WO1995002361A1

WO1995002361A1 - Doppler ultrasound trigger for use with mr

Info

Publication number: WO1995002361A1
Application number: PCT/US1994/007888
Authority: WO
Inventors: Stevan D. Zimmer; Claire T. Hovland
Original assignee: Zimmer Stevan D; Hovland Claire T
Priority date: 1993-07-15
Filing date: 1994-07-14
Publication date: 1995-01-26

Abstract

This is an apparatus and method for monitoring blood flow for use in triggering an MR procedure. A magnetic resonance (MR) device (52) is arranged for conducting a predetermined MR procedure on a predetermined body portion of an examined body, and the MR device (52) produces a strong static magnetic field. A Doppler ultrasound probe (12) for providing blood flow velocity information in the strong static magnetic field is arranged to sense blood flow in a vessel (14, 16, 18) in the examined body. The blood flow velocity information is monitored and analyzed (50, 51) for determining when to trigger the desired MR procedure.

Description

Doppler Ultrasound Trigger For Use With MR Background of the Invention

This invention relates to the use of a Doppler ultrasound probe which monitors blood flow in a vessel to determine when to trigger an MR procedure such as imaging, spectroscopy, angiography etc.

Magnetic Resonance Imaging (MRI) has become widely accepted in the medical community because of the sharp detail provided in the images obtained by the technique compared to X-ray imaging. MRI brain, spine and other parts of the body are routinely scanned in patients to determine the extent of injury or disease, such as the presence of cancerous growths. More recent advances in the development of computers and image generation have allowed processing and display of three dimensional images of the acquired MRI data. These computer enhanced images permit a surgeon to view the exact location of a brain tumor, for example, prior to surgery to optimize the approach to surgical removal of the diseased tissue.

A more difficult application for the MRI technique is imaging of a moving organ such as the heart. The potential applications of obtaining clear, high resolution images of the heart are great, including the possibility of determining the extent of wall thickening and coronary artery occlusion. The currently used methods of MRI data acquisition have inherent limitations which restrict realization of the full potential of the MRI technique for this application.

Another application of MR is MR angiography. According to Biomedical Business International (January, 1993), if the heart and coronaries can be imaged, "the most exciting new application of MRI is MR angiography which makes images of blood vessels without the use of contrast agents. MR angiography exploits the fact that normal blood flow transports excited nuclei to unexcited regions, providing a means to recognize flow without contrast agents. "

"Unlike a conventional angiogram, where all blood vessels are projected onto a single plane so that some vessels can obscure other vessels lying along the same projection line, MR angiograms produce their data in full 3-D. The data can be projected, producing an image that resembles a conventional angiogram, but can also be presented in 3-D. Although the technique has been applied to the coronary arteries, success there has been limited because motion of blood is a combination of flow and cardiac wall motion and because the coronary arteries are relatively small. "

MR technology utilizes three types of electromagnetic fields: 1) a strong static magnetic field, 2) a time-varying gradient field; and 3) a radiofrequency (RF) field which consists of RF pulses used to produce an image. MRI technology is based on the physical principle that when placed in a very strong magnetic field, the protons in atoms such as hydrogen, will tend to align with the magnetic field lines. Application of a radio frequency pulse perpendicular to these lines at frequencies selective to certain atoms will cause these protons to absorb energy and precess about the field lines. Removing the radio frequency pulse causes the protons to cease their precession slowly and to give up energy following the field cessation. The energy given up by the proton in this process is characteristic of the atomic and chemical environment in which it exists.

MR imaging is done by detecting the emitted proton energy using radial detectors in the MR system. Typically, the protons in hydrogen, from the water in the body, are used to image tissue and blood with a spatial resolution of 1 mm³ in the current systems. Chemical MR, or spectroscopy, can be done by selecting a specific energy and spatially imaging for specific nuclei in a certain chemical environment. For example, the imaging of phosphates in the heart may provide vital information about the presence and location of ischemic or oxygen deficient tissue in the heart. This technology could further advance the non-invasive screening of patients to determine the need for coronary artery bypass surgery or balloon angioplasty to open occluded coronary arteries in the heart.

