Peter D. Jakab

Prototype miniature endoluminal MR imaging catheter.

PURPOSE The feasibility of a miniature endoluminal magnetic resonance (MR) detection coil was investigated for imaging mural and perimural anatomy of small, tubular structures. MATERIALS AND METHODS To this end, remotely tunable, single-loop, multiturn, receive-only radio-frequency coils, housed in 6-9-F arterial sheaths, were built. A 1.9-T imager was used. Phantom excitation was accomplished with a 62-mm-diameter bird-cage quadrature coil, and ex vivo specimen excitation was accomplished with a single-turn, untuned wire loop. Phantom images obtained with use of a 9-F catheter coil showed a signal-to-noise improvement on the border of 20 dB compared with images obtained with the quadrature coil. An 8-F catheter coil was used to obtain high-resolution (100 microns in-plane pixel size, 500 microns section thickness) spin-echo images (repetition time = 2,400 msec, echo time = 53 msec) of the wall of a fresh ex vivo human popliteal artery. RESULTS Prospectively, these images were suggestive of the presence of diffuse intimal hyperplasia, medial calcification, and focal atherosclerotic plaque. These findings were confirmed histologically. Three-dimensional restacking of the axial images simplified examination of the normal layers and pathologic changes within the wall. The improved signal-to-noise characteristics of these miniature coils permit fast high-resolution imaging, allowing visualization of microscopic anatomic details. CONCLUSIONS With further development, this technology may be useful for studying atherosclerosis and for providing imaging guidance during endoluminal MR interventions.

Swimming capsule endoscope using static and RF magnetic field of MRI for propulsion

Capsule endoscopy is a promising technique for diagnosing diseases in the small intestines. Here we propose a miniature swimming mechanism that uses MRIs magnetic fields for both propulsion and wireless energy delivery. Our method uses both the static and radio frequency (RF) magnetic field inherently available in MRI to generate propulsion force. The propulsion force is produced by a swimming tail containing waving beam consisting of three coils in a row. Alternating current in the coils acting on the static magnetic field of the MRI will generate waving movement to produce a propulsion force. RF magnetic field will provide power to generate the alternating currents in the coils. We developed a theoretical model to predict sinusoidal waves produced by the waving beam using the Euler-Bernoulli beam equation and multiple- input multiple-output system were solved using antenna design theory. This numerical model predicted that the maximal propulsion from a 10 mm long tail can produce a velocity of 7.9 mm/s force of 5.5 mN when placed in a 3T static magnetic field. A validation study with a single coil demonstrated that the theoretical and numerical model predicts well the proposed swimming mechanism and it is useful for the fabrication of swimming tails.

Flagellar swimming for medical micro robots: Theory, experiments and application

Flagellar swimming is one of the swimming methods used by micro-organisms to advance in aquatic environment. The undulating motion of the flagella was successfully imitated to create propulsion for a medical swimming micro robot. The influence of a head section on the performance of such a robot is shown analytically. Swimming experiments demonstrate forward and backward swimming of a novel magnetically driven swimming tail. We intend to use the flagellar swimming tail in a three tail configuration for neurosurgical intervention in the ventricular system.

Magnetic resonance imaging of interstitial laser photocoagulation

We have previously demonstrated the detection of reversible and irreversible changes on MR images oflaser energy deposition and tissue heating and cooling1. It is possible to monitor and control energy deposition during interstitial laser therapy. This presentation describes some first steps toward optimizing the power and total energy deposited in various tissues in vivo, by analyzing the irreversible tissue changes and their spatial distribution as revealed by spin echo imaging. We used various power settings of an Nd.YAG laser delivered by a fiber optic inserted into several tissues (brain, muscle, liver) of anesthetized rats and rabbits. MR imaging was performed at 1.9 T. Photothermally-produced lesions were seen on both T1- and Ta-weighted images. The overall size of the lesions correlated with the magnitude of the energy applied. The MR image appearance depended not only on the laser energy but also on the way it was delivered, on the type of tissue, and the MR pulse sequence applied. While Ti-weighted images adequately demonstrated an area of tissue destruction, T2- weighted images showed a more heterogeneous and more extensive lesion which could be better correlated with the complex histological representation of these lesions. Typically, when rabbit brain, liver, and muscle had been exposed to laser power of 2.5 Watts for a range of 55 to 120 seconds, depending on the tissue, a central area of signal void was surrounded by an inner hypointensity and an outer hyperintensity on T2-weighted images. The 3D extent of the lesions was well demonstrated on multislice images, providing correlation of the affected volumes seen on MRI with volumes seen in histological or histochemical preparations. We are developing an analytical model of laser heating and its effect on MR images to assess whether heating during imaging will produce unacceptable artifacts during surgery. The effect of heating is modeled as a change in magnetization during image acquisition. The region in which the change occurs is blurred by the Fourier transform of the change in magnetization as a function of time. Thus, blurring is minimized when changes occur slowly, compared to image acquisition times. We conclude that MRI can demonstrate the 3D extent of the lesions induced by lasers and can be used to investigate and optimize the control of induced tissue change within the affected volume.

Magnetic resonance imaging and spectroscopy to monitor and control experimental laser surgery

We have proposed the use of MRI for monitoring and control of interstitial laser surgery, in order to improve the accuracy and reduce the invasiveness of these procedures. To expand the knowledge base about the MR appearance of laser-induced tissue damage, we applied MR imaging and phosphorus-31 MR spectroscopy to detect the changes induced in various tissues by radiation from an Nd:YAG laser at 1060 nm wavelength delivered interstitially through a fiber optic waveguide. A range of laser energies was applied, and laser pulse parameters were varied. Proton MR images of the laser-produced lesions were compared with the histological appearance in brain and liver tissue of experimental animals. The spatial extent of laser effects differed among tissue types, and this was well reflected on MR images. The distribution of MR signal change resulting from different laser exposures was also demonstrated. Experimental laser surgery was performed in animal brain and bladder. Images taken before, during, and after laser irradiation allowed us to distinguish between reversible thermal and permanent effects. This information was utilized to tailor the destruction of preselected targets while minimizing damage to surrounding tissues. Qualitative changes were also revealed on phosphorus spectra. Irreversible lesions were characterized by overall line broadening and a decrease in AT?. There was also a large relative increase in the inorganic phosphate region of the spectrum. These demonstrations are a big step toward achieving our ultimate goal, the development of MR-controlled laser surgery.