ABSTRACT:

1. MR Microimaging at 7 T: Pulse Sequence Development & Implementation Ms. Tracy L. Doyle Mr. Ramchand Maharaj Upper St. Clair High School, Pittsburgh, PA Blanche Ely High School, Pompano Beach, FL Vinyl Nut in Water (Resolution 50x50 mm):Used for gradient calibration • PROCEDURE: • Basic spectroscopy and imaging techniques were performed on several samples immersed in water through the development of the following pulse sequences. • - A Free Induction Decay (FID) • - A Spin Echo • - Gradient calibration factors in the x, y and z directions • - One-dimensional imaging (a single gradient in the read direction) • Two-dimensional imaging with phase encoding (gradients in the read & phase directions) • - Two-dimensional imaging with slice selection (gradients in the read, phase & slice directions) • Three-dimensional imaging with multiple slices • Cleaner images with less noise (crusher gradients, phase rewrapping & gradient ramping) • IDEALIZED PULSE SEQUENCE: Spin-echo Imaging • The following diagram displays a 3D spin-echo imaging sequence. Using this template, the spin-echo pulse sequence was implemented in NTNMR code for the Tecmag console. Equations were created so that, by defining only a few variables (TE, TR, FOV, slice thickness, etc.), gradient strengths and timing parameters were calculated automatically. • EXAMPLE OF AN NTNMR PULSE SEQUENCE: • EXAMPLE OF EXPRESSIONS USED: • Below are the expressions used for creating multiple slices in the axial plane. • d1 = [te]/2-[pw90]/2-[pw180]/2-[dphase]-[dcrush] • d2 = [te]/2-[pw180]/2-[Acq. Time]/2-[ad]-[dcrush] • Last Delay = ([tr]-[te]-[Acq. Time]/2-7u-[pw90]/2-[dphase])/[Scans 1D] • Gfocus = val([Gread]*[Acq. Time]/(2*[d1])) • Gread = 2*[SW +/-]*10000/42570000*1/[FOVread cm]*[x calibration] • Gphase = val(117.5*[Points 3D]/[FOVphase cm]/[dphase]*[y calibration]) • Gslice = val(2*1000000/[pw90]*1/4258*1/[slicethickness]*[z calibration]) • Gslicefocus = -1*[Gslice]*val([pw90]/(2*[d1])) • INTERPRETING IMAGES: • Sample preparation: All samples for this study were placed in a 5-mm diameter glass tube and immersed in water. • Hyperintense areas: Represent the water around an object; the high intensity of the signal in the surrounding water is due to the high concentration of mobile proton nuclei. • Hypointense areas: Represent the object in the water or the air around the tube; the low signal is due to the absence of mobile proton nuclei. As a result, these areas display only noise. APPLICATIONS: The pulse sequences developed here are to be used for imaging with the Keck 25-T magnet and Series Connected Hybrid magnet currently in development (~36 T). As such, these pulse sequences will generate the highest field MR images yet achieved. The pulse sequences that were developed can be used as a training tool for students of NMR and MRI. The Tecmag console provides a relatively inexpensive and flexible tool for these studies, although certain problems were encountered during the implementation of the 3D spin-echo sequences (e.g. expression handling). Future work will seek to resolve these issues and implement additional pulse sequences to enhance the capabilities of this high field MR microscopy system. ACKNOWLEDGEMENTS: William W. Brey, Ph.D., Associate Scholar Scientist, CIMAR Samuel C. Grant, Ph.D., Assistant Professor, CIMAR & Chemical & Biomedical Engineering Kiran K. Shetty,Assistant in Engineering, CIMAR REFERENCES: 1. Joseph P. Hornak, Ph.D., The Basics of MRI, (2006). www.cis.rit.edu/htbooks/mri/ 2. Zhi-Pei Liang and Paul Lauterbur, Principles of Magnetic Resonance Imaging: A Signal Processing Perspective, (1996). 3. William Faulkner, Rad Tech’s Guide to MRI: Basic Physics, Instrumentation, and Quality Control, (2002). 4. Eiichi Fukushima and Stephen Roeder, Experimental Pulse NMR: A Nuts and Bolts Approach, (1981). 5. www.simplyphysics.com ABSTRACT: The purpose of this project was to take an existing 300MHz magnet and Tecmag Discovery console used for Nuclear Magnetic Resonance (NMR) spectroscopy, and create a pulse sequence that would allow the existing NTNMR software to collect 3D images, a process referred to as Magnetic Resonance Imaging (MRI). This project required a basic understanding of the NMR and MRI processes, calibration of the system components, and interpretation & manipulation of data. BACKGROUND: NMR SPECTROSCOPY NMR spectroscopy is a technique used by scientists to obtain physical, chemical, electronic and structural information about a molecule. The nucleus of an atom, composed of protons and neutrons, can behave like a small magnet due to a property known as “spin”. The hydrogen atom is a good example. It’s nucleus is a single proton that has a spin of ½. In a tube of water (H2O) the proton spins are randomly oriented and so their magnetic fields act in all different directions, cancelling each other. When placed in a magnetic field, the proton spins align and can be measured through the NMR effect. The interaction of these nuclei with each other and with the larger external magnetic field provides the information of the NMR process. NMR active elements include hydrogen, phosphorus, sodium, carbon, nitrogen and fluorine. Of these, hydrogen (1H) is the most sensitive and concentrated (i.e. water molecules), and is most often utilized in MRI. Below is a simplified explanation of the NMR spectroscopy process. 1 mm 1 mm Multiple Sagittal Slices of Vinyl Screw (50x50x370 mm) Step 2: The sample is placed in a powerful magnetic field. Once inside the magnetic field, the spinning protons will align with the magnetic field. Although some protons may become aligned against the field, a slight excess will point in the direction of the field, as shown below. Step 1: In NMR spectroscopy, a sample is selected to be analyzed. For example, hydrogen nuclei have one spinning proton. Initially, these nuclei spin in different directions. The different spins create different magnetic fields in a variety of directions. Zebra Fish Imaged at 11.75 T using Bruker software & console Step 3: A Radio Frequency (RF) pulse is then applied to the sample for a short period of time. The nucleus absorbs energy from the pulse, causing the spin of the nucleus to change its orientation in the magnet. In the diagram, the absorbed energy causes the excited nucleon to move from the +z-direction to the x-y plane, or even to the –z-direction. Step 4: Following the RF pulse, the nucleus releases energy as it returns to its original orientation, aligned with the magnetic field (lower energy state). The energy released from the nucleus is in the form of a RF wave. The frequency of this wave is used to identify the chemical make up of the sample. Different elements release this pulse at unique frequencies. Multi-slice axials: 78x78x500 mm Multi-slice coronals: 86x86x250 mm BACKGROUND: MRI Encoding MRI uses the basic principles of NMR spectroscopy but incorporates spatial information. NMR gives an overview of an entire sample, what the sample is made up of but not where the different components are located within the sample. MRIs indicate the what and where of a sample. To spatial encode a sample, MRI uses additional external magnetic fields in the x, y, and z-directions. These linearly varying magnetic fields are referred to as gradients. Due to these gradients, each point in 3-dimensional space has a unique magnetic field strength, to which the nuclei at that point react. The signal and contrast of an MR image depends on several factors, including the concentration of nuclei at a given point, the interactions of those nuclei with the environment and the interaction of those nuclei with other nuclei. MRI is three dimensional, and slices of any object can be acquired in any direction. The views obtained from orthogonal slices are called sagittal (z-y plane), coronal (z-x plane)& axial (x-y plane).