MR imaging can be done throughout the heart cycle with the use of a method to determine the cycle and compensate for the motion, because the signals are strong. To be able to obtain the chemical image of the heart by MR, it is necessary to make measurements while the heart is not rapidly moving because the chemical signals are very weak and take time to acquire. The relatively motionless period is called diastole and occurs while the ventricles of the heart are filling with blood. To obtain good spectroscopic information of the heart, it is necessary to acquire the MR images during diastole, and to have a non-artif actual means of determining when the diastolic period begins and ends. A signal indicating the occurrence of diastole must be provided to the image acquisition electronics of the MRI system.

MR imaging, angiography and spectroscopy, as well as other MR procedures where changes are induced by cardiac motion or blood volume (e.g. brain), require triggering the MR equipment at the appropriate time. The MRI trigger is used to identify when to start data acquisition relative to the cardiac cycle. Current methods of providing this trigger are the Electrocardiogram signal (ECG or EKG) or pulse oximetry, which are both very susceptible to artif actual signal generation. The pulse oximetry method suffers from motion artifact and decreased signal due to peripheral vasoconstriction. Both of these sources of artifact make pulse oximetry relatively unreliable for MR triggering. Electrocardiogram devices (ECG) or (EKG) are commonly used to determine the characteristics of the electrical impulses which cause the heart to beat in the proper cycle. The ECG uses conductive, adhesive electrodes attached to the skin to conduct these small electrical impulses through electrical connections to a monitor. Two parts of the ECG signal are used to indicate the start of systole and diastole. The QRS complex is used to indicate the start of systole, while the T-wave is used to indicate the start of diastole.

The ECG has serious limitations when used during any MR procedure. The limitation primarily occurs because the high static magnetic field strength required for either imaging or spectroscopy degrades the performance of the ECG device. The noise which can mask or obliterate the underlying ECG signals can be produced by several sources: 1) muscle artifact, 2) induced electromotive force (EMF), and 3) the RF noise from the RF pulse. Induced EMF is caused by a conductor moving in a static magnetic field, such as when an ECG lead is moved by the patient breathing, patient movement or heart beat movement. Smaller induced EMF may also be caused by the time variant/graded magnetic field. The leads, electrodes and tissue between the electrodes also comprise a winding in which the RF pulse generates EMF. The magnitude of the induced EMF caused by even relatively small motions such as respirations is enough to significantly mask the underlying ECG signal.

Another important source of interference with the ECG signal is caused by the Hall effect. As charged particles comprising the blood accelerate out of the heart and into the vasculature during systole, they produce EMF because of the strong magnetic field of the MR system. The EMF is invariably recorded by the ECG. The amplitude of this EMF is such that the underlying ECG signal is obliterated. The timing of the Hall effect unfortunately occurs during the ST and T- wave portions of the ECG. These portions typically are used as markers for the beginning of diastole. Thus, there are no reliable ECG markers of diastole within a typical magnetic field used in MR applications.

In summary, while the ECG signal provides reliable indicators of the cardiac cycle (systole and diastole) in the non-magnetic environment, it has fundamental limitations at the high static field strengths used in MR applications. Triggering on particular aspects of the cardiac cycle is necessary to obtain MR images and spectroscopic data which is relatively free of cardiac motion or cardiac cycle related changes in blood volume. In particular, cardiac applications of MR have been delayed because of a lack of a reliable triggering system.

With the development and installation of stronger static fields which offer many improvements in imaging and spectroscopy the relative inability of the ECG signal at 1.5T become virtually absolute at, for example, 4.0T. The inherent problems of motion-induced EMF and the Hall effect are proportionally magnified at higher field strengths. Thus there is an obvious need for a technology which overcomes these inherent limitations. U.S. Patent No. 5,218,532 entitled "Method And System For

Acquiring MR Data In MRI", issued to Mori on June 8, 1993 is an example of the conventional method of using the R-wave to trigger acquisition of MR data. This reference teaches monitoring the R-wave to determine if cardiac arrhythmia occurs. This reference also teaches monitoring the movement of the patient. If cardiac arrhythmia occurs or if the patient moves, the MR data must be reacquired.

U.S. Patent No. 5,217,010 entitled "ECG Amplifier And Cardiac Pacemaker For Use During Magnetic Resonance Imaging", issued to Tsitlik et al on June 8, 1993 is directed to a modified pacemaker design which is safe to use during MRI. This reference discusses EMF induced in pacemaker leads. This is similar to the EMF induced in any ECG apparatus when placed in a strong static magnetic field. Movement of the patient caused by heartbeat, breathing etc. will cause the conductor to move through the magnetic field, causing EMF. This reference is directed to a filter which provides an R-wave which can be used to synchronize a MRI system or cardiac pacemaker.

