FRCR Physics Notes

A deck covering basics of FRCR Physics

Sample Data

Front

Gradient (Recalled) Echo Sequence

Back

Spin echo sequences work fine for sequences of a long TR. If a short TR is needed (for example, in T1 weighted scans), we need to cut down the scan time. We do this by forgoing the 180° RF pulse and, instead, using a gradient to rephase the spins. This is a gradient echo sequence.1. RF pulse applied2. Slice-select gradient applied3. Phase-encoding gradient applied4. Frequency-encoding gradient applied‣ A negative GFE is applied. The spins dephase, some faster than others.‣ The positive GFE is applied. The spins start to rephase until they are againin phase and a signal is created - the Gradient Echo

The other aspect of a GRE sequence is that you don’t have to use a 90° RF pulse at the start of the cycle, an RF pulse of any flip angle can be used. If an RF pulse with a smaller flip angle is used, it will take less time for the spins to regain all their Mz as they are closer to 0°. However, this also means that the Mxy signal is not as high as if a 90° flip angle was used.

Front	Gamma Camera
Back	The gamma camera is the equipment used to detect the distribution of radiopharmaceutical within the patientComponents:Components: Collimator Radiation detector‣ Scintillation crystal‣ Photomultiplier tubes Electronics‣ Preamplifier

Front

Turbo Spin Echo (TSE) Sequence

Back

We can take the multi-echo sequence further. We can repeat the 180° RF pulses many times, creating many echoes, within one cycle. If we apply a different GPE each time we can fill up different lines of k-space. The number of echoes we create is called the “echo train length (ETL)”. In the above example, the ETL is 5 but we can use an ETL of 212. The TE measured is taken to be the echo created when the GPE is zero and is called the “effective TE (Teff)”.N.B. The phase-encoding gradient is reversed prior to the next 180° RF pulse to rephase the spins.This will shorten the sequence time:Normal spin echo = TR x no. GPE x number of slicesTurbo spin echo = TR x no. GPE x number of slices / ETLAdvantages Very fast - useful for MR angiography in which very fast scan times are needed. Can create two images of different contrasts by filling two different k-spaces, e.g. if we have an ETL of 14, we can use the first 7 echoes for a PD image (first k-space) and the last 7 echoes for a T2 image (second k-space). This is called a Double-Echo TSE sequence Disadvantages Only really able to achieve heavily T2 weighted images Mix of contrasts: Each echo that fills a different line of k-space is at a different time and, therefore, a different contrast.