BASIC PRINCIPLES OF MR IMAGING
John R. Hesselink, MD, FACR
Magnetic resonance (MR) is a dynamic and flexible technology that allows one to tailor the imaging study to the anatomic part of interest and to the disease process being studied. With its dependence on the more biologically variable parameters of proton density, longitudinal relaxation time (T1), and transverse relaxation time (T2), variable image contrast can be achieved by using different pulse sequences and by changing the imaging parameters. Signal intensities on T1, T2, and proton density-weighted images relate to specific tissue characteristics. For example, the changing chemistry and physical structure of hematomas over time directly affects the signal intensity on MR images, providing information about the age of the hemorrhage. Moreover, with MR's multiplanar capability, the imaging plane can be optimized for the anatomic area being studied, and the relationship of lesions to eloquent areas of the brain can be defined more accurately. Flow-sensitive pulse sequences and MR angiography yield data about blood flow, as well as displaying the vascular anatomy. Even brain function can be investigated by having a subject perform specific mental tasks and noting changes in regional cerebral blood flow and oxygenation. Finally, MR spectroscopy has enormous potential for providing information about the biochemistry and metabolism of tissues. As an imaging technology, MR has advanced considerably the past 10 years, but it continues to evolve and new capabilities will likely be developed.
An MR system consists of the following components: 1) a large magnet to generate the magnetic field, 2) shim coils to make the magnetic field as homogeneous as possible, 3) a radiofrequency (RF) coil to transmit a radio signal into the body part being imaged, 4) a receiver coil to detect the returning radio signals, 5) gradient coils to provide spatial localization of the signals, and 6) a computer to reconstruct the radio signals into the final image.
The signal intensity on the MR image is determined by four basic parameters: 1) proton density, 2) T1 relaxation time, 3) T2 relaxation time, and 4) flow. Proton density is the concentration of protons in the tissue in the form of water and macromolecules (proteins, fat, etc). The T1 and T2 relaxation times define the way that the protons revert back to their resting states after the initial RF pulse. The most common effect of flow is loss of signal from rapidly flowing arterial blood.
The contrast on the MR image can be manipulated by changing the pulse sequence parameters. A pulse sequence sets the specific number, strength, and timing of the RF and gradient pulses. The two most important parameters are the repetition time (TR) and the echo time (TE). The TR is the time between consecutive 90 degree RF pulse. The TE is the time between the initial 90 degree RF pulse and the echo.
The most common pulse sequences are the T1- weighted and T2-weighted spin-echo sequences. The T1-weighted sequence uses a short TR and short TE (TR < 1000msec, TE < 30msec). The T2-
weighted sequence uses a long TR and long TE (TR > 2000msec, TE > 80msec). The T2-weighted sequence can be employed as a dual echo sequence. The first or shorter echo (TE < 30msec) is proton density (PD) weighted or a mixture of T1 and T2. This image is very helpful for evaluating periventricular pathology, such as multiple sclerosis, because the hyperintense plaques are contrasted against the lower signal CSF. More recently, the FLAIR (Fluid Attenuated Inversion Recovery) sequence has replaced the PD image. FLAIR images are T2-weighted with the CSF signal suppressed.
The TR, matrix size and NEX are the only parameters that affect scan time. Increasing any one of these parameters increases the minimum scan time. Spatial resolution is determined by matrix size, FOV and slice thickness. Increasing matrix size or decreasing FOV and slice thickness increases spatial resolution, but at the expense of either decreased signal-to-noise or increased scan time. To obtain images of high resolution with high signal-to-noise requires longer scan times. All of the scan parameters affect signal-to-noise. The signal within an image can be improved by increasing TR, FOV, slice thickness and NEX or by decreasing TE and matrix size. The most direct way to increase signal is by increasing NEX, but one must keep in mind that increasing NEX from two to four, for example, doubles the scan time, but increases the signal by only the square root of two. Finally, TE does not affect scan time, however, it does determine the maximum number of slices in multislice mode. Increasing the TE or shortening TR decreases the number of slices that can be obtained with one pulse sequence.
Specialized techniques for reducing motion and artifacts on the images also have applications for brain imaging. Gradient moment rephasing or flow compensation techniques effectively reduce ghost artifacts resulting from CSF flow. They should be used for T2-weighted spin-echo and gradient-echo acquisitions, but not with T1-weighted imaging because they increase the signal from CSF. Flow compensation techniques do not contribute to SAR (a measure of power deposition), but the extra gradient pulses lengthen the minimum TE, and gradient heating may limit the number of slices, the minimum FOV and slice thickness. Cardiac gating also reduces artifacts from CSF pulsations, resulting in superior object contrast and resolving power in the temporal lobes, basal ganglia and brain stem.
Saturation (SAT) techniques use extra RF pulses to eliminate artifacts from moving tissues outside the imaging volume, such as swallowing or respiratory artifacts, and from unsaturated protons that enter the imaging volume through vascular channels. SAT should be used for T1-weighted imaging of the sella, internal auditory canals, and the spine. The extra RF pulses cost SAR and take time, lengthening the minimum TR or decreasing the maximum number of slices in a multislice mode.