U.S. Patent No. 4,413,233 entitled "Dynamic NMR Measurement", issued to Fossel on November 1, 1983 is directed to apparatus for triggering spectroscopy. This reference teaches using either the R-wave (QRS complex) of an electrocardiogram (ECG) or a blood pressure wave to determine systole in a rat heart which has been removed from the rat body and is pumped artificially for study purposes. This reference is directed to experimental usage of MR spectroscopy and is not suitable for use with human patients. Summary of the Invention

All of the prior art methods of triggering an MR procedure such as spectroscopy teach the use of an ECG signal to determine systole or diastole in the heart. Fossel teaches that ECG and blood pressure wave are equivalent for determining systole in a specific experimental situation where a rat heart has been removed from the body and artificially pumped for study purposes. None of the references recognize the benefits provided by using blood flow velocity information to trigger a MR procedure, such as spectroscopy.

Applicants' inventive apparatus and method replaces the currently used ECG devices with Doppler ultrasound technology for heart cycle determination. This Doppler ultrasound device will permit cardiac cycle detection at high static magnetic fields to develop the full potential of MR imaging, angiography, spectroscopy or other procedure.

The need for a new method has lead to the identification of Doppler ultrasound measurement of blood flow in the vasculature as a more effective trigger for the MRI. Doppler ultrasound measurement of blood flow velocities in the carotid or other arteries may be obtained inside the MRI apparatus without artifacts induced by the various sources of EMF. This new method provides the opportunity to improve the operating efficiency of the MRI units, which may cost several million dollars each, and yield additional improvements in imaging as well. Use of Doppler ultrasound has become routine in many aspects of medical practice. This technology is commonly used for fetal imaging or imaging of the heart. Pulsed Doppler ultrasound is also commonly used to obtain color flow images of the heart to indicate blood flow direction and velocity. These Doppler ultrasound images are routinely obtained by an ultrasound scan head attached to an imaging monitor. The scan head is placed on the patient's skin near the site to be studied and ultrasound is transmitted through the skin, muscle, and tissue. Some of the transmitted ultrasound is reflected back to the scan head, detected, and converted to an image. The reflected sound can also be used to measure the velocity of flowing blood by analyzing the "Doppler shift, " which is a frequency shift proportional to the velocity of the blood.

Placement of a Doppler ultrasound probe may also be done to continuously measure the flow of blood in a specific vessel. This measurement results in a blood velocity waveform from the analyzed vessel. A pulsatile flow waveform may be obtained through the skin on the neck from the carotid artery which flows from the ascending aorta to the brain. This blood velocity waveform shows peak flow during systole while the aortic valve is open and the heart is pumping oxygenated blood to the brain. Low or no blood velocity is measured while the aortic valve is closed during diastole.

The Doppler ultrasound method of measuring flow will not be affected by the various causes of EMF discussed above. As is well known in the art, because of the high frequency of the transmitted Doppler ultrasound, typically 5MHz, transformer coupling of the detected signal into the signal processing electronics is commonly done. EMF induced voltages are decoupled from the ultrasound signal and do not interfere with it. This transformer is designed and tested to decouple voltages above 1500 volts, far in excess of 20-100 V of EMF (induced EMF discussed for example in U.S. Patent No. 5,217,010 at Col. 3 lines 40-58). While in the body, the ultrasound is not affected by the magnetic fields, and when converted to electrical impulses by the piezoelectric transducer, the signals can be shielded from the effects of the RF fields associated with MR procedures. Shielding prevents stray signals from being emitted or received between the RF of the MR and ultrasound systems.

