Patent Application
20130231551
The invention relates to cardiac stress testing, and more particularly relates to cardiac stress testing using diagnostic medical imaging techniques. In its most immediate sense, the invention relates to cardiac exercise stress testing using magnetic resonance (“MR”) imaging.
In an electrocardiogram (ECG) cardiac exercise stress test, a patient is connected to an ECG monitor and instructed to walk on a treadmill following an exercise protocol such as the Bruce treadmill protocol. As the test continues, the speed and inclination of the treadmill are gradually increased until the patient reaches peak effort (thereby generating peak cardiovascular stress). The test stops when the patient reaches his or her maximum heart rate, an ECG abnormality is detected, or when the patient cannot continue the exercise, whichever occurs first. The results of the test are then used to assess the patient's heart function.
Cardiac exercise stress tests are also carried out using medical diagnostic imaging techniques. Such techniques typically require the acquisition and comparison with each other of two images of the heart; a pre-exercise image acquired when the patient's heart is at rest is compared with a post-exercise image acquired after the patient has exercised.
For example, when nuclear medicine is used as the imaging modality, a first dose of a radioisotope is introduced into the patient's bloodstream while the patient is at rest. The blood circulates and perfuses the patient's myocardium, and the radioisotope is taken up in the resting myocardium. The patient's heart is then imaged in a gamma camera, which responds to the radioisotope to form a pre-exercise image showing myocardial perfusion while the patient is at rest. Then, the patient is exercised on a treadmill, and instructed to tell the clinician when (s)he is about to reach maximum effort. A second dose of radioisotope is then administered, and the patient continues to exercise for a period (perhaps 90 seconds) during which the blood perfuses the patient's myocardium and the radioisotope is taken up in the patient's myocardium while the heart is under stress. Several minutes later (perhaps 30 minutes later) the patient's heart is then once again imaged in the gamma camera to form a post-exercise image showing post-exercise myocardial perfusion. (Significantly, a nuclear medicine cardiac exercise stress test does not require that the patient's heart be imaged while the patient's heart is at peak stress. This is because the nuclear medicine image detects the distribution of the radioisotope within the myocardium and this distribution does not change rapidly.)
Echocardiography can also be employed as the imaging technique. In a cardiac exercise stress test using echocardiography, the two sets of heart images are acquired using an ultrasound imager, with the pre-exercise images of the heart being compared with the post-exercise images of the patient's heart acquired immediately following the time the patient has reached peak effort by exercising on a treadmill.
An MR imager can produce high-resolution images quickly and for these reasons researchers have tried various approaches for using MR imaging in cardiac stress testing. All have been unsatisfactory. One approach was to induce stress pharmacologically rather than by exercise on a treadmill. This was unsatisfactory because pharmacologically induced stress does not link physical activity to symptoms as treadmill exercise does. Another approach required the patient to exercise on a supine bicycle ergometer located inside the imager. While this was useful for certain research studies, it was unsuitable for cardiac stress testing. This is because the diagnostic value of the post-exercise MR image is maximized when that image is acquired at peak effort. Patients were unable to reach peak effort by cycling in a supine position because it was uncomfortable and because leg fatigue caused patients to cease exercising before they reached peak effort. In still another approach by Rerkpattanapipat et al., a treadmill was placed outside the scan room in which the imager was located. (The necessity for placing the treadmill outside the scan room is discussed in more detail below.) Patients exercised on the treadmill and then walked twenty feet to the imager in order to acquire the post-exercise heart image. Although the results of this experiment were encouraging, the Rerkpattanapipat et al. experimental setup is not practical for clinical use. This is because a patient's heart rate slows down between the time that exercise ceases and the time that the patient is imaged in the MR imager, whereby the post-exercise image of the heart is not an image of the heart at peak effort. Additionally, use of a treadmill outside the scan room raises safety concerns with moving a patient such a distance after (s)he has reached peak effort.
