Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
2002-11-19
2004-11-09
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
Reexamination Certificate
active
06816266
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to vital sign monitors for detecting physiological parameters such as heartbeat, respiration, physical movement, blood pressure and other bodily activities of a patient for use in a magnetic resonance imaging (MRI) environment, confined care facilities (e.g., geriatrics) and in-hospital during surgery, postoperative recovery and intensive care units.
DESCRIPTION OF RELATED ART
1. Background of Vital Sign Monitoring in Magnetic Resonance Imaging Labs
Use of Magnetic Resonance Imaging (MRI) is rapidly growing in the U.S. and other parts of the world for investigations and diagnosis of many diseases. Statistical data published by In-vivo Research shows that over 18 million scans are performed per year in the U.S. alone. To better understand the problems of monitoring patients undergoing MRI scanning, a summary of the key steps required in generating a patient's image is provided:
1. A strong magnetic field, on the order of 1.5 to 2 Teslas (1 Tesla=10,000 Gauss, earth's magnetic field is 1 Gauss), is required to align all randomly oriented nuclei cells of the patient;
2. Radio frequency (RF) pulses, directed at the patient, are used in the presence of the external magnetic field, to cause the cell nuclei to absorb more energy producing magnetic resonance. This is generally referred to as super charging of the nuclei, which further changes their alignment from the original state;
3. The RF supercharged cell nuclei recover their original state of alignment within the magnetic field by re-emitting the absorbed RF energy. The RF signal re-emitted by each tissue is proportional to the difference between the energized magnetic resonance states and the original alignment states. Tissue imaging contrast develops as a result of the different rates of realignment;
4. Time varied magnetic field (TVMF) gradients are applied briefly to spatially encode the RF signals emitted from the patient tissues;
5. The RF coils in the MRI pick up these spatially encoded RF signals emitted from the tissues and are transformed by a computer into 2 or 3 dimensional images.
The strong magnetic field, RF pulses and/or TVMF gradients are referred to in this disclosure as “the MRI environment.”
Of the 18 million MRI scans done per year, approximately 10% of the patients are sedated during scanning for a variety of reasons. These patients are sedated using general, conscious intravenous (IV, spinal and epidural), orally administered (chewing tablets) or local anesthesia. If anesthesia is administered during MRI scanning, the law generally mandates that the patient's vital signs be monitored continuously. Monitoring of different vital parameters depends on the patient condition such as heart patient, pediatric or claustrophobic and the type of anesthesia administered. In the past, the attending anesthesiologist made the decision. More recently, the American Society for Anesthesia has published guidelines describing both the physiological monitoring equipment and parameters that must be measured for different patient types (“Both sedated and critically III require Monitoring during MRI”, Mark Schiebler, MD, et al. www.invivoresearch.com/topics/vital signs/survey.html) and White Paper on “MRI Safety”, Charlotte Bell, MD, et al., American College of Radiology,
AJR
2002; 178:1335-1347). According to these guidelines, the key parameters that must be continuously monitored include: EKG, Pulse Oximetry, Blood pressure, and Respiration by end tidal CO
2
/Capnograph or other methods.
Generally the three broad categories of problems are experienced when monitoring the vital signs of sedated patients in the MRI: 1) MRI environment induced interference in the vital sign monitoring equipment; 2) inadequate monitoring of respiration due to long separation between the patient and equipment producing latencies, and blockages in capnograph equipment lines; and 3) use of conventional ferrous-based EKG electrodes and lines cause burns to patients. Therefore the real time control is compromised.
Each of the above problems has been addressed in light of the monitoring equipment. Because the key-monitoring equipment used in detecting the vital signs in the MRI environment is the EKG, a brief interpretation of the EKG waveforms and problems associated with them during the scanning is provided below.
The electrocardiogram (EKG) measures changes in skin electrical voltage/potential caused by electrical currents generated by the myocardium. This electrical activity is typically represented by PQRST waveforms. The P wave reflects atrial depolarisation, while the QRS complex represents ventricular depolarisation, and the T wave ventricular repolarisation. Repolarisation is a process that occurs in many cells where the electrical potential across the cell membrane returns from the value during the action potential to that of the resting state (the resting membrane potential). Although the EKG shows heart rate and rhythm and can indicate myocardial damage, it does not directly give information on the adequacy of contraction. Normal electrical complexes can exist in the absence of cardiac output, a state known as pulseless electrical activity or electromechanical dissociation (EMD). The pulseless behavior is a special case of the myocardium but generally there is a direct correlation between the electrical activity as measured by the EKG with the mechanical activity as measured by phonocardiography. The foregoing is known to those skilled in the art and described in, “Phonocardiography: Measurement of Heart Sounds”, www.seas.smu.edu/~cd/EE5340/lect20/tsld011.htm, which is incorporated herein by reference in its entirety.
The EKG is generated using the 3, 5 or 12 lead configuration depending on the circumstances. For example in the MRI, usually 3 or 5 lead EKG is used because the patient is imaged while sedated but does not undergo surgery. At the end of each lead is an electrode that measures the small potential difference produced as a result of heart's electrical activity. By measuring for example the Rate, Rhythm, Impulse Axis, Hypertrophy and Infarction, information about the heart condition can be determined. These characteristic parameters are determined from the data manifested in V1 through V6 leads placed on specific locations on the chest and 1, 2, 3, AVR, AVL and AVF leads placed on the limbs, etc. Normal and abnormal rate and rhythm EKG waveforms that could be used to monitor vital signs as well as to determine other heart conditions are known to those skilled in the art and described in the article “Normal Sinus Rhythm”, www.rchc.rush.edu/rmawebfiles/abnl%20rhythm%20for%20parent %20body.htm and www.rchc.rush.edu/rmawebfiles/EKG%20for%20parents%20body.htm, which are incorporated herein by reference in their entireties.
Using the empirically correlated data not only provides clinical information about the five aspects of the heart's electrical activity but also provides variations that reflect other heart conditions associated within each of the five categories. It is well documented in the literature that the P wave signifies the generation of electrical impulses from the SA node, which travels down the AV node into the myocardial cells. The QRS complex represents the electrical impulse when it travels from the AV node into the Purkinje fibers into the myocardial cells and produces ventricular contractions. This signal can therefore provide information about the mechanical contraction of the heart's ventricles, which is followed by its relaxation (process of repolarization). This characteristic signal when used by itself or in conjunction with other waveforms reveals many heart conditions such as arrhythmias, abnormal rates and infarctions; provided the EKG waveforms are not corrupted.
A number of manufacturers such as HP, Colin Medical etc. make vital sign monitoring systems that are frequently used in operating rooms and outpatient surgical environments. These systems provide continuous monitoring capability of the EKG, pulse oxi
Jeffers Larry A.
Maida, Jr. John L.
Varshneya Deepak
Connolly Patrick
Pearne & Gordon LLP
Turner Samuel A.
Varshneya Deepak
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