Applicants' have developed a Doppler ultrasound device to measure the flow of blood in certain locations in the arterial tree, such as the carotid, brachial, or radial artery, as well as from the ascending aorta from the suprasternal notch. The measured velocity of blood in the vessel produces a pulsatile waveform indicative of the blood flow from the heart. A delay in the measured signal from the actual heart pulse can be factored into the trigger signal. For example, the blood may require 80 milliseconds to travel from the heart to the region of the carotid artery where the flow is measured by the Doppler ultrasound device. Measurements further from the heart, such as the brachial or radial artery in the arm, will add additional delay to the pulse. Greater variability in the time delay may occur farther from the heart because the resistance of the peripheral vascular system may change. The preferred location for the ultrasound measurement is from the carotid artery. The carotid artery provides a very strong signal, has minimum delay for the trigger, and is an optimum position without patient discomfort or invasion of the body. Applicants' ultrasound device measures blood flow from a peripheral vessel, consists of an electronic monitor and a reusable ultrasound probe. The probe is placed on the skin in a location to obtain velocity measurements from the carotid artery. The probe and cable are shielded to permit operation in the RF field of the MRI system. Electrical interference is not emitted from or received by the probe. The electronics monitor will be developed to permit operation in the strong magnetic field near the MRI magnet. The electronic monitor may be designed to operate in the strong magnetic field near the MR system. This design requires the use of non¬ magnetic components, shielding or components which are sensitive to strong magnetic fields, and use of battery or d.c. power. Increasing the distance of the monitor from the magnet reduces the field effects. The monitor will provide a velocity waveform trigger to the MRI image acquisition electronics. The trigger will be gated to diastole for spectroscopy. Applicants' Doppler ultrasound device can also be used to monitor patient heart rate, if desired. For example, if the ECG signal does not provide the necessary information, as in a strong magnetic field, the heart rate can be obtained from the ultrasound data.

The ultrasound trigger for the MRI system relies on using the principle of the Doppler shift to measure velocity of a moving object. An ultrasound wave, transmitted into the body and vessel, will reflect from the moving red blood cells in the blood. The velocity of these blood cells will cause a measurable shift in the frequency of the reflected sound. This frequency shift is measured by the electronic monitor. The measurement of the reflected sound results in a continuous display of the blood velocity in the vessel. The resultant pulsatile waveform can be electronically obtained and transmitted to the MR imaging electronics. The advent of systole, where the heart is contracting, is indicated by the ejection of blood into the arterial system and into the carotid artery. The beginning of diastole, where the heart is relatively motionless, and imaging can occur, is indicated by the end of the systolic pulse. Brief Description of the Figures

Figure 1 shows a subject to be examined and shows the location of the carotid arteries and the brachial arteries;

Figure 2 shows the inventive ultrasound probe;

Figure 3 is a block diagram of the inventive apparatus; Figure 4 shows the triaxial cable in more detail;

Figure 5 shows a systolic velocity pulse waveform and its timing in relation to the EKG. Detailed Description of the Invention

While this invention may be embodied in many different forms, there are described in detail herein specific preferred embodiments of the invention. This description is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiments illustrated.

Figure 1 shows a patient 10 to be examined using a MR device. The disclosure will describe either imaging MR or chemical MR (spectroscopy) as the MR procedure to be performed on the patient. However, it should be understood that the inventive trigger apparatus can be used to trigger any desired procedure used in connection with NMR equipment.

Reference numeral 12 indicates the inventive ultrasound probe, which can be positioned on the skin over any vessel. In the preferred embodiment probe 12 will be placed over the left or right common carotid artery (14 or 16), or over the brachial artery 18 in an arm of the patient. The preferred embodiment of the invention uses a non-invasive probe. However, the inventive probe could also be implanted or invasively obtain its information.

The preferred location for probe 12 is over a carotid artery 14 or 16. The probe 12 is designed to allow the ultrasound transducer(s) to be positioned over the artery while the body of the probe may be secured to the collarbone 20 of the patient. As is well known in the Doppler ultrasound art, a single transducer, multiple transducers, or a transducer array may be used in this invention. Figure 2 shows the probe 12 in more detail. Non-magnetic materials are used in the construction of probe 12. Probe 12 includes transducer positioning sphere 22 and transducer securing portion 24. Sphere 22 contains a socket 26 which matingly receives ball 28 to allow the transducer securing portion 24 to rotate omnidirectionally with respect to transducer(s) 30 on sphere 22. Piezoelectric transducers which convert the echoed ultrasound to electrical impulses are well known in the art. Spherical joints which provide three degrees of rotational freedom are well known in the art. Screw 32 is used to lock the securing portion 24 in place. Many methods are well known in the art to lock a spherical joint in a desired position. Any structure which allows the transducer(s) 30 to be located where desired relative to securing portion 24 can be used.