As stated, in the Rerkpattanapipat et al. experiments, the treadmill was located outside the scan room in which the MR imager was located. This was necessary because it would have been unwise and indeed unsafe to place a conventional treadmill in the scan room. An MR imager necessarily generates an intense magnetic field and a conventional treadmill contains ferromagnetic components (particularly the electric motor used to drive the treadmill belt). If a ferromagnetic object is in the scan room, the object will likely be pulled towards the imager with great force, potentially causing great damage to the imager and even causing serious injury to the patient. For example, in one reported instance an oxygen tank located in the scan room was turned into a projectile when the imager was energized, causing injury to a pediatric patient. Additionally, even if a ferromagnetic component were to be firmly fixed in position so that it could not come loose to cause equipment damage or patient injury, its presence in the scan room would be highly undesirable because it would distort the main magnetic field created by the imager. This distortion would generate artifacts in the reconstructed MR image and possibly reduce the diagnostic utility of that image. For these reasons, Rerkpattanapipat et al.'s treadmill was located outside the scan room.
It would be advantageous to provide method and apparatus for conducting a cardiac exercise stress test using MR imaging, the method and apparatus permitting a patient to exercise on a treadmill located inside the scan room immediately adjacent the MR imager.
It is therefore one object of the invention to provide method and apparatus for conducting a cardiac exercise stress test using MR imaging, which permit a patient to exercise on a treadmill located inside the scan room immediately adjacent the MR imager.
Another object is, in general, to improve on known method and apparatus of this general type.
The invention proceeds from the realization that an electric motor is the most massive ferromagnetic element in a conventional treadmill and that replacing that element with a non-ferromagnetic motor can greatly simplify the task of providing a treadmill that can be used inside the scan room. The invention further proceeds from the realization that if the treadmill motor that drives the belt is designed to produce rotational motion from pressurized fluid supplied to the motor, the motor can be made of non-ferromagnetic components and the fluid can be pressurized by a pump located outside the scan room.
In apparatus in accordance with the invention, a treadmill is provided. The treadmill is located in the scan room and has a belt driven by a motor that produces rotational motion from pressurized fluid supplied to the motor. A pump is further provided. The pump is located outside the scan room, produces pressurized fluid, and is operatively connected to the motor. Advantageously, and in the preferred embodiment, the fluid is water-based, and the treadmill is entirely made of non-ferromagnetic materials.
Apparatus in accordance with the invention can be safely put inside the scan room because there is little (advantageously, no) ferromagnetic material to be attracted to the magnet or to distort the main field of the MR imager. Any appreciable quantity of ferromagnetic material is relocated out of the scan room where it cannot be attracted to the magnet of the MR imager and cannot adversely affect the quality of the MR image.
In a method in accordance with the invention, a treadmill is located inside the scan room adjacent the MR imager. The treadmill has a motor that drives the belt. The motor produces rotational motion in response to pressurized fluid supplied to the motor. Fluid is pressurized outside the scan room and the patient is caused to stand on the belt. The pressurized fluid is supplied to the motor to drive the belt, thereby exercising the patient inside the scan room. Then, the supply of pressurized fluid to the treadmill motor is terminated, whereby the patient's exercise ceases. The patient is then moved from the treadmill belt into the MR imager without leaving the scan room, and the MR imager is used to conduct an imaging study on the patient.
By using a method in accordance with the invention, the patient is very close to the MR imager and there is only a short time lapse between cessation of the patient's exercise and acquisition of the post-exercise MR image. It is therefore possible to acquire an MR image that is closely representative of an image of the patient's heart at peak effort.
Referring to
A hydraulic powered treadmill 110 and MR imager 112 are contained in the scan room 102. The hydraulic powered treadmill 110 is connected to a hydraulic power pack 124 via hydraulic hoses 122. In
Because as discussed below the treadmill 110 is advantageously fully MR compatible, it may be placed immediately adjacent to the MR imager 112 or in any desired position within the scan room 102. Complete MR compatibility also allows the treadmill 110 to be used adjacent to MR imagers operating at high field strengths, e.g., 3.0 Tesla. The resulting configuration of the scan room 102 may be very similar to the setup for a standard exercise echocardiography lab.
An exemplary embodiment of a treadmill 110 may comprise a support and a belt 111 rotatably mounted within the support. Furthermore, the treadmill 110 may include programmable components so as to be controlled independently through a standard PC as well as with leading treadmill stress testing software. The programmable components may execute any of a variety of exercise protocols, including the standard Bruce Treadmill Exercise protocol. The Bruce Treadmill Exercise protocol automatically advances a patient through set stages of speed and elevation as shown in Table 1. The treadmill may also or alternatively be controlled manually.
