 Illustration of the spin echo sequence The Bloch simulator Tissue in the MRI scanner is magnetized by the strong magnetic field. The magnetization is subsequently manipulated by use of radio waves and field gradients. The dynamics are described by the Bloch equations.  Below is shown a screen dump from a Bloch simulator written for educational purposes by Lars G. Hanson (M.Sc, Ph.D) who got the ESMRMB software award for this work, October 2009. The software is used to explore fundamental aspects of MRI such as precession, resonance, excitation, inhomogeneity and relaxation. Important concepts such as rotating frames, weightings, spoilers, spin-echoes, stimulated echoes and driven equilibrium can also be demonstrated using the program. Finally, the fundamentals of MR imaging can be shown, i.e. how the similarity between induced phase roll patterns and the structure of the imaged object is reflected in the MR signal. The simulator and related ressources is useful during lectures for both technical and non-technical students (radiographers, MDs,...). It has also proven useful for student exercises. Examples are given in the software documentation which is distributed with the software (choose "Help" and "Challenges"). At the bottom of this page, you will find a download version and example movies recorded using the program. Answers to frequently asked questions are in the FAQ .  Above: Screen dump illustrating the use of the program. Click image one or more times to enlarge. Initial conditions are chosen via menues as shown. Subsequently radio-frequency (RF) pulses can be employed to illustrate MR techniques and sequences. The screen shot is captured during a demonstration of excitation in the presence of field inhomogeneities. The spin isochromates shown in white are pushed by the RF field as indicated by the red bars. Example animations with soundtracks published in RadioGraphicsThese animations with soundtracks show examples of interactive use of the simulator for teaching. Additional movies (no soundtrack)
The following additional simulations made with the add-on OpenGL viewer are representative of the programs capabilities to explore more complicated matters. They were recorded by choosing appropriate initial conditions and pulse sequence elements (RF pulses, spoilers,...) via the graphical user interface. - Stimulated echo. A more complicated sequence is shown in this movie, where the x-axis is used to indicate the position along the direction of a field gradient. Hence, a row of spin isochomates precessing at different frequencies are shown. After a 90 degree excitation pulse, a phase roll accumulates due to the variation in Larmor precession frequency. The wavelength of the phase roll decreases until a second 90 degree pulse is employed. This rotates the disc of spins and the transversal component of the magnetization dephases. After spoiling (i.e. projection of the magnetization on to the z-axis) it becomes clear that the residual longitudinal magnetization forms a sinusoidal pattern. A third 90 degree RF pulse rotates this sinusoidal pattern into the transversal plane. It is partially refocused by the gradient after a waiting period equal to the first dephasing period.
- Spin ensemble. The net magnetization (shown in green) is made up from many small contributions pointing in different directions. This is a result of the imperfect alignment caused by the relatively weak fields used for MRI (the field is strong compared to the earth field, for example, but the thermal energies by far exceeds the energy differences associated with nuclear orientation). The near isotropic spin-distribution is rotated as a fixed structure by RF. Hence it is sufficient to consider only the dynamics of the net magnetization for regions where the field variations are insignificant.
Note: The spins are not only pointing up or down, as often said. This and other MRI myths are discussed here.
Download The program described above is available for download as an IDL "save" file at http://www.drcmr.dk/Bloch/bloch.sav . It does not require a full IDL licence, but can be run from a IDL virtual machine, that can be downloaded from the ITT homepage free of charge (or from here: Mac, Windows, Linux (version 6.3. Later is needed for Vista)). Install guide for running bloch.sav in an IDL Virtual Machine: - Download IDL VM either from here ( Mac, Windows, Linux) or by following steps 1-4 below:
- Go to the ITT homepage
- Select Products in the top menu and choose IDL VM (virtual machine).
- Click on download IDL VM (you need to register to download the program from the ITT homepage).
- Select the appropriate version for your computer.
- Install IDL VM. It takes up approximately 200 MB of space depending on version. Default install options are appropriate. Mac users: Avoid spaces in install path.
- Download bloch.sav
- Run the file with IDL VM:
- Windows: Double click the bloch.sav file and click anywhere on the IDL startup window to get past it.
- Linux: Do the same or type "idl -vm=bloch.sav" to start the program.
- Mac: Do the same or click "bloch.sav" to associate it with the application "idlvm.app" in the IDL install directory.
- Now test the simulator. If you like it, you should definitely consider downloading the free Bloch viewer that greatly improves the clarity by exploiting the capabilities of modern 3D graphics cards. Other supplemental ressources are at http://www.drcmr.dk/MR
- In case of problems, please consult the trouble-shooting guide that covers both the simulator and the viewer.
The new version 3.1 published July 2008 resolves a Vista compatibility problem, avoids window clutter, and improves the visualization of spoiling. If you are already using the simulator, only one file bloch.sav needs to be updated to get the new features - check window title to see which version, you are currently running. Contact Lars, if you have comments, questions or wish to be notified of future updates (address: larsh @ drcmr.dk). Links to related resource |
