Locating steel needles in the human body using a SQUID magnetometer.

A technique has been developed, based on magnetic field measurements, to localize, in three dimensions, hypodermic and sewing needles lost in the human body. A theoretical model for the magnetic field generated by needles has been elaborated and experimentally validated. Using this model, the localization technique gives information about needle's centre, orientation and depth. The clinical measurements have been made using a SQUID system, with patients being moved under the sensor with the aid of an X-Y bed. The magnetic field associated with the remanent magnetization of the needle is acquired on-line and mapped over a plane. In all six cases that occurred, the technique allowed surgical localization of the needles with ease and high precision. This procedure can decrease the surgery time for extraction of foreign bodies by a large factor, and also reduce the generally high odds of failure.

Detecçäo magnética do circuito de ativaçäo de arritmia cardíaca induzida experimentalmente / Magnetic detection of activation circuit of cardiac arrhythmia experimentally induced

Com a finalidade de estudar o processo de ativação tecidual durante o "flutter atrial", foi desenvolvido um protocolo experimental para acompanhar de forma controlada a propagação circular da frente de onda de ativação elétrica em tecido isolado de coração de coelho através da técnica não-invasiva de detecção do campo magnético. As medidas foram feitas com blindagem magnética e com parâmetros de posição do tecido sob controle. Estudos da mgnitude e fase dos seis primeiros da série de Fourier foram realizados para melhorar a relação sinal/ruído e através de um processamento por mínimos quadráticos foi localizado o dipolo circular equivalente. A frente de onda de despolarização descreveu um movimento circular, caracterizando o padrão reentrante como mecanismo subjacente à arritmia

Abstract - ln order to study the tissue activation process during atrial flutter, an animal experimentation protocol has been developed to allow controlled studies of circular motion of the evolution front in rabbit atrium tissue through the non-invasive magnetic field detection technique. The measurement were performed inside a shielded chamber with the tissue position parameters under control. Noise-free time series were obtained by using the magnitude and phase of the first six harmonics of the Fourier Series and the position of the center of the equivalent circulating dipole were calculated using a least square procedure. The electrical activation of the tissue indeed appeared to move along a circular path, characterizing the rotating pattern of propagation as the underlying mechanism ofthe arrhythmia.

Magnetic localisation of a current dipole implanted in dogs.

In order to evaluate the difficulty in localising a current dipole due to volume conductor current contributions to the magnetocardiogram, accuracy of depth localisation of a commercial coaxial pacemaker cable, used as a current dipole, was studied in two experimental situations: immersed in a prismatic container with NaCl solution and introduced into the lower oesophagus of dogs. Isofield contour maps were obtained by interpolation of the magnetic field measured over a plane and perpendicular to it with a third-order gradiometer coupled to a SQUID. The dipole can be accurately localised in the prismatic container. The observation of an isofield map that is symmetric about the maximum-minimum axis when the dogs are in the dorsal decubitus position with the dipole in the cephalocaudal direction implies that internal inhomogeneities in the dog's volume conductor produce no appreciable effect on the magnetic field. Nevertheless, a large distortion of the magnetic field lines is observed and can be explained by calculations using models that take into account the external boundary of the volume conductor.