Traditional treadmills include electric motors having ferromagnetic components with significant mass. As explained above, ferromagnetic items can pose a severe hazard to MR imagers and patients if they are close to an operating MR imager. An electric motor must contain ferromagnetic parts. For this reason, in an example embodiment of the present invention, a hydraulic motor 113 made of stainless steel (which is not ferromagnetic) is used to power the treadmill 110. The other components in the treadmill 110 may be made from non-ferromagnetic materials including, but not limited to, stainless steel or aluminum such that the treadmill 110 may be safely operated in close proximity to the MR imager 112 while the MR imager 112 is operating. Material choices may depend on tradeoffs between the necessary strength of the material compared to the increased cost of using stainless steel, for example.
In this example, hydraulic power pack 124, comprising an electrical motor driven pump 200 as shown in
In an MR imaging study in accordance with a method in accordance with the invention, a patient completes all or part of an exercise protocol on the treadmill 110 and then takes his or her place within the MR imager 112. Medical staff, who may be present in the room during the stress test, may assist the patient in transferring from the treadmill 110 to the MR imager 112. The lift system of the treadmill 110 may be used to assist in transferring the patient by positioning the height of the treadmill 110 to allow easy transfer of the patient to the patient table 750 of the MR imager 112. Because the treadmill 110 may be directly adjacent to the MR imager 112, some patients may not require assistance while transferring from the treadmill 110 to the MR imager 112. Cardiac image data is acquired by the MR imager 112 and analyzed by a medical professional following patient exercise.
The MR imager 112 is controlled via in-room wireless monitor 114 and wireless keyboard/mouse 116 such as are available from Siemens Medical Solutions, Malvern, Pa., and a start button located on the front panel of the MR imager 112. The wireless monitor 114 and wireless keyboard/mouse 116, designed primarily for interventional MRI applications, duplicates the functionality of the computer 106. A power contrast injector 118 such as one from Medrad Corp., Pittsburgh, Pa. may be outfitted with a manual control switch for operation from within the MRI scan room 102. The injection protocol may be pre-programmed and loaded so that it can be executed immediately at the start of the perfusion scan from within the scan room 102. In an example embodiment of the present invention, all equipment necessary to conduct the treadmill exercise test with continuous ECG 108 and blood pressure monitoring 120, as well as the equipment necessary to control the MR study, is positioned to allow the test to be performed within the scan room 102. As a result, the stress testing team is able to remain in the room and in direct communication with the patient at all times.
In this example, hydraulic fluid flowing from power pack 124 through the hoses 122 powers the hydraulic motor 113 (
An example of the operation of one embodiment will now be described. A variable speed electric motor in the power pack 124 drives the hydraulic pump 200 (
In further description of one example of an operation of one embodiment, the Bruce Treadmill Exercise protocol requires the treadmill to attain a maximum grade of 22% to accommodate patients with a wide range of physical conditions. The grade (gradient) of the treadmill belt 111 is controlled by an ancillary circuit located in the power pack 124. An accumulator 500 of power pack 124 is charged with a volume of fluid sufficient to operate the treadmill 110 for a complete patient test session. At each protocol stage, a portion of the stored fluid is directed to the non-ferromagnetic treadmill lift cylinder 412 by way of valves and conductors (hoses in communication with cylinder 412 are not shown in
The remaining design elements of an exemplary system include components suitable for a safe, reliable machine. With particular reference to this example, a pressure relief valve is installed at the pump outlet. Non-ferromagnetic hoses and couplers comprise the required fluid conductors. The couplers are sized and polarized to prevent incorrect connection during equipment setup. System cooling is provided by the reservoir. Filtration is built into the power pack 124 to filter the fluid returning from the circuits as well as fluid added to the system. Level sensors and pressure switches are used to complete the control circuits.
Referring to
In this example, the treadmill control system is located in the control room 100 outside the scan room 102 as shown in
Referring to
A speed signal is routed to a motor controller that controls the speed of the electric motor located in the power pack 124 outside the MRI exam room. The electric motor controls the speed of a pump 200, which in turn controls the rate of fluid flow delivered to the hydraulic motor 113 located in the treadmill 110. The signal from the motor shaft speed sensor 410 is fed into a feedback loop where it is compared with the intended speed of the hydraulic motor 113. A signal is sent to the electric motor control, which alters the speed of the electric motor in the power pack 124.
