| Popis: |
Anastomotic leakage (AL) is the leading cause of morbidity and mortality after bowel anastomosis, a surgical procedure used to restore luminal continuity after bowel tumour resection. Even after years of research, its occurrence has not decreased, and new methods of monitoring the wound and predicting anastomotic failure are therefore urgently required. Here we propose the use of an internal coil to increase the signal-to-noise ratio (SNR) during Magnetic Resonance Imaging (MRI ) and/or Spectroscopy (MRS). Both methods may be used to identify ischemia and oedema, considered to be clinical indications of AL. The annular nature of the anastomotic surgical wound suggests the use of a coil with an annular field-of view, mounted on a Biodegradable Anastomosis Ring (BAR), a surgical device commonly used as a temporary mechanical support that is fragmented and excreted from the body after wound healing. The proposed solution is a Magneto-Inductive (MI) ring resonator, based on a set of magnetically coupled L-C resonators. Its advantages are that its separate elements fit comfortably inside the BAR, are not mechanically connected, and consequently may be fragmented and excreted with the BAR itself. A coupled pair of 8-element MI ring resonators is proposed, operating on an anti-symmetric spatial mode to avoid coupling to the B1 field during the excitation phase of MRI. However, the electrical response of an early prototype shows that insufficient rejection of uniform fields is achieved using the most obvious arrangement. Therefore, a search of the effect of design parameters on the spectra of resonant modes supported by the electrical system is carried out to identify an arrangement offering improved decoupling. A suitable design is developed, based on physical overlap between adjacent elements in the same ring, which alters the sign and magnitude of a key magnetic coupling coefficient. MRI fields-of-view are theoretically estimated for several different arrangements for signal extraction, including devices that are mutually coupled to an external read coil and directly coupled devices. Difficulties with combining mutual coupling and B1 field rejection are identified, and wired connections are proposed as a solution. It is found that a device with a single such connection gives a sensitivity pattern with partial symmetry, whereas a quadrature tap restores full symmetry. In vitro 1H MRI is then carried out at 1.5 T and 3.0 T using agar gel immersion phantoms, both for mutually coupled systems and for directly coupled systems. As expected, mutual coupling is found to be an unsuitable readout method for a device operating on its anti-symmetric mode, but does allow analysis of the effectiveness of B1 field decoupling. Directly coupled devices operate essentially as expected, providing up to 15-fold local enhancement in SNR, compared to the system body coil. |
