What determines slice thickness in MRI?

Slice thickness is determined by the slope of the slice select gradient and the transmit bandwidth (it affects in-plane resolution). Thin slices require a steep gradient and narrow tBW. This results in increased resolution. Thick slices require a shallow gradient and broad tBW, resulting in decreased resolution.

What is the sampling frequency in MRI?

In most modern MR scanners, the MR signal is sampled 512-1024 times per echo (even though the display resolution in the frequency-encode direction is usually taken to be 256). In other words, the Nyquist sampling rate is 2-4 times the highest frequency in the signal.

How is MRI SNR calculated?

The measured SNR = S/N must then be multiplied by the 0.66 Rayleigh distribution correction factor to calculate the true SNR. If more than one receive coil is used for data collection, an additional correction factor of up to 8% (depending on number of coils) may also need to be applied.

How many slices does an MRI take?

Single MRI images are called slices. The images can be stored on a computer or printed on film. One exam can produce thousands of images.

What is the flip angle in MRI?

The flip angle is an MRI phenomenon by which the axis of the hydrogen proton shifts from its longitudinal plane (static magnetic field B0) Z axis to its transverse plane XY axis by excitation with the help of radiofrequency (RF) pulses.

What is slice selection How is it typically accomplished in MRI?

Slice selection in MRI is the selection of spins in a plane through the object. The principle behind slice selection is explained by the resonance equation. Slice selection is achieved by applying a one-dimensional, linear magnetic field gradient during the period that the RF pulse is applied.

How is the bandwidth of a MRI measured?

MR manufacturers have slightly different methods for defining receiver bandwidth. GE Healthcare uses the total bandwidth across the entire image, while Siemens and Canon use bandwidth per pixel (Px). In the example above, GE would report a rBW = 50 kHz regardless of the spatial resolution chosen.

What does BW stand for in a MRI?

Bandwidth (BW) is the range of frequencies (measured in Hz) involved in the transmission or reception of an electronic signal. In MRI the term may be used to refer to the frequencies associated either with RF-excitation ( transmitter bandwidth, tBW) or signal reception ( receiver bandwidth, rBW ).

Why does a high bandwidth MRI have a higher SAR?

This is a high bandwidth RF pulse with compromised slice profile and will give a higher SAR compared to the other modes. This RF pulse type will provide the user an opportunity to reduce TEs and TRs. It has shorter echo spacing (ES) and very minimal susceptibility and distortion artefacts.

How does the receiver bandwidth affect the scan time?

Increasing the receiver bandwidth shortens the scan time by allowing shorter TR and TEs. For example, a T1 sequence with a bandwidth of 130 with 600 TR and 25 TE is usually acquired in about 3.30 minutes.