"During the image acquisition process, a radiofrequency (RF) pulse is emitted from the scanner. When tuned to the Larmor frequency, the RF pulse is at resonance: it creates a phase coherence in the precession of all the proton spins. The duration of the RF pulse is chosen such that it tilts the spin magnetisation perpendicularly to the magnetic field. When a receiving coil (an electrical conductor) is put in the vicinity of the tissue, the transverse magnetisation, that still rotates as the Larmor precession, will generate an electric current in the coil by Faraday induction: this is the nuclear magnetic resonance (NMR) signal."
"Multiple image sets are obtained in a standard examination protocol (which varies from facility to facility). Exam times vary according to the part of the anatomy being studied, pathology expected, radiologist preferences, and the scanner hardware and software used. Occasionally, a contrast medium may be used to enhance images, this will also usually prolong the scan time. Typically, exams are ordered without and with contrast for comparison purposes. Very rarely, and only in certain circumstances are exams ordered with contrast only. After the MRI exam the patient is removed from the scanner and given post-procedure instructions (information about contrast medium and/or sedation if used)."
Expected headings
"Preparation"
"Excitation"
"Spatial encoding"
"Signal acquisition"
"Standard exam"
"The physics of MRI are complicated and much harder to understand than those underpinning image generation in plain radiography, CT or ultrasound."
"The NMR signal is attenuated due to two simultaneous relaxation processes. The loss of coherence of the spin system attenuates the NMR signal with a time constant called the transverse relaxation time (T2). Concurrently, the magnetisation vector slowly relaxes towards its equilibrium orientation that is parallel to the magnetic field: this occurs with a time constant called the spin-lattice relaxation time (T1). The contrast in MR images originates from the fact that different tissues have, in general, different T1 and T2 relaxation times; as this is especially true for soft tissues, it explains the excellent soft tissue contrast of MRI."