Securing portion 24 includes ball 28, securing strip 34 and probe body 36. Securing strip 34 in the preferred embodiment is designed for securing the probe body to the collarbone of a patient. Strip 34 is approximately 1/2 inch wide and two inches long and relatively flat in the preferred embodiment. In use, piezoelectric transducer(s) 30 is rotated until it is positioned to direct the beam of ultrasound into the carotid artery (14 or 16). The securing portion 24 can then be rotated so that the transducer(s) is still in place, but allowing the securing portion 24 of probe 12 to lay along the collarbone 20 of the patient. Securing strip 34 can be used to secure the probe 12 in place using adhesive tape, or other suitable method. The entire probe 12 may be covered in RF shield material 40, which can be thin metal film on plastic or other conductive coatings as are well known in the art. RF shielding is well known in the art. Triaxial lead 42 connects transducer(s) 30 to the monitor. Triaxial lead 42 runs in a channel in ball 28 and strip 34 to prevent movement of lead 42. The triaxial cable used for lead 42 is available from W.L. Gore and is called hybrid round cable and is shown in more detail in Figure 4. As is well known in the art, the hybrid round cable is constructed with a conductor, surrounded by a dielectric, surrounded by a braided tin plated copper shield, surrounded by a binder, surrounded by a second braided tin plated copper shield, and finally surrounded by a jacket material. The use of triaxial cable and its connection to transducer(s) 30 are well known in the art. RF shield material 40 is conductively coupled to the shield in triaxial cable 42. Because the entire probe and cable are RF shielded, they are shielded from EMF caused by the RF pulses as well as preventing significant emissions which could interfere with the RF pulses and cause image distortion.

Figure 3 shows a block diagram of the inventive apparatus. Transducer(s) 30 is shown connected to monitor 50, which in turn is connected to computer 51, which drives and controls the MR system 52. The computer is programmed to monitor the signal provided by transducer(s) 30 and monitor 50 and trigger the procedure at the desired time. In the preferred embodiment the procedure is spectroscopy which is gated to diastole in the heart, although MRI or any other MR procedure could also be triggered, if desired. Figure 5 shows the typical blood flow velocity waveform provided by transducer(s) 30. The determination of systole and diastole based on such a waveform are well known in the art. It should be understood that while spectroscopy is discussed as being gated to diastole, it can be gated to any desired part of the waveform. Also it should be understood that the MR procedure can vary from imaging, angiography etc. and each procedure could be gated to a different part of the waveform, as is well known in the art. It should also be understood that this apparatus can be used to provide data for a single point in the waveform, or for many points in the waveform, as desired.

Figure 5 shows the timing of the blood velocity pulse as observed in the aorta in relation to an EKG. The P-wave is shown at 60, the QRS complex is shown at 62, the T-wave is shown at 64 and A_p and V_p represent peak acceleration and velocity. As seen in figure 5, the rapid blood velocity increase measured by Doppler ultrasound (66) allows a close estimate of end diastole and the start of systole (the QRS complex 62). The dicrotic notch, shown at 65 signals the end of systole and the beginning of diastole. These two points along with suitable delays from these points provide accurate triggering based on systole or diastole for MR procedures.

Transducer(s) 30 will not be affected by the various causes of EMF discussed above. ECG signals are very low frequency signals, approximately 60Hz, and are usually in the range of 1-10 millivolts. This low frequency ECG signal does not allow for capacitive or transformer decoupling. Therefore, the induced EMF voltages of 20-100 V can very easily mask or obliterate the ECG signal. The signal of transducer(s) 30, even though in the 4-10 microvolt range operates at 5MHz, plus or minus 5KHz. This high frequency does allow capacitive or transformer decoupling, the use of which is old in the art of Doppler ultrasound.

The inventive apparatus has been used experimentally with a 4T Siemens NMR with spectrographic capability, but can easily be adapted to be used with any MR equipment desired.

The above disclosure is intended to be illustrative and not exhaustive. These examples and description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the attached claims. Those familiar with the art may recognize other equivalents to the specific embodiments described herein which equivalents are also intended to be encompassed by the claims attached hereto.

Claims

What is claimed is as follows:

1. Apparatus for monitoring blood flow for triggering an MR procedure, comprising: magnetic resonance (MR) means for conducting a predetermined MR procedure on a predetermined body portion of an examined body, the MR means producing a strong static magnetic field;

Doppler ultrasound probe means for providing blood flow velocity information in the strong static magnetic field, the means being arranged to sense blood flow in a vessel in the examined body, and trigger means connected to the Doppler ultrasound probe means for triggering the predetermined MR procedure at a predetermined time related to blood flow velocity.