At higher elevations of the treadmill belt 111, depending mainly on the weight of the patient, the work of the patient running on the treadmill 110 acts to drive the motor 113 to a speed that is higher than desired. At this point, the valve 502 is activated, creating back pressure on the hydraulic motor 113. The hydraulic motor 113 then acts as a brake, enabling the system to maintain the prescribed speed. An emergency stop button located on the body of the treadmill 110 provides a motor shutoff signal if needed.
The treadmill elevation signal is output to the elevation mechanism. The mechanism may be a pre-charged accumulator that outputs the desired quantity of fluid through a valve either to a non-ferromagnetic hydraulic cylinder or to a master-slave cylinder system in which a traditional hydraulic cylinder located outside the room controls a slave cylinder located on the treadmill. A feedback signal is received from the elevation sensor, which may be either from a linear potentiometer located on the elevation cylinder or a fluid filled tilt sensor located on any flat surface of the treadmill. This signal enters a separate feedback loop where it is compared to the intended elevation.
Referring to
Referring to
In an example embodiment of the present invention, slice localization by single-shot steady-state free precession (SSFP) imaging is followed by resting cine imaging in step 802. The cine function sequence is configured to scan each slice position for approximately 2 seconds, while the temporal resolution varies depending on the size of the patient and the resulting field of view. A test acquisition for first-pass perfusion may be performed without contrast agent. Pulse sequences are queued for stress imaging such that they may be executed automatically from the scan start button located on the MR imager 112. The patient is then removed from the MR imager 112. Certain makes and models of MR imagers may require medical staff to use extra care when removing the patient so as not to pull the patient table 750 all the way out of the MR imager 112, and not to move the surface array coil too drastically. Either of these actions may cause certain systems to repeat adjustments prior to the start of the stress scan, causing delays.
Next, the patient exercises on the treadmill 110 positioned inside the scan room 102. In an example test, the speed and elevation of the treadmill 110 are progressively increased every three minutes following the standard Bruce protocol. 12-lead ECG is continuously monitored during exercise. Blood pressure is measured and a hard copy of the ECG is obtained at the midpoint of each Bruce protocol stage. As with conventional stress testing, patients are continuously monitored by a nurse and/or physician who may stop the test at any time based on recognition of adverse endpoints or in response to the patient's request.
After reaching his or her exercise limit or the maximum predicted heart rate (MPHR) based on age (220-age), the patient is moved onto the patient table 750 of the MR imager 112 (step 806). The surface coil is placed on the chest, the contrast injector 118 is connected to a previously inserted IV in the patient's arm, and the patient table 750 is withdrawn to move the patient inside the MR imager 112. The previously prepared cine and first-pass perfusion scans are started using the start button located on the MR imager 112 (step 808); stress function is executed first, followed by stress perfusion in step 810. The time from end of exercise to start of imaging (Tstart) is recorded. A member of the medical team starts the injection protocol as soon as an audible change from the cine pulse sequence to the first-pass pulse sequence is detected. The patient remains inside the MR imager 112 for approximately 90 seconds for stress imaging.
Following exercise on the treadmill 110, MR studies are executed to evaluate cardiac function and myocardial perfusion at peak stress. In an example embodiment of the present invention, cardiac function is evaluated using a real-time steady-state free precession (SSFP) pulse sequence with TR/TE of 2.3/1.0 msec and Temporal Sensitivity Encoding (TSENSE) acceleration factor of 3. Five slices are acquired in the short axis (SAX) direction, and one slice each in horizontal (HLA) and vertical (VLA) long axis directions. Temporal resolution of 57 msec and spatial resolution of 3.0 mm×3.8 mm×8 mm may be achieved with no breath-hold and no ECG gating. Each slice position is scanned for approximately two seconds depicting three or more cardiac cycles, depending on heart-rate. Thus, cine images depicting three or more cardiac cycles in each of seven slice locations including short-axis and long-axis views may be acquired in approximately 14 seconds at peak stress. Other data acquisition methods may be used such as improved array coils to accelerate scanning, scanning more slices, or scanning each slice for more heartbeats, or using segmented k-space acquisition methods to improve temporal resolution even further.
