Arnulf Oppelt

On the heating of linear conductive structures as guide wires and catheters in interventional MRI

The interest in performing vascular interventions under magnetic resonance (MR) guidance has initiated the evaluation of the potential hazard of long conductive wires and catheters. The objective of this work is to present a simple analytical approach to address this concern and to demonstrate the agreement with experimental results. The first hypothesis is that a long conductive structure couples with the electric field of the radio frequency (RF) transmit coil. The second hypothesis is that this coupling induces high voltages near the wire ends. These voltages can cause tissue heating due to induced currents. The experimental results show an increase in coupling when moving a guide wire toward the wall of an RF transmit coil, documented with a temperature increase of a saline solution in close proximity to the tip of the guide wire. The coupling of the wire not only presents a potential hazard to the patient, but also interferes with the visualization of the wire. A safe alternative would be the use of nonconducting guide wires. J. Magn. Reson. Imaging 2001;13:105–114.

System for generating images and spacially resolved spectra of an examination subject with nuclear magnetic resonance

For providing nuclear magnetic resonance equipment with reduced noise generation, gradient coils are disposed in the vacuum of the cryostat which accepts the superconducting magnetic coils for generating the fundamental field. Under given conditions, the radio-frequency transmission and reception coils or antennae are also located in the vacuum.

Intraoperative MRI with a rotating, tiltable surgical table: A time-use study and clinical results in 122 patients

OBJECTIVE The objective of our study was to evaluate intraoperative low-field MRI for the frequency and duration of imaging sessions needed during surgery, the direct additional procedure time attributable to imaging, and the proportion of cases in which information provided by intraoperative MRI led to a change in the procedure or otherwise was deemed valuable by operating surgeons. MATERIALS AND METHODS One hundred twenty-two patients (65 males, 57 females; age range, 6-77 years; mean age, 43.8 years) underwent 130 neurosurgical and ENT procedures (106 craniotomies, 17 transsphenoidal pituitary resections, three biopsies, three intracranial cyst aspirations or injections, and one skull base resection) in a specially designed surgical MRI suite equipped with a 0.2-T imager and a prototype rotating, tiltable surgical table. The intraoperative MR sequences included free induction with steady-state precession (fast imaging with steady-state precession [FISP]), steady-state free precession T2-weighted, reverse fast imaging with steady-state free precession (PSIF), FLASH, spin-echo T1-weighted, turbo spin-echo (TSE) T2-weighted, and TSE FLAIR. Each case was analyzed for the number of imaging sessions, duration of each session, total imaging time during surgery, and impact of imaging information on procedure. RESULTS Each patient underwent between one and five intraor postoperative imaging sessions. Imaging times were 1.7 seconds-8 minutes 31 seconds per sequence. The mean total imaging time was 35 minutes 17 seconds per surgical procedure. Imaging was continuous during biopsy and cyst aspiration procedures and averaged 200.67 and 54.66 minutes, respectively. Additional surgical resection based on intraoperative imaging findings was performed in 72.8% of the cases. CONCLUSION Intraoperative low-field MRI provides valuable information for surgical decision making that is predominantly related to detection of residual tumor and the exclusion of complications. The benefits of this technology surpass the time cost associated with its implementation when using proper imaging strategies.

Device for generating images of an object under examination using nuclear magnetic resonance

The invention involves a device for generating images using nuclear magnetic resonance in which coils are provided for applying magnetic fields to the patient and for recording the displacement of the atomic nuclei of the patient from their equilibrium state by a high-frequency magnetic excitation pulse. Calibration devices are attached in the coils so that, for each image plane selected, they are at least partially intersected by the image plane during measurement of a patient.

