Principles of MR imaging

MR imaging is founded on the principle that certain nuclear species, such as hydrogen, possess inherent magnetic properties. When placed in a high-strength external magnetic field these nuclei will precess, which is a resonance phenomenon, according to the axis of the external field. In order to obtain diagnostic information from these precessing protons, radiofrequency pulses are sent into the patient, which convert these nuclei to a higher energy state, by 'flipping' them from the longitudinal to the transverse plane. Once the disturbing radiofrequency (RF) pulse is turned off, thermal equilibrium is restored and the released electromagnetic energy induces a voltage in a receiver coil that is placed around the body part of interest. Based on the resistance of the imaging coil, this generates a current. This current, when sent through an analog to digital converter, creates digitized data that undergo a mathematical progression known as Fourier transformation, yielding an MR image.

Table 5.1.3 Simplified MR signal characteristics of joints.

^-Weighted

T2-Weighted

Cortical bone

Low

Low

Fat (marrow,

High

High

subcutaneous)

Type I collagen (normal

Low

Low

ligament,tendon,

meniscus,labrum)

Fluid (normal joint)

Low

High

Muscle

Intermediate

Intermediate

Articular cartilage*

Intermediate

Slightly lower

* Signal characteristics are pulse-sequence and image-parameter dependent.

By varying the time between successive disturbing RF pulses (repetition time or TR) and the time between the disturbing RF pulses and the time at which echo information is recorded (echo time or TE), pulse sequences of varying soft tissue contrast may be generated. A simplified table of MR signal characteristics is provided in Table 5.1.3. Tj relaxation time refers to the recovery of the initial magnetization following the disturbing RF pulse, which fat protons achieve quickly, thus yielding bright signal on a Tj-weighted sequence. T2 relaxation time refers to the decay process which occurs in the transverse plane.

Commonly used MR pulse sequences for orthopedics include spin echo and fast or turbo spin echo, the latter of which yield rapid acquisition of images, often with superior soft tissue contrast. As the latter images are obtained more quickly, superior spatial resolution may be achieved by use of a higher resolution matrix. It should be remembered that all MR images are digitized; therefore, spatial resolution is dependent on the size of the picture element or pixel, which is in turn determined by the field of view, or area of study, divided by the imaging matrix. Maintaining an adequate field of view to cover diagnostic information may be performed with no sacrifice to spatial resolution, when an appropriate imaging matrix is chosen.

Gradient echo sequences are also commonly utilized, which provide a coarsened appearance to trabecular bone, as they have little correction for field inhomogeneities. Such pulse sequences typically have poor contrast between fat and muscle (in the absence of additional fat suppression techniques) and yield a phenomenon known as flow-related enhancement, which may be exploited for vascular MR pulse sequences.

Fat suppression techniques 'rescale' the contrast range, as fat is characteristically conspicuous on most MR pulse sequences. Such sequences are essential in orthopedic imaging to disclose subtle areas of trabecular microfracture or subchondral edema secondary to osteochondral impaction. They also accentuate the high signal from soft tissue fluid collections such as ganglion cysts of the menisci or shoulder labrum.

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