In an example embodiment of the present invention, immediately following the acquisition of cardiac function images, first-pass cardiac perfusion images are obtained during intravenous infusion of a contrast agent of 0.1 mmol/kg gadolinium-DTPA at a rate of 4 mUs. Other doses or rates may be used. A gradient-echo echo-planar (GRE-EPI) imaging sequence with TR/TE of 5.8/1.2 msec and TSENSE acceleration rate of 2 is used to acquire three short-axis slices each cardiac cycle. A saturation recovery time of 30 msec may be used and an acquisition time per slice of 70 msec (96×160 matrix, 3.0 mm×2.4 mm×10 mm resolution). These sequence parameters appear to be optimal, but there are many more options that are feasible.
Other imaging options that may be used in conjunction with the present invention include: cine only covering more slices and views; perfusion only; cine followed by perfusion; perfusion followed by cine; real-time blood flow velocity mapping; real-time myocardial velocity mapping; real-time cardiac tagging for myocardial strain measurement; real-time displacement encoded stimulated echo (DENSE) for myocardial strain measurement; MR spectroscopy measurement of myocardial metabolism at peak stress; and MR spectroscopy measurement of skeletal muscle metabolism at peak stress.
Following imaging at peak stress, the patient table is removed from the bore of the MR imager 112 in step 812 and diagnostic 12-lead ECG and blood pressure monitoring is performed during the supine recovery period lasting approximately 6-10 minutes. Following this recovery period, the patient is moved again into the bore of the MR imager 112 for additional imaging. Resting cardiac function images and resting first-pass perfusion images are acquired in step 816 using the methods previously described. After another ten minutes to allow the contrast agent to reach equilibrium, delayed myocardial enhancement (DME) (step 818) images are acquired to detect any regions of myocardial infarction or fibrosis. Additional scans may be performed to evaluate valve function, diastolic dysfunction, atrial function, size and compliance of the aorta, and a variety of other common cardiovascular MRI techniques.
During the test, a supervising cardiologist may review interim findings, particularly if they warrant termination of exercise such as severe ischemic ECG changes accompanied by worrisome symptoms. Upon completion of the test, the supervising cardiologist assimilates all of the information including the patient's history, any symptoms recorded during exercise, ECG tracings recorded before/during/after exercise, and the CMR images. Software that displays all the images in a format suitable for rapid review and comparison is used. A comprehensive interpretation of the test results may include assessment of the patients exercise capacity, symptoms and their time of onset as well as mode of resolution, ECG changes, and stress-induced contractile and perfusion response. In addition, CMR allows direct visualization of scarred myocardium that can be incorporated into both segmental and patient-level interpretations of normal/no ischemia, fixed infarction, or ischemic response to stress.
A detailed list of features of the present invention and related advantages are summarized in Table 3.
Currently, nearly 10 million cardiovascular stress imaging studies performed annually using echocardiography and nuclear scintigraphy. The present invention allows the superior imaging provided by MR imaging to be used for cardiovascular stress imaging studies. The MR-compatible treadmill system of the present invention supports the use of MR imaging which provides a diagnostic advantage over current echocardiography and nuclear scintigraphy. The present invention allows rapid acquisition of MR images following exercise to more accurately diagnosis cardiovascular disease while increasing patient safety by minimizing the travel required between exercise equipment and the MR imager table.
While certain embodiment(s) of the present invention have been described in detail above, the scope of the invention is not to be considered limited by such disclosure, and modifications are possible without departing from the spirit of the invention as evidenced by the following claims:
This application is a continuation under 35 USC §120 of application Ser. No. 12/424,835 filed Apr. 16, 2009, which is a continuation of application number PCT/US07/81948, filed Oct. 19, 2007, titled SYSTEM AND METHOD FOR CARDIOVASCULAR EXERCISE STRESS MRI, which is in turn entitled to benefit of a right of priority under 35 USC §119 from U.S. Ser. No. 60/862,107, filed Oct. 19, 2006, titled MAGNETIC RESONANCE COMPATIBLE TREADMILL, both of which are herein incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60862107 | Oct 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12424835 | Apr 2009 | US |
| Child | 13601483 | US | |
| Parent | PCT/US07/81948 | Oct 2007 | US |
| Child | 12424835 | US |
