Calculation of MRI-induced heating of an implanted medical lead wire with an electric field transfer function

PURPOSE: To develop and demonstrate a method to calculate the temperature rise that is induced by the radio frequency (RF) field in MRI at the electrode of an implanted medical lead. MATERIALS AND METHODS: The electric field near the electrode is calculated by integrating the product of the tangential electric field and a transfer function along the length of the lead. The transfer function is numerically calculated with the method of moments. Transfer functions were calculated at 64 MHz for different lengths of model implants in the form of bare wires and insulated wires with 1 cm of wire exposed at one or both ends. RESULTS: Heating at the electrode depends on the magnitude and the phase distribution of the transfer function and the incident electric field along the length of the lead. For a uniform electric field, the electrode heating is maximized for a lead length of approximately one-half a wavelength when the lead is terminated open. The heating can be greater for a worst-case phase distribution of the incident field. CONCLUSION: The transfer function is proposed as an efficient method to calculate MRI-induced heating at an electrode of a medical lead. Measured temperature rises of a model implant in a phantom were in good agreement with the rises predicted by the transfer function. The transfer function could be numerically or experimentally determined.     

Improvement in diagnosis of breast tumour using ultrasound elastography and echography: A phantom based analysis

Due to the isoechoic nature of lesions and their poor contrast with neighbouring tissue, a lesion may remain undetected in ultrasound B mode imaging for cancerous tissue. Imaging of the elastic properties of tissue provides new information which is collateral to tissue pathology. This study provides quantitative analysis of improvements in tumour diagnosis when the ultrasound B mode imaging is combined with elastography. Quantification was based on the textural parameters measured from the ultrasound B mode image and strain measured from the elastogram. The ability of a parameter to discriminate between diseased cases and normal cases was evaluated using receiver operating characteristic (ROC) analysis. Polyacrylamide gel based tissue mimicking phantoms with embedded inclusions of varying stiffness were used for the analysis.     

两种固体模体水等效特性的比较研究

目的 对两种固体模体的水等效性及相关校准因子进行检测.方法 对美国MED-TEC公司虚拟水和德国PTW公司RW3两种同体模体不同深度处进行X线、电子线照射测量,得到深度电离量曲线与水模体中的进行比较,而水等效因子两种模体直接比较.按照国际原子能机构定义得到不同能量电子线深度比例因子(Cpl)与注量比例因子(hpl),比...     

牙体牙髓病学仿真实验教学的探究

从牙体牙髓病学专业特点出发,阐述了仿真人头模拟系统在实验教学中的应用。认为此系统不但能够增强学生的学习积极性、巩固基础理论知识,而且能够提高临床操作技能,该系统已成为现代口腔医学实验教学必备的设备和手段,值得进一步在口腔教学中推广应用。     

Quantitative micro-computed tomography: a non-invasive method to assess equivalent bone mineral density

One of the many applications of micro computed tomography (microCT) is to accurately visualize and quantify cancellous bone microstructure. However, microCT based assessment of bone mineral density has yet to be thoroughly investigated. Specifically, the effects of varying imaging parameters, such as tube voltage (kVp), current (microA), integration time (ms), object to X-ray source distance (mm), projection number, detector array size and imaging media (surrounding the specimen), on the relationship between equivalent tissue density (rhoEQ) and its linear attenuation coefficient (micro) have received little attention. In this study, in house manufactured, hydrogen dipotassium phosphate liquid calibration phantoms (K2HPO4) were employed in addition to a resin embedded hydroxyapatite solid calibration phantoms supplied by Scanco Medical AG Company. Variations in current, integration time and projection number had no effect on the conversion relationship between micro and rhoEQ for the K2HPO4 and Scanco calibration phantoms [p>0.05 for all cases]. However, as expected, variations in scanning tube voltage, object to X-ray source distance, detector array size and imaging media (referring to the solution that surrounds the specimen in the imaging vial) significantly affected the conversion relationship between mu and rhoEQ for K2HPO4 and Scanco calibration phantoms [p<0.05 for all cases]. A multivariate linear regression approach was used to estimate rhoEQ based on attenuation coefficient, tube voltage, object to X-ray source distance, detector array size and imaging media for K2HPO4 liquid calibration phantoms, explaining 90% of the variation in rhoEQ. Furthermore, equivalent density values of bovine cortical bone (converted from attenuation coefficient to equivalent density using the K2HPO4 liquid calibration phantoms) samples highly correlated [R2=0.92] with the ash densities of the samples. In conclusion, Scanco calibration phantoms can be used to assess equivalent bone mineral density; however, they cannot be scanned with a specimen or submerged in a different imaging media. The K2HPO4 liquid calibration phantoms provide a cost effective, easy to prepare and convenient means to perform quantitative microCT analysis using any microCT system, with the ability to choose different imaging media according to study needs. However, as with any liquid calibration phantom, they are susceptible to degradation over time.     