2. The apparatus of claim 1 wherein the Doppler ultrasound probe is RF shielded to prevent induced EMF from an RF pulse used by the MR means for imaging and where the RF shielding prevents the probe from emitting radiation which would interfere with the RF pulse.

3. The apparatus of claim 2 wherein the MR means is a chemical MR means for performing spectroscopy.

4. The apparatus of claim 2 wherein the MR means is an imaging MR means for performing imaging.

5. The apparatus of claim 2 wherein the MR means is an angiography MR means for performing angiography.

6. The apparatus of claim 3 wherein the predetermined body portion is the heart.

7. The apparatus of claim 6 wherein the predetermined time for triggering spectroscopy is during diastole, during which the heart is not moving rapidly.

8. The apparatus of claim 7 wherein the trigger means is a computing means which continuously monitors the blood flow velocity information to determine the beginning and end points of diastole.

9. The apparatus of claim 8 wherein the computing means is comprised of a transducer monitor which is constructed of non-magnetic materials so that it may operate in or near the strong magnetic fields and which is connected to a computer which controls the chemical MR means.

10. The apparatus of claim 9 wherein the Doppler ultrasound probe means is non-invasively arranged for providing the blood flow velocity information.

11. The apparatus of claim 10 wherein the Doppler ultrasound probe means is arranged for providing blood flow velocity information from an artery, through the skin, to continuously measure the flow of blood in the artery and provide a pulsatile blood flow waveform.

12. The apparatus of claim 11 wherein the Doppler ultrasound probe means is constructed and arranged to place an ultrasound transducer on the skin over the carotid artery, to lay along the collarbone of the body to be examined and wherein the ultrasound probe means is secured to the collarbone.

13. The apparatus of claim 12 in which the Doppler ultrasound probe means is comprised of: an ultrasound transducer portion comprising: a sphere containing a socket constructed to matingly receive a ball; a locking means for locking the ball in place; ultrasound transducer means attached to the sphere and located approximately opposite the locking means for providing blood flow velocity information, and a probe means securing portion comprising: a ball mating with the socket in the sphere, and elongate flat securing means used to secure the Doppler ultrasound probe means to the collarbone, whereby the probe means securing portion is omnidirectionally rotatable such that the ultrasound transducer means may be located were desired, while positioning the elongate flat securing means along the collarbone for securing the probe in place.

14. The apparatus of claim 13 wherein the ultrasound transducer means is comprised of an array of transducers, each connected to a shielded cable, and wherein the array may be used to image the vasculature accessible where the probe is positioned and select a vessel from which to obtain blood flow velocity information.

15. The apparatus of claim 10 wherein the Doppler ultrasound probe means is arranged for providing blood flow velocity information of a brachial artery in an arm of the body to be examined, to continuously measure the flow of blood in the brachial artery and provide a pulsatile blood flow waveform.

16. The apparatus of claim 9 wherein the Doppler ultrasound probe means is invasively arranged for providing the blood flow velocity information.

17. The apparatus of claim 1 wherein the Doppler ultrasound probe means is constructed from non-magnetic materials.

18. Apparatus for monitoring blood flow for determining heart rate during an MR procedure, comprising: magnetic resonance (MR) means for conducting a predetermined MR procedure on a predetermined body portion of an examined body, the MR means producing a strong static magnetic field;

Doppler ultrasound probe means for providing blood flow velocity information in the strong static magnetic field, the means being arranged to sense blood flow in a vessel in the examined body, and computing means conductively connected to the Doppler ultrasound probe means for determining the heart rate from the blood flow velocity information during an MR procedure.

19. A method of triggering an MR procedure using magnetic resonance (MR) means for conducting a predetermined MR procedure on a predetermined body portion of an examined body, the MR means producing a strong static magnetic field, Doppler ultrasound probe means for providing blood flow velocity information in the strong static magnetic field which is arranged to sense blood flow in a vessel in the examined body, and trigger means connected to the Doppler ultrasound probe means for triggering the predetermined MR procedure at a predetermined time, comprising the steps of: continuously monitoring the blood flow velocity of a vessel using the Doppler ultrasound probe means; determining the predetermined time to trigger the predetermined MR procedure by analyzing the blood flow velocity information, and triggering the predetermined MR procedure.

