What is resonance (simple explanation)?  What determines resonance in MR and how is it used to select slice. (Ch3, q5, 8, 10, 12)

Resonance is a phenomenon in physics that can be applied to many systems, but in essence deals with principles of energy transfer at particular frequencies. In MRI, the frequency of the electromagnetic pulses is the same, or resonates with, the frequency of the precession of nuclear spins in a magnetic field, which allows the application of small amounts of energy to the nuclei to result in large changes in the system overall. Resonance in MR is determined by the Larmor frequency. It is the frequency at which certain atomic nuclei or particles are excited (absorb photons) to a different energy level, and thus allows the exchange of energy with the magnetic pulse. In MRI, this nucleus is the hydrogen atom, which consists of a single proton. The Larmor frequency also describes the rate at which the proton is precessing (which happens because it is a charged particle in a magnetic field). This frequency depends upon the gyromagnetic ratio, which in turn depends upon magnetic and rotational properties (charge and mass) of the nucleus.   

In MR, energy is transmitted via RF pulses, set at the resonant frequency, or the Larmor frequency. There are two states that the nuclear spin can be in, and when energy is absorbed at just the right frequency, the state switches into the higher-energy state. This is the excitation of the protons, which is done to only a thin slice by adjusting the frequency and magnetic field. Because only those protons receiving the RF pulses at the resonant frequency will be excited, it allows the excitation of a slice of the brain instead of the whole area at one, because other locations will experience slightly different frequencies due to the gradient.

How is location determined within slice (e.g. see ch4 q 7, 16)

Spatial information is determined from both the phase of the magnetization and the frequency of the MR signal. By using a gradient magnetic field, the phases and frequencies of protons in different locations can be localized. The two methods that can localize the signal being detected by the receiving coil are phase encoding and frequency encoding, which can be separated according to their timing during the image formation process.

Phase encoding is based upon the different rates of precession of spins. A stronger magnetic field will cause the precession to speed up. This gradient is applied prior to data acquisition.  In contrast, frequency encoding relies upon the differential effects of a gradient magnetic field that applied during data acquisition. When the frequency encoding gradient is switched on during the data acquisition, different regions experience different strengths of magnetic fields. The amplitude of the signal that is detected depends upon how many spins in a certain location are aligned with the gradient.

Information from 3 dimensions, provided by 1) slice excitation, 2) frequency encoding, and 3) phase encoding, is used to create an image of spatial location based on frequencies and phases. This is encoded, in that a Fourier transform is used to create the image itself from the raw data.

Be able to explain difference between T1 vs. T2 (ch3, q14-16; ch 5, q6-10)

T1 measures the decay of the signal over time as the longitudinal magnetization returns to its equilibrium state. Different tissues have different T1 values (meaning the protons in different tissues take different amounts of time to return to equilibrium from an excited longitudinal magnetization). An intermediate TR is needed, along with a small TE to minimize contrast due to T2 and T2*. 

T2-weighted images show the different times between tissues in how long it takes for transverse magnetization to return to an equilibrium state. The length of time this takes differs according to the type of tissue, and the decay of the signal at different points in time can thus give information about which type of tissue is sending the signal depending on its strength. An intermediate TE is and a long TR is needed. 

T2 measures larger scale inhomogeneities and generally takes longer than T1, because transverse magnetization is caused by larger variations in the magnetic field. T1 is sensitive to smaller changes. A T1 weighted image is useful for an anatomic reference scan, while T2 is used in functional studies.

What is a fourier transform? (simple explanation - current answer Ch4 q8 on wiki not adequate – look it up)

A Fourier transform is the conversion of data in the time domain (which is how it is collected), into the frequency domain, by modeling it as a sine wave. The magnetic gradients and RF pulses focused on a slice allow differentiation of locations based on the phase and the frequency data collected from each coordinate. The Fourier transform is needed to convert the raw values into phase and frequency information that can be displayed as a wave

How does k-space differ from 'normal space' illustrate this with some concrete examples (see ch4, q's 5, 11, 12, 14)

            The biggest difference is that the location of coordinates in k-space does not correspond in a straightforward manner to that of ‘normal’ space. In short, k-space is a graph of spatial frequency. That is, k-space is a representation of normal space that cannot be visualized as a translation or any simple functional operation. Although a k-space image may be shown as, for instance, a 3x3 inch 2D diagram that is transformed into a 3x3 inch image, a coordinate on the k-space graph does not determine the coordinate in that same location in Cartesian space on the image.

Each coordinate of k-space represents the summation of the gradient fields’ effects on the MR signal for all of the voxels at that location. Data is transformed from k-space to normal space via a Fourier transform, which depends on phase and frequency data. Large values in k-space are found in the center, and the smaller the value the further from the center the coordinate will be located. The larger the dimensions to the k-space image (the larger the amount of information in the periphery), the more detailed the resulting image will appear. In short, the intensity of the signal determines the central part of k-space, and the level of detail determines the periphery.

Why are slices interleaved, how does this affect temporal characteristics of fMRI? (for first part, see Ch4, q15) Slices are interleaved in order to prevent surrounding areas outside of a slice becoming saturated with MR signal, thus eliminating meaningful data from being collected. It does increase the time needed, as samples are not as spatially close to one another and it takes time to change locations back and forth.

What is TR and TE (Ch5, q3,4)

TR is repetition time, or the time between the beginning of each pulse sequence. Adjusting TR influences the contrast that shows up on an image.  

 TE is echo time, or the time between excitation and data collection, when the MR signal is of the greatest amplitude before it begins to decay over time. The TE also influences the contrast in an image.

What is a susceptibility artifact? (Ch5, q18)

A susceptibility artifact is a distortion on an image that is the result of a change in the strength, or inhomogeneity, of a magnetic field. This is likely to occur at the boundary of tissue types, for instance between brain and cavities filled with air (e.g. nose, etc.) Long echo times and gradient echo sequences increase the likelihood of these types of artifacts.