Rectum dose analysis employing a multi-purpose brachytherapy phantom

PURPOSE: It is difficult to reproduce a brachytherapy measurement because of changes in the rectal shape during inter-fraction. We constructed a multi-purpose brachytherapy phantom (MPBP) and reproduced the same conditions found in actual therapy. We further attempted to apply the measured optimal dose to reduce rectal complications. METHODS: A measured dose was administered at rectal reference point R1 using a diode detector in four patients who used a tandem and ovoid in brachytherapy for carcinoma of the cervix. A total number of 20 rectal dose measurements were performed five times per patient. In addition, discrepancies in the set-up of the diode detector were analyzed with each repetitive measurement. After reproducing the same conditions as found in actual therapy using a multi-function applicator (MFA) in the multi-purpose brachytherapy phantom constructed for this study, the dose was measured at reference points in the rectum using a thermoluminescence dosimeter (TLD). RESULTS: According to the discrepancies measured in the set-up using a diode detector, Patient 1 showed a maximum value of 11.25 +/- 0.95 mm in the Y direction, Patients 2 and 3 exhibited 9.90 +/- 2.40 mm and 20.85 +/- 4.50 mm in the Z direction, respectively. Patient 4 showed 19.15 +/- 3.33 mm in the Z direction. In addition, values of the mean dose according to the position of the diode detector were recorded as 122.82 +/- 7.96-323.78 +/- 11.16 cGy. In the measured results for TLD in an MPBP, relative error for Patients 1 and 4 at the rectal reference point R2 were a maximum of 8.6 and 7.7%, respectively. For Patients 2 and 3 they were 1.7 and 1.2%, respectively. Furthermore, the dose measured at point R1 and R2 exhibited values approximately 1.7-8.6% higher than the dose calculated in advance, excluding point R1 in Patient 2. The discrepancies in the set-up owing to repetitive measurements and alterations in dosage according to these changes were not analyzed. It was evident that the relative error between the calculated and measured value was within 15%, which was allowable according to the recommendations by the American Association of Physicists in Medicine (AAPM). CONCLUSIONS: The multi-purpose brachytherapy phantom constructed for this study successfully reproduced an optimal dose measured under the same conditions found in actual therapy in which the dose was precisely analyzed at a rectal reference point. In addition, these results were considered reliable and applicable for dose optimization before applying therapy using the measured data from the phantom in order to reduce rectal complications.     

Estimation of the transverse strain tensor in the arterial wall using IVUS image registration

Intravascular ultrasound (IVUS) elastography is an imaging technique that obtains the local mechanical properties of the artery wall and atherosclerotic plaques through strain measurements using IVUS. Knowledge of these mechanical properties may provide crucial information that can help in estimating plaque composition and its vulnerability. Here, we present a new method to estimate the transverse strain tensor of the arterial wall based on nonrigid image registration using IVUS images. This method registers a pair of images acquired at a vessel site under different levels of luminal pressure. The 2-D displacement field in the vessel cross-section is estimated from image registration; then the displacement field is used to calculate the 2-D local strain tensor. From the strain tensor, the strain in any direction in the cross-section can be obtained; here, the radial and circumferential strain distributions are presented. This strain estimation method has been validated with synthetic motion IVUS images and evaluated using the IVUS images of a polyvinyl alcohol cryogel phantom. The accuracy of the estimated strain and the ability of the method to overcome IVUS system noise are demonstrated.     

Construction of a novel port wine stain phantom and measurement of colour by digital imaging and reflectance spectrophotometry

A novel skin phantom is described that is constructed with quantifiable amounts of skin pigments, as well as a light scattering medium in the form of barium sulphate suspension. The two predominant skin pigments (melanin and haemoglobin) are varied in controlled amounts within the phantoms to simulate skin colour in different situations. The phantoms were devised in order to simulate the changes in skin colour particularly after laser treatment of port wine stains, where superficial cutaneous vascularity is reduced. Preliminary investigations of two techniques for skin colour assessment were subsequently carried out so that their suitability for measuring colour in the skin phantoms could be considered. A specifically designed device was constructed to enable repeatable digital image capture of the phantoms. Further development of this skin phantom may enable comparison of techniques for skin colour assessment.     

21. Phantom Pain

Abstract: Phantom pain is pain caused by elimination or interruption of sensory nerve impulses by destroying or injuring the sensory nerve fibers after amputation or deafferentation. The reported incidence of phantom limb pain after trauma, injury or peripheral vascular diseases is 60% to 80%. Over half the patients with phantom pain have stump pain as well. Phantom pain can also occur in other parts of the body; it has been described after mastectomies and enucleation of the eye. Most patients with phantom pain have intermittent pain, with intervals that range from 1 day to several weeks. Even intervals of over a year have been reported. The pain often presents itself in the form of attacks that vary in duration from a few seconds to minutes or hours. In most cases, the pain is experienced distally in the missing limb, in places with the most extensive innervation density and cortical representation. Although there are still many questions as to the underlying mechanisms, peripheral as well as central neuronal mechanisms seem to be involved. Conservative therapy consists of drug treatment with amitriptyline, tramadol, carbamazepine, ketamine, or morphine. Based on the available evidence some effect may be expected from drug treatment. When conservative treatment fails, pulsed radiofrequency treatment of the stump neuroma or of the spinal ganglion (DRG) or spinal cord stimulation could be considered (evidence score 0). These treatments should only be applied in a study design.