat equation (PBHE) is experimentally validated in homogeneous tissue-mimicking phantoms. Simulated three-dimensional temperature profiles are compared to volumetric MR thermometry imaging (MRTI) of FUS sonications in the phantoms, whose acoustic and thermal properties are independently measured. Additionally, Monte Carlo (MC) uncertainty analysis is performed to quantify the effect of tissue property uncertainties on simulation results. The mean error between simulated and experimental spatiotemporal peak temperature rise was +0.33°C (+6.9%). Despite this error, the experimental temperature rise fell within the expected uncertainty of the simulation, as determined by the MC analysis. The average errors of the simulated transverse and longitudinal full width half maximum (FWHM) of the profiles were -1.9% and 7.5%, respectively. A linear regression and local sensitivity analysis revealed that simulated temperature amplitude is more sensitive to uncertainties in simulation inputs than in the profile width and shape. Acoustic power, acoustic attenuation and thermal conductivity had the greatest impact on peak temperature rise uncertainty; thermal conductivity and volumetric heat capacity had the greatest impact on FWHM uncertainty. This study validates that using the HAS and PBHE method can adequately predict temperature profiles from single sonications in homogeneous media. Further, it informs the need to accurately measure or predict patient-specific properties for improved treatment planning of ablative FUS surgeries.

Flipping the classroom: a national pilot curriculum for physiology in neonatal-perinatal medicine

OBJECTIVE

To explore interest, feasibility, perceived effectiveness, and acceptability of a standardized, national physiology curriculum for neonatal-perinatal medicine (NPM) fellows using online videos for knowledge acquisition paired with flipped classrooms (FCs) for knowledge application.

STUDY DESIGN

Two educational programs pairing online videos with FCs were developed and peer-reviewed. These programs were piloted at five institutions. Fellows completed surveys, and fellows and educators participated in focus groups after their FC experiences.

RESULTS

Thirty-five fellows responded to the survey. Forty-one fellows and six educators participated in focus groups. Fellows and educators preferred online videos paired with FCs over didactic teaching and perceived them to be effective for knowledge acquisition and application.

CONCLUSION

Fellows and educators preferred FC learning over traditional didactics and reported that FCs facilitated creation of a learning community, fostering active learning. The favorable response toward this pilot project and the feasibility of its use supports further development of a standardized NPM physiology curriculum for fellowship training.

Experimental assessment of phase aberration correction for breast MRgFUS therapy

PURPOSE

This study validates that phase aberrations in breast magnetic resonance-guided focussed ultrasound (MRgFUS) therapies can be corrected in a clinically relevant time frame to generate more intense, smaller and more spatially accurate foci.

MATERIALS AND METHODS

Hybrid angular spectrum (HAS) ultrasound calculations in an magnetic resonance imaging (MRI)-based tissue model, were used to compute phase aberration corrections for improved experimental MRgFUS heating in four heterogeneous breast-mimicking phantoms (n = 18 total locations). Magnetic resonance(MR) temperature imaging was used to evaluate the maximum temperature rise, focus volume and focus accuracy for uncorrected and phase aberration-corrected sonications. Thermal simulations assessed the effectiveness of the phase aberration correction implementation.

RESULTS

In 13 of 18 locations, the maximum temperature rise increased by an average of 30%, focus volume was reduced by 40% and focus accuracy improved from 4.6 to 3.6 mm. Mixed results were observed in five of the 18 locations, with focus accuracy improving from 6.1 to 2.5 mm and the maximum temperature rise decreasing by 8% and focus volume increasing by 10%. Overall, the study demonstrated significant improvements (p < 0.005) in maximum temperature rise, focus volume and focus accuracy. Simulations predicted greater improvements than observed experimentally, suggesting potential for improvement in implementing the technique. The complete phase aberration correction procedure, including model generation, segmentation and phase aberration computations, required less than 45 min per sonication location.

CONCLUSION

The significant improvements demonstrated in this study i.e., focus intensity, size and accuracy from phase aberration correction have the potential to improve the efficacy, time-efficiency and safety of breast MRgFUS therapies.

Multiecho pseudo-golden angle stack of stars thermometry with high spatial and temporal resolution using k-space weighted image contrast

PURPOSE

Implement and evaluate a 3D MRI method to measure temperature changes with high spatial and temporal resolution and large field of view.

METHODS

A multiecho pseudo-golden angle stack-of-stars (SOS) sequence with k-space weighted image contrast (KWIC) reconstruction was implemented to simultaneously measure multiple quantities, including temperature, initial signal magnitude M(0), transverse relaxation time ( T2*), and water/fat images. Respiration artifacts were corrected using self-navigation. KWIC artifacts were removed using a multi-baseline library. The phases of the multiple echo images were combined to improve proton resonance frequency precision. Temperature precision was tested through in vivo breast imaging (N = 5 healthy volunteers) using both coronal and sagittal orientations and with focused ultrasound (FUS) heating in a pork phantom using a breast specific MR-guided FUS system.