An integrated head‐holder/Coil for intraoperative MRI in open neurosurgery

With the invention of “open” magnetic resonance imaging (MRI) systems, access to the patient is possible during the imaging procedure. An important application of these systems is intraoperative MRI to control the extent of resection during tumor surgery. Up to now flexible surface coils wrapped around, or placed at each side of the head, were used for imaging. These flexible coils have several disadvantages such as unreliability, interindividual problems, difficult handling, poor hygienic properties, and often unsatisfactory or inhomogeneous image quality. To solve most of these problems, an MR‐compatible head‐holder in combination with an integrated surface coil for use in a 0.2 T C‐shaped magnet was developed. Forty‐eight patients with known cranial tumors underwent MRI intraoperatively. In 32 patients (67%), residual tumor was found, and additional surgical resection was performed. The integrated head‐holder/coil is a safe and practical tool for intraoperative MRI, providing efficient and reliable resection control during neurosurgical procedures. J. Magn. Reson. Imaging 2000;11:564–567.

Method for the operation of a nuclear magnetic resonance apparatus for the fast identification of the longitudinal relaxation time T1

A method for the operation of a nuclear magnetic resonance apparatus excites the spin system of an examination subject by applying a fundamental magnetic field as well as by irradiation with a sequence of radio-frequency pulses and the nuclear magnetic resonance signals emitted by the spin system are measured. The chronological development of the longitudinal magnetization of the spin system can be tracked by a sequence of partial read pulses each having a flip angle of less than 90° and thus a fast identification of the longitudinal relaxation time T1 can be achieved. In combination with applied gradient fields, the method can be applied in imaging nuclear magnetic resonance tomography.

Sicherheitsaspekte bei der interventionellen MRT

Because of its high soft-tissue contrast, Magnetic Resonance Imaging (MRI) is used increasingly for guidance and control of minimal invasive and neurological surgical procedures. Besides common precautions during an MRI investigation, special attention has to be paid to the consequences of MR compatibility, accuracy of localisation of interventional tools and geometrical distortions. As a new application of interventional MR intravascular procedures are developing that involve the introduction of guidewires, catheters and miniaturized coils with their leads into the blood vessels. Resonant currents and high electric fields can develop at the conductor ends, possibly causing burning lesions.

Surface coil with calibration substance for use in a nuclear magnetic resonance apparatus

A coil for conducting examination of a patient in a nuclear magnetic resonance apparatus has a flat surface coil for spectroscopic in vivo examination of the patient and a calibration substance disposed within the coil. The coil is placed in the patient during examination, and the calibration subject supplies a nuclear magnetic resonance signal measurable by the coil as a reference.

A new pulse sequence for determining T1 and T2 simultaneously.

Determination of the relaxation times T1 and T2 which are important for tissue characterization generally requires the use of different pulse sequences in magnetic resonance imaging. In this study, a new pulse sequence which facilitates simultaneous determination of the T1 and T2 times is presented. Determination takes place in this case pixel by pixel from the measured images. The measuring time corresponds in this case approximately to that of a normal spin-echo sequence with long repetition time and two data acquisitions. The functional dependence of the accuracy of the T1 and T2 determination upon external errors, e.g., angle of rotation errors, is discussed. The tissue contrast behavior of the individual echoes is shown and its dependence on pulse parameters is explained.

Magnetic resonance imaging of aneurysms and thrombi

This is a report of the first systematic investigation of the qualitative and quantitative diagnosis of aneurysms in the regions of the left ventricle and thoracic and abdominal aorta plus proof of intracavitary thrombi in the heart and the aorta, as well as aneurysms in the superior and inferior vena cava. For diagnosis of the heart, ECG gating is an absolute necessity, but for the analysis of abdominal aortic aneurysms it only leads to a considerable improvement of the spatial resolution. For differential diagnosis of the blood flow and intracavitary clots in the heart and the aorta, use of a second or even multiple echoes is needed. Also, digital subtraction between the first and second echoes (magnetic resonance digital subtraction) can assist in assessing flow. When dissecting aortic aneurysm is suspected and in cases when risk of perforation of ventricular and aortic aneurysms is present, MR offers particular advantages, since it is noninvasive and few scans can provide all the information that is required.