BACKGROUND

Bio impedance measurements are known to be used to monitor fluid content in a subject's thorax.

Some bio impedance measurement implementations suggest using implanted electrodes. For example, bio impedance measurement is utilized in some pacemakers, where the bio impedance associated with tissues/organs located in-between the pacemaker electrodes and/or case is measured. For example, Medtronic Plc of Dublin Ireland offers OptiVol®, which provides fluid status monitoring through intrathoracic impedance measurements. OptiVol® Fluid Status Monitoring is utilized with implantable cardioverter defibrillator (ICD) or cardiac resynchronization therapy device (CRT-D). External devices, which are able to perform Bio impedance measurements, are also used, for example the Zoe fluid status monitor from NMT medical. The electrical principle behind bio impedance is Ohm's law. When a sinusoidal electrical current passes through a medium, a sinusoidal voltage drop in magnitude is generated. The electrical resistance gives the relationship between the electrical current and the corresponding voltage. An electrical current that passes through biological tissue is impeded by biological tissue resistance, which causes a phase shift between the sinusoidal electrical current and the sinusoidal voltage. Therefore, the tissue impedance that is calculated depends on both the magnitude of the signal and the phase shift that is generated. Depending on the frequencies that are used, different parameters can be measured within the body. High frequency signals are able to pass through the cell membrane and as a consequence both the intracellular and extracellular impedance is measured. Low frequency signals on the other hand only measure the impedance of the extracellular compartment. These properties can be used to measure different parameters. Depending on the desired output, two different bio impedance measuring techniques can be distinguished, i.e. single-frequency measurements and multi-frequency measurements. Multi-frequency measurements, also known as bio impedance spectroscopy (BIS), employ electrical currents of multiple frequencies through the body and can be used to determine static parameters (e.g. body composition, fluid status, etc.). Single-frequency signals on the contrary, can be used to measure more dynamic parameters, such as respiration (2). Single-frequency bio impedance devices that measure respiration can be used in patients suffering from pulmonary problems such as chronic obstructive pulmonary disease, sleep apnea or asthma.

Bio impedance technology has limitations due to the use of electrical currents for the measurements, causing accuracy, specificity and stability issues limiting the use of bio-impedance in relative and trend analysis.

SUMMARY

According to some embodiments of the present invention, there is provided a system. The system comprises a plurality of electrodes adapted to measure bio impedance measurements using electrical currents passing in a target thorax area of a target therebetween during a learning phase, at least one radiofrequency (RF) sensor adapted to measure RF interaction measurements of RF radiation interacting with the target thorax area during the learning phase. The processor(s) are adapted to calculate calibration function according to the bio impedance measurements and the RF interaction measurements and determine a target thorax area value by adjusting subsequent bio impedance measurements using subsequent electrical currents passing in the target thorax area during an operational learning phase using the calibration function.

Optionally, the system comprises the RF interaction measurements and the bio impedance measurements are a sequence of pairs of time correlated measurements, each the pair comprises one of the RF interaction measurements and one of the bio impedance measurements; wherein the calibration function is calculated according to the sequence of pairs.

More optionally, the pairs are taken during different times when different fluid levels are found in the target thorax area.

Optionally, the system comprises the target thorax area is a lungs area.

Optionally, the system comprises the calibration function is selected from a group consisting of a linear model, a logarithmic model, a logistic model, a sigmoid model and a generalized logistic function.

Optionally, the system comprises the calibration function is adapted according to at least one anatomical parameter of the target.

Optionally, the system comprises the calibration function is adapted according to at least one physiological parameter of the target.

According to some embodiments of the present invention, there is provided a method of determining a target thorax area value. The method comprises calculating calibration function according to bio impedance measurements using electrical currents passing in a target thorax area during a learning phase and RF interaction measurements of RF radiation interacting with the target thorax area during the learning phase and determining a target thorax area value of the target thorax area according to an adjustment of subsequent bio impedance measurements using subsequent electrical currents passing in the target thorax area during an operational phase by the calibration function.

Unless otherwise defined, all technical and/or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the invention, exemplary methods and/or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting.

