How is an MRI scan created?
Hydrogen atoms in the body are individual charged atoms that rotate around a random axis, so that the total magnetic field of the body is zero. In an MRI scan, the patient is placed in a magnetic field that causes the nuclei of the hydrogen atoms to align ipsilaterally. The emission frequency pulse causes the nuclei of the hydrogen atoms to enter a high-energy state. When the RF pulse ends, the activated nuclei release energy and return to a lower energy state, a process that becomes relaxation. The energy released during the transition is detected by the surrounding MR receiver and generates signal data that is subsequently displayed on the screen. The interval between repeated excitation is called the repetition time, and the time between the radiation of the RF pulse and the acquisition of the return signal is called the echo time. The process of relaxation is described as T1 and T2 according to two independent time constants.
What is signal intensity?
Signal intensity is used to describe the brightness of tissue in MRI images. Tissue can be described as high signal (bright), medium signal (gray), or low signal (dark). When the diseased tissue is contrasted with the surrounding normal tissue, it can be described as high intensity, equal intensity or low intensity. The magnetic resonance signal intensity depends on the T1, T2 and proton density (number of free hydrogen ions) images of the tissue to be evaluated.
Explain the difference between T1 and T2 phases in magnetic resonance images.
T1 (longitudinal relaxation time) and T2 (transverse relaxation time) are inherent physical properties of tissues. Different tissues have different T and T2 properties due to the fact that the nuclei of hydrogen atoms in different tissues respond differently to RF pulses during MRI scanning. The contrast in MR images is determined by various scan constants (TE and TR).
T1 images: produced by short TR (<1000ms) and short TE (<30ms), T1 images are fat-weighted, with fat as a bright signal on T1 images and a brighter signal on T2 images. t1 images are suitable for the evaluation of structures such as those containing fat, hemorrhage, or protein-containing fluid. All of these structures have short T1 high signal in T1 images, and T1 images show tissue structures well due to their high signal-to-noise ratio.
T2 images: produced by long TR (>1500ms) and long TE (>45ms), T1 images are water weighted, water is bright signal on T2 images and dark signal on T1 images. the signal intensity on T2 images is related to the water content of the tissue, water-rich tissues such as cerebrospinal fluid, cysts, and normal intervertebral discs are high signal on T2 phase. T2 images are very useful in comparing normal tissue with abnormal tissue. Usually, diseased tissues (e.g., tumors, infections, acute fractures) with increased water content are high signal on T2 images and low signal on T1 images.
Describe the signal intensity of common tissues in the T1 and T2 phases of magnetic resonance.
Mineralized tissues show low signal on both T1 and T2 phases because they contain few free hydrogen ions. Air does not contain free hydrogen ions and therefore does not produce a magnetic resonance signal. The correlated signal intensities of different tissues in T1 and T2 phase images are shown in Table 3-1.
Tissue T1 phase T2 phase
Cerebrospinal fluid Low-Medium Bright
Bone cortex Low Low
Tendon/ligament Low Low
Muscle Moderate Moderate
Fat High Moderate
Red bone marrow Moderate Moderate
Yellow bone marrow High High
Intervertebral disc (central) Medium Light
Intervertebral disc (peripheral) Low Moderate
How do I know if I’m looking at a T1-phase or T2-phase MRI image?
One way is to look at the TE (echo time) and TR (repetition time) in the scan.
Image type TE TR
T1 15-30ms 400-600ms (<1000)
T2 60～120ms 1500～3000ms(>1000)
Proton density weighted 15 to 30ms 1500 to 2000ms
Another simple method is to review the characteristics of water in the image and look for structures containing water (e.g. cerebrospinal fluid surrounding the spinal cord). If the water is bright, the image may be in T2 phase, and if the water is dark, the image may be in T1 phase.
The above criteria refer to the most basic spin-echo (SE) sequences; other sequences have more complex signal comparisons.
What is a pulse sequence?
A pulse sequence refers to a special method of collecting MRI data. Spin-echo (SE) pulse sequences are widely used. Other methods have been developed and used to shorten scan times and reduce artifacts and improve the quality of imaging at specific lesion sites, such as fast spin-echo sequences, gradient-echo sequences, and short T1 inversion recovery sequences. When special pulse sequences are widely used in the daily practice of spinal MRI, their advantages and disadvantages will be properly evaluated.
What are the contraindications to magnetic resonance scanning?
Patients with endosseous implants in the body that are susceptible to magnetic field interference that would disable them or show potentially harmful activity are contraindications to MRI scanning. These include certain cochlear or intraocular implants, pacemakers, certain prosthetic valves, implantable analgesic pumps and neurostimulators, certain Swan-Ganz catheters, carotid clips, periorbital metallic materials and certain penile prostheses, and cerebral aneurysm clips. When MRI is performed on patients in the ICU, MRI-compatible ventilators and monitors are required. Women in early pregnancy are a relative contraindication to MRI. Patients who are extremely claustrophobic or uncooperative are also relative contraindications to MRI, and these patients may require pre-test sedation.
Internal spinal fixations are not a contraindication to MRI; however, if the fixation is close to the area to be examined, significant artifacts can be produced, making the segment with the fixation undiagnostic, but useful images can still be shown above or below the fixation, whereas stainless steel fixations usually produce an excessive amount of artifacts. Patients with stainless steel internal fixation need to be evaluated by CT or CT-spine imaging.
When do I need an MRI to evaluate spinal disease?
MRI shows the full spinal cord and spinal sequence (distal to the sacrum from the greater occipital foramen). MRI in patients with spinal deformities is used to identify the presence of neurological deformities. Examples include left-sided thoracic scoliosis, youthful scoliosis, and spinal cord dysplasia. Spinal fistulas, Arnold-Chiar malformations, spina bifida, spinal cord tumors, spinal cord tethering syndrome, and congenital spinal stenosis may be detected in these patients.
Magnetic resonance screening is also important for preoperative assessment of metastatic spinal tumors for the presence of multisegmental involvement.
Terms used in MRI to describe abnormal disc morphology.
Fibrous annulus rupture: rupture of the ligament surrounding the periphery of the disc.
Bulge: diffuse surrounding rather than localized disc tissue beyond the intervertebral space.
Bulge: Uneven displacement of localized disc tissue beyond the intervertebral space. The displaced disc tissue is attached to the whole. The basal diameter of the displaced disc tissue (where it is attached to the whole) is greater than the longest diameter of the portion beyond the intervertebral space.
Protrusion: Uneven local displacement of disc tissue beyond the intervertebral space. The longest diameter of the portion beyond the intervertebral space is greater than the basal diameter of the displaced disc tissue.
Free prolapse: A fragment of a disc that is not attached to the whole, another common term for a free disc fragment. A disc is present in every free prolapsed disc; not every herniated disc is free and prolapsed.