MRI: Basic Principles and Applications - Essay Example

?Question One In MRI applications, the main magnetic field is homogeneous so the same Larmor frequency makes all spins resonate. Consequently one precessional frequency for the entire body is available for all spins. The strength of the magnetic field determines the Larmor frequency so to produce discernable data, gradient are applied in each direction. The application of differing gradients ensures that different spins and thus different frequencies can be achieved that can be easy to read. Read Direction Encoding Nuclei are spatially encoded in the read direction by utilising a pulse and collect sequence. The application of the sequence produces a FID which is read to get the gradient required. Multiple frequencies are produced as the read gradient is applied. The variation of frequencies is linearly connected. The total change of frequency experienced depends on the position within the gradient. After the FID is acquired, it is treated with a Fourier transform. This produces a spectrum that displays peaks corresponding to different frequencies. The sum total of all signal intensity values at one single place of observation become individual peaks. A one dimensional quantity is produced by the application of the read gradient as it is independent of time. (Weishaupt et al., 2006) Phase Direction Encoding A phase gradient is applied after applying a read gradient and slice selection. This is otherwise known as phase encoding and tends to increase the nuclei’s frequency such that it precesses at different angles that all match up with the Larmor frequency. The increase of frequency due to the application of a phase gradient directly impacts the total phase change displayed by nuclei. However there is a need to discern different nuclei which can be done by the application of Fourier transforms. (Westbrook et al., 2005) Question Two Using the Fourier transforms helps to convert the available data from the time domain to the frequency domain. This can then be utilised to form two dimensional or three dimensional images based on available data. Data is spatially encoded before becoming a part of the k space and so its position within the k space can be determined accordingly. Application of the first Fourier transforms aids in interpreting the data values that were encoded in the read direction. This is useful in identifying the frequency (alternatively signal intensity) within the plane selected for the application of the read gradient. This makes it simple to differentiate the positions within the k space’s horizontal trajectory. The data obtained in this way has its units changed from m-1 to m. Consequently only a one dimensional image is formed. (Woodward, 2001) Application of the second Fourier transform helps to differentiate various frequencies that were encoded along the phase direction after the application of a phase gradient. This transform separates all the values and lists them accordingly. The vertical k space trajectories are dealt with this transformation. The units again change from m-1 to m and the resulting image becomes two dimensional. (IMAIOS, 2009) The total k space contains data encoded from two directions that are the read and the phase directions. The read direction’s data is displayed as horizontal trajectories in the k space while the phase direction’s data is displayed as vertical trajectories in the k space. Fourier transforms aid in creating a complete two dimensional image of the concerned nuclear spin densities in relation to the slice positions. (Hashemi et al., 2004) Question Three Various experimental factors affect transverse spin coherence as well as the k space. These factors and their effects are listed below. Radio Frequency Pulse: A radio frequency pulse at 90o is utilised along with the chief magnetic field to produce magnetism such that the Z direction vector reorients itself into the X plane the Y plane. The magnetism produced is subsequently de-phased both in the X plane and the Y plane. This requires one more re-phasing at 180o. Read Gradient: Read gradients are applied during the acquisition of data particularly as signal intensities reach their maximum in an echo period. Read gradient application aids in de-phasing a signal with loose coherence and takes it to zero whenever data acquisition is required. Signal intensity is retraced through the application of a read gradient. The preparation gradient is half the value of a read gradient and is applied before or after the 180o pulse is applied. Application before the pulse ensures that the preparation gradient is in the same direction and vice versa. taken as a whole in k space terms, one horizontal trajectory in the k space denotes a single echo which signifies the application of one read gradient. (Westbrook & Kaut-Roth, 2005) Slice Gradient: Slice gradient can be applied to both the 90o and the 180o pulses. Slices are chosen through gradient selection through alteration of the frequency depending on its position in the gradient. Application of the slice gradient can cause a loss of magnetisation. This causes loss of spin which in turn causes loose coherence to be displayed in the X-Y plane. The signal is then retraced by applying a gradient with opposing orientation for the same amount of time. A single k space is composed of one slice. The slice gradient being considered is placed in the centre of the total k space. Phase Gradient: Phase gradients are applied prior to the 180o pulse. It is a strong gradient as well and is comparable to the slice gradient. It causes de-phasing as well (loss of magnetisation) which causes a decrease in the total signal and coherence. The term TR symbolises the time for reparation observed between the first 90o and the first 180o pulses. Phase gradients are changed every TR so as to reimburse the loss of signal as a phase gradient is applied. The k space and the vertical k space trajectories tend to store the encoding created by the application of the phase gradient. A single vertical trajectory seen in the k space is actually the application of one phase gradient. (Woodward, 2001) Signal Application: A single echo is produced in a single TR. The maximum signal loss is experienced at the peak within any echo. Echoes are in turn produced by the application of 90o pulses followed by 180o pulses. A single line in a k space symbolises an echo. (Brown & Semelka, 1999) References Brown, M.A. & Semelka, R.C., 1999. MRI, basic principles and applications. New York: Wiley-Liss. Hashemi, R.H., Bradley, W.G. & Lisanti, C.J., 2004. MRI: The basics. 2nd ed. Lippincott : Williams & Wilkins. IMAIOS, 2009. 2D Fourier transform and MRI image reconstruction. [Online] Available at: HYPERLINK "http://www.imaios.com/en/e-Courses/e- MRI/image-formation/2D-Fourier-transform" http://www.imaios.com/en/e-Courses/e- MRI/image-formation/2D-Fourier-transform [Accessed 24 August 2011]. Weishaupt, D., Marincek, B. & Koechli, V.D., 2006. How does MRI work? An introduction to the physics and function of magnetic resonance imaging. New York: Springer. Westbrook, C. & Kaut-Roth, C., 2005. MRI in Practice. 3rd ed. Wiley-Blackwell. Westbrook, C., Kaut-Roth, C. & Talbot, J., 2005. MRI in practice. Malden, MA: Blackwell Publishers. Woodward, P., 2001. MRI for technologists. New York: McGraw-Hill Medical Publishing Division. Read More