Animal Ventilator for Gated Hyperpolarized Helium MRI Ashley Anderson III, Micah Brown, Matt Smith, Chris Wegener, Dr. Sean Fain1 Abstract- The use of hyperpolarized helium (He-3) as a contrast agent in functional magnetic resonance imaging (fMRI) is an emerging and promising technique for diagnosing diseases and abnormalities in the respiratory tract. Current methodology for animal studies allows fMRI imaging to be performed during inhalation of 100% He-3 every fourth breath. This limitation leads to average scan times of approximately 8 minutes for a diagnostic scan (scan parameters with breath gating). This paper will discuss the production of a ventilation device that delivers an 80:20 ratio of hyperpolarized helium and oxygen gas mixture so that image acquisition can be performed with every breath, decreasing scan time by a factor of 4. This device is needed to function as an oxygen ventilator and serve as the means to integrate He-3 into the respiratory tract of anesthetized small animals with every breath. Index Terms- animal ventilator, hyperpolarized helium, He-3 MRI, fMRI I. Introduction Medical imaging systems have proven their capabilities in diagnostic means and physiological verification. Magnetic Resonance Imaging (MRI) is superior in detecting soft tissue contrast. Another advantage with MRI, unlike Computed Tomography (CT), is the ability to image in oblique planes. This is very advantageous in a number of Manuscript received May 4th, 2007. This work is supported by the laboratory of Dr. Sean Fain, from the University of Wisconsin-Madison in Madison, Wisconsin. A. Anderson III is with the Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin. M. Brown is with the Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin. M. Smith is with the Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin. C. Wegener is with the Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, Wisconsin. S. Fain M. Brown is with the Department of Biomedical Engineering, Department of Medical Physics, Department of Radiology, University of Wisconsin-Madison, Madison, Wisconsin.2different applications, such as interventional MRI and the diagnosis of abnormal tissues. Due to the highly configurable nature of MRI, various pulse sequences can be written to highlight different aspects of the subject anatomy or physiology. A relatively newer technique in MRI is the use of hyperpolarized Helium (He-3) as a source of signal. In order for Helium to have the magnetic properties that allow it to be imaged with MRI it must be hyperpolarized (He-3). The hyperpolarization gives the Helium a heightened spin state that is required to give an MR signal. Although there is no natural He-3 in the human body, it can be safely inhaled to achieve signal in the respiratory tract and lung systems, which are often difficult areas to image with conventional MRI because of the air interfaces. Oxygen has a paramagnetic effect that depolarizes He-3, destroying the property that allows it to be imaged1. Therefore, great care is taken to avoid mixing the two gases prior to the scanning. The conventional clinical MRI detects signal from the H1 protons in the body that resonate within the main magnetic field at 42.58 MHz/T. There are other potential sources of signal in the body, like 31P (11.26 MHz/T) and 13C (10.71 MHz/T) that can be imaged with MRI as well3. He-3 MRI utilizes the same magnet except instead of tuning the receiver to detect the signal from H1, it is tuned to He-3 which resonates at 32.43 MHz/T. Therefore, the scanner detects the signal specifically and solely from He-3. The gas is typically inhaled while the MR scanner acquires the signal. Conventional image reconstruction techniques can be performed with the acquired data to yield images of the signal. During the onset of He-3 inhalation, it is possible in the time-resolved image set to view the signal traveling down the trachea in humans much like the bolus of contrast in Contrast Enhanced MRI (CE-MRI). Thus, He-3 MRI can reveal physiological information about both the structure and therefore function of the respiratory system. Current animal studies are being performed to test the ability and effectiveness of He-3 MRI in diagnosing respiratory diseases. The He-3 MRI interpretation of a respiratory disease can be compared to both Positron Emission Tomography (PET) images and histological examination (see Figure 1). During current studies the anesthetized small animals are vitally sustained through an oxygen ventilator system (MRI-1 Volume Ventilator, Ardmore PN) that uses pneumatic valves to control breathing rates and tidal volume gas delivery. Previous3integration of He-3 into the animal suppressed the ventilator every fourth breath to allow a stepper motor controlled syringe to deliver the appropriate tidal volume of He-3. Through this method, image acquisition can only be performed every fourth breath when the He-3 is the animal’s airways. In order to acquire He-3 signal with every breath and decrease necessary scan time by a factor of four, a dual syringe system was developed. This system integrated oxygen at 20% of the tidal volume through one syringe to sustain life and He-3 at 80% of the tidal volume through the second syringe to allow adequate signal detection from the respiratory system. The following sections will discuss the production of the He-3/Oxygen delivery system, the software developed for user interface and parameter control, the calibration techniques used, and validation testing results. II. Ventilator Design The ventilator described in the following sections was designed to provide the same Oxygen delivery as the conventional ventilator and thus is used both to maintain the animal breathing during anesthesia and to provide the He-3 signal for imaging. Until the imaging is performed during the study, the anesthetized animal receives Oxygen through the original ventilator. Once ready, the Oxygen delivery responsibility is transferred to our ventilator. The ventilator design (see Figure 2) incorporates the use of two sliders traveling on two aluminum rods respectively. Each slider is controlled by a rack and pinion gear system. The diameters of each of the two gears are integral dimensions for determining the ratio of He-3 and Oxygen delivered to the animal.
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