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Ebook MRI at a glance: Part 2

(BQ) Part 2 book "MRI at a glance" presentation of content: Data acquisition and scan time, signal to noise ratio, spatial resolution, magnetic susceptibility, flow phenomena, phase contrast MR angiography, phase contrast MR angiography, contrast enhanced MR angiography, screening and safety procedures,. | 35 Data acquisition and frequency encoding one cycle A. sampled twice per cycle, waveform interpreted accurately B. sampled once per cycle, misinterpreted as straight line C. sampled less than once per cycle, misinterpreted as wrong frequency (aliased) Figure 35.1 The Nyquist theorem. TE 90° 180° frequency-encoding (readout) gradient sampling time minimum TE increased 90° 180° sampling time increased Figure 35.2 Sampling time and the TE. 70 Chapter 35 Data acquisition and frequency encoding The application of RF excitation pulses and gradients produces a range of different frequencies within the echo. This is called the receive bandwidth as a range of frequencies are being received. All of these frequencies must be sampled by the system in order to produce an accurate image from the data. The magnitude of the frequency encoding gradient, along with the receive bandwidth, determines the size of the FOV in the frequency encoding direction i.e. the distance across the patient into which the frequencies within the echo must fit. Every time frequencies are sampled, data is stored in a line of K space. This is called a data point. The number of data points in each line of K space corresponds to the frequency matrix (e.g. 256, 512, 1024). After the scan is over, the computer looks at the data points in K space and mathematically converts information in each data point into a frequency. From this the image is formed. As the frequency-encoding gradient is always applied during the sampling of data from the echo, it is often called the readout gradient (although the gradient is not collecting the data, the computer is doing this). • The time available to the system to sample frequencies in the signal is called the sampling time. • The rate at which frequencies are sampled is called the sampling rate. • The sampling rate is determined by the receive bandwidth. If the receive bandwidth is 32 kHz this means that frequencies are sampled at a rate of 32,000 times