20. The method of claim 19 wherein the predetermined MR procedure is based on spectroscopy.

21. The method of claim 19 wherein the predetermined MR procedure is based on imaging.

22. The method of claim 19 wherein the predetermined MR procedure is based on angiography.

23. The method of claim 20 wherein the predetermined body portion is the heart.

24. The method of claim 23 wherein the predetermined time for triggering spectroscopy is during diastole, during which the heart is not moving rapidly.

25. The method of claim 24 wherein the trigger means is a computing means which continuously monitors the blood flow velocity information to determine the beginning and end points of diastole.

26. The method of claim 25 wherein the computing means is comprised of a transducer monitor which is constructed of non-magnetic materials so that it may operate in or near the strong magnetic fields and which is connected to a computer which controls the MR means.

27. The method of claim 26 wherein the Doppler ultrasound probe means is non- invasively arranged for providing the blood flow velocity information.

28. The method of claim 27 wherein the Doppler ultrasound probe means is arranged for providing blood flow velocity information from an artery, through the skin, to continuously measure the flow of blood in the artery and provide a pulsatile blood flow waveform.

29. The method of claim 28 wherein the Doppler ultrasound probe means is constructed and arranged to place an ultrasound transducer on the skin over the carotid artery and to lay along the collarbone of the body to be examined and is secured to the collarbone.

30. The method of claim 29 in which the Doppler ultrasound probe means is comprised of: an ultrasound transducer portion comprising: a sphere containing a socket constructed to matingly receive a ball; a locking means for locking the ball in place; ultrasound transducer means attached to the sphere and located approximately opposite the locking means for providing blood flow velocity information, and a probe means securing portion comprising: a ball for mating with the socket in the sphere, and elongate flat securing means used to secure the Doppler ultrasound probe means to the collarbone, whereby the probe means securing portion is omnidirectionally rotatable such that the ultrasound transducer means may be located were desired, while positioning the elongate flat securing means along the collarbone for securing the probe in place.

31. The method of claim 30 wherein the ultrasound transducer means is comprised of an array of transducers, each connected to a shielded cable, and wherein the array may be used to image the vasculature accessible where the probe is positioned and select a vessel from which to obtain blood flow velocity information.

32. The method of claim 27 wherein the Doppler ultrasound probe means is arranged for providing blood flow velocity information of a brachial artery in an arm of the body to be examined, to continuously measure the flow of blood in the brachial artery and provide a pulsatile blood flow waveform.

33. The method of claim 26 wherein the Doppler ultrasound probe means is invasively arranged for providing the blood flow velocity information.

34. The method of claim 19 wherein the Doppler ultrasound probe means is RF shielded to prevent induced EMF from an RF pulse used by the MR means for imaging and where the RF shielding prevents the probe from emitting radiation which would interfere with the RF pulse.

35. The method of claim 34 wherein the Doppler ultrasound probe means is constructed from non-magnetic materials.

36. A method of determining heart rate during an MR procedure, using magnetic resonance (MR) means for conducting a predetermined MR procedure on a predetermined body portion of an examined body, the MR means producing a strong static magnetic field, Doppler ultrasound probe means for providing blood flow velocity information in the strong static magnetic field which is arranged to sense blood flow in a vessel in the examined body, and computing means connected to the Doppler ultrasound probe means for determining the heart rate from the blood flow velocity information, comprising the steps of: continuously monitoring the blood flow velocity of a vessel using the Doppler ultrasound probe means; determining the heart rate from the blood flow velocity information during an MR procedure.

37. Apparatus for triggering an MR procedure by measuring blood flow velocity, comprising: magnetic resonance (MR) means for conducting a predetermined MR procedure on a predetermined body portion of an examined body, the MR means producing a strong static magnetic field;

Doppler ultrasound probe means for measuring blood flow velocity in a predetermined vessel in the examined body in the strong static magnetic field, the Doppler ultrasound probe means providing an electrical output signal indicative of the measured blood flow velocity, and trigger means conductively connected to the output signal provided by the Doppler ultrasound probe means for triggering the predetermined MR procedure at a predetermined time related to the measured blood flow velocity in the predetermined vessel.