Advances in Coronary ImagingBasic principles of magnetic resonance imaging☆
Section snippets
The source of the MR signal
A correct description of what happens when tissue is subjected to a magnetic field relies on quantum mechanics. Fortunately, all the theory necessary for MRI can be based on a simple classical model in which certain nuclei that spin around their own axes behave like small magnets. For clinical imaging, hydrogen is the most frequently used nucleus, but other possible nuclei are carbon-13, sodium, and phosphorus. Under normal circumstances these tiny magnets are randomly distributed in space, the
Excitation
The net magnetization vector from the nuclei inside the magnet in its equilibrium state is static and does not produce a measurable signal. To obtain information from the spins, the direction of the net magnetization vector has to be altered. For this the precessing spins are excited by applying energy, in the form of radiofrequency (RF) energy pulses of exactly the Larmor frequency (resonance frequency). When an RF signal is given at the resonance frequency into the patient, two phenomena
Return to equilibrium
After the RF frequency transmitter is switched off, the equilibrium state will be sought (high energy back to low energy). This means that the magnetization decays over time, which is represented by a decreasing magnitude of Mz in the transverse plane. Consequently, the induced signal in the receiver coil will decrease in time. This decreasing signal is called the free induction decay (FID) (Fig 4).
Spatial encoding
To create an image, the MR signal from the H-protons has to contain information about where these H-protons are positioned in the patient. This is done in three steps: slice selection, frequency encoding, and phase encoding.
To select an imaging slice through the body, a magnetic gradient (for example 25 milliT/m) is added along the main magnetic field in the caudal to cranial direction. Because the frequency of precession, and thus the frequency at which the spins can be excited, is dependent
The echo signal, spin-echo imaging
There are several reasons why the FID signal is not used for clinical imaging. First there is a certain time necessary to perform the spatial encoding, and even with present ultra-fast MR scanners this can not be performed before the FID declines. Second, the creation of a second AC signal gives opportunities to modify the contrast in the images depending on the T1 and T2 values of the tissues.
To evoke a second AC signal, a second RF pulse is applied that flips the spin by 180°, and also
Contrast
MRI has the potential to visualize the difference in T1 and T2 of different tissues. Using these differences, contrast between different soft tissues in MRI is superb compared with x-ray computer tomography. If the time for the next repetition of RF pulses (time of repetition = TR) is shorter than the time necessary for total longitudinal relaxation, the contrast in the image will be mainly influenced by the difference in T1 value of the tissues (Fig 12).
Resolution
In digitized imaging, such as MRI, pictures are composed of a matrix of elements, called picture elements or pixels. The image represents the field of view (FoV). The image matrix defines the number of pixels used to construct an image that is determined by the number of frequency encodings (128 or 256 on the x-axis) and the number of phase-encoding steps used (128 or 256 on the y-axis) for a certain FoV (Fig 13).
Summary
To obtain a single MR image, a complex combination of RF pulses and magnetic gradient switches have to be applied. In spin-echo imaging, two RF pulses combined with gradient switches in three directions result in one echo signal with spatial information. For an electrocardiogram-triggered cardiac image, in each heartbeat one combination is completed and thus one echo is acquired (Fig 14).
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Address reprint requests to Robert-Jan M. van Geuns, MD, Department of Cardiology, Thoraxcenter, BD 377, University Hospital Rotterdam, PO Box 2040, 3000 CA Rotterdam, The Netherlands; e-mail: [email protected].