RESULTS

Temperature measurement precision was significantly improved after echo combination when compared with the no echo combination case (spatial average of the standard deviation through time of 0.3-1.0 and 0.7-1.9°C, respectively). Temperature measurement accuracy during heating was comparable to a 3D seg-EPI sequence. M(0) and T2* values showed temperature dependence during heating in pork adipose tissue.

CONCLUSION

A self-navigated 3D multiecho SOS sequence with dynamic KWIC reconstruction is a promising thermometry method that provides multiple temperature sensitive quantitative values. Magn Reson Med 79:1407-1419, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Effect of applied energy in renal sympathetic denervation with magnetic resonance guided focused ultrasound in a porcine model

Background

Past catheter-based and focused ultrasound renal denervation studies have indicated that procedure efficacy is related to the number of ablations performed or the amount of energy used for the ablation. This study extends those prior results and investigates energy level effects on the efficacy of MR guided focused ultrasound renal denervation performed in a porcine model.

Methods

Twenty-four normotensive pigs underwent unilateral denervation at three intensity levels. The applied intensity level was retrospectively de-rated to account for variability in animal size. Efficacy was assessed through evaluating the norepinephrine present in the kidney medulla and through histological analysis. The treatment was performed under MRI guidance including pre- and post-procedure T1-weighted and quantitative T1 and T2 imaging. During treatment, the temperature in the near field of the ultrasound beam was monitored in real time with MR temperature imaging. Energy delivery in the regions surrounding the renal artery was independently confirmed through an invasive fiberoptic temperature probe placed in the right renal artery.

Results

Animals that underwent denervation at a de-rated acoustic intensity of greater than 1.2 kW/cm² had a significantly lower norepinephrine concentration in the kidney indicating successful denervation. Images obtained during the treatment indicated no tissue changes in the kidneys as a function of the procedure but there were significant T1 changes present in the right lumbar muscles, although only one animal had indication of muscle damage at the time of necropsy.

Conclusions

While MR guided focused ultrasound renal denervation was found to be safe and effective in this normotensive animal model, the results indicated the need to incorporate patient-specific details in the treatment planning of MRgFUS renal denervation procedure.

Effect of a Prior History of Overweight on Return of Menses in Adolescents With Eating Disorders

PURPOSE

The purpose of this study was to determine whether a history of overweight, weight suppression, and weight gain during treatment have an effect on return of menses (ROM) in adolescents with eating disorders (EDs).

METHODS

Retrospective chart review of female adolescents presenting to an ED program from January 2007 to June 2009.

RESULTS

One hundred sixty-three participants (mean age, 16.6 ± 2.1 years) met eligibility criteria. The mean median body mass index percent at ROM for those previously overweight was 106.1 ± 11.7 versus 94.2 ± 8.9 for those not previously overweight (p < .001). Both groups needed to gain weight for ROM. Greater weight suppression (odds ratio, 0.90; 95% confidence interval, 0.84-0.98; p = .013) was associated with lower likelihood of ROM, and greater weight gain during treatment (odds ratio, 1.20; 95% confidence interval, 1.07-1.36; p = .002) was associated with higher likelihood of ROM in those not previously overweight.

CONCLUSIONS

Previously overweight amenorrheic patients with EDs needed to be at a higher median body mass index percent for ROM compared to those who were not previously overweight.

Enhanced efficacy of combination heat shock targeted polymer therapeutics with high intensity focused ultrasound

Combination of polymer therapeutics and hyperthermia has been shown to enhance accumulation in selectively heated tumor tissue. The additional use of heat shock (HS)-targeting towards tumor tissues can further enhance accumulation and retention, and improve therapeutic outcomes. In this work, high intensity focused ultrasound (HIFU) was used to generate hyperthermia in prostate tumor tissue. Upregulation of the cell surface HS receptor glucose regulated protein 78 kDa (GRP78) was observed after treatment with HIFU hyperthermia which was then targeted by specific HS-targeting peptides. We used the peptide sequence WDLAWMFRLPVG attached to the side chains of water-soluble N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers containing docetaxel (DOC) conjugated via a lysosomally degradable linker. It was shown that HIFU-mediated HS-targeted copolymer-DOC conjugates improved treatment efficacy in a murine prostate tumor xenograft model. These results show that the use of HIFU hyperthermia in combination with HS-targeted polymer-drug conjugates has potential to improve therapeutic outcomes in prostate cancer treatment.