Methods for eliminating wrap-around or aliasing should be prescribed when imaging small anatomic areas, such as the sella and internal auditory canals, with smaller FOR's. The "no phase wrap" option is most effective in the anterior-posterior direction for sagittal and axial scans. On newer MR systems, many of theses specialized techniques are automatically added to the appropriate sequences.
MR IMAGE CONTRAST
When reviewing an MR image, the easiest way to determine which pulse sequence was used, or the "weighting" of the image, is to look at the cerebrospinal fluid (CSF). If the CSF is bright (high signal), then it must be a T2-weighted imaged. If the CSF is dark, it is a T1-weighted image. Next look at the signal intensity of the brain structures.
On MR images of the brain, the primary determinants of signal intensity and contrast are the T1 and T2 relaxation times. The contrast is distinctly different on T1 and T2-weighted images. Also, brain pathologies have some common signal characteristics.
Pathologic lesions can be separated into five major groups by their specific signal characteristics on the three basic images: T2- weighted, proton density-weighted (PD)/FLAIR, and T1-weighted.
BRAIN IMAGING PROTOCOL
Since studies have shown that T2-weighted images are most sensitive for detecting brain pathology, patients with suspected intracranial disease should be screened with T2-weighted spin-echo and FLAIR images. The axial plane is commonly used because of our familiarity with the anatomy from CT. The other scan parameters include a 256 x 256 matrix, 1 NEX, 22 cm FOR and 5 mm slice thickness for a scan time of less than 4 minutes and a voxel size of 5 x 0.86 x 0.86 mm. A 1-2 mm interslice gap prevents RF interference between slices.
If an abnormality is found, additional scans help characterize the lesion. Noncontrast T1-weighted images are needed only if the preliminary scans suggest hemorrhage, lipoma, or dermoid. Otherwise, contrast-enhanced scans are recommended. Gadolinium-based contrast agents for MR are paramagnetic and have demonstrated excellent biologic tolerance. No significant complications or side effects have been reported. It is injected intravenously at a dose rate of 0.1 mmol/kg. The gadolinium contrast agents do not cross the intact blood-brain barrier (BBB). If the BBB is disrupted by a disease process, the contrast agent diffuses into the interstitial space and shortens the T1 relaxation time of the tissue, resulting in increased signal intensity on T1-weighted images. The scans should be acquired between 3 and 30 minutes postinjection for optimal results.
Contrast enhancement is especially helpful for extra-axial tumors because they tend to be isointense to brain on plain scans, but it also identifies areas of BBB breakdown associated with intra-axial lesions. Gadolinium enhancement is essential for detecting leptomeningeal inflammatory and neoplastic processes. Contrast scans are obtained routinely in patients with symptoms of pituitary adenoma (elevated prolactin, growth hormone, and so forth) or acoustic neuroma (sensorineural hearing loss). To screen for brain metastases in patients with a known primary, contrast-enhanced T1-weighted scans alone are probably sufficient.
Gadolinium does not enhance rapidly-flowing blood. If vascular structures are not adequately seen on plain scan, the positive contrast provided by gradient-echo techniques or MR angiography may be helpful to confirm or disprove a suspected carotid occlusion or cerebral aneurysm, to evaluate the integrity of the venous sinuses, and to assess the vascularity of lesions. Gradient-echo imaging also enhances the magnetic susceptibility effects of acute and chronic hemorrhage, making them easily observable, even on low and mid-field MR systems.
Although the axial plane is the primary plane for imaging the brain, the multiplanar capability of MR allows one to select the optimal plane to visualize the anatomy of interest. Coronal views are good for parasagittal lesions near the vertex and lesions immediately above or below the lateral ventricles (corpus callosum or thalamus), temporal lobes, sella, and internal auditory canals. The coronal plane can be used as the primary plane of imaging in patients with temporal lobe seizures. Sagittal views are useful for midline lesions (sella, third ventricle, corpus callosum, pineal region), and for the brain stem and cerebellar vermis.
Scan techniques are slightly different for the sella and cerebellopontine angle. For the sella, the plain and contrast enhanced scans are obtained in the coronal and sagittal planes using a smaller FOR and thin (3mm or less) contiguous or overlapping sections. For patients with a sensorineural hearing loss or suspected acoustic neuroma, contrast enhanced scans with T1-weighting are obtained through the internal auditory canals, again using thin overlapping sections.
As imaging techniques of the brain, MR and CT are both competitive and complimentary. In general, CT performs better in cases of trauma and emergent situations. It provides better bone detail and has high sensitivity for acute hemorrhage. Support equipment and personnel can be brought directly into the scan room. CT scanning is fast. Single scans can be done in 1 second, so that even with uncooperative patients, adequate scans usually can be obtained. CT is more sensitive than MR for subarachnoid hemorrhage. CT is also more sensitive for detecting intracranial calcifications.
MR, on the other hand, functions best as an elective outpatient procedure. Proper screening of patients, equipment, and personnel for ferromagnetic materials, pacemakers, etc. is mandatory to avoid possible catastrophe in the magnet room. If proper precautions are in place, emergency studies can be done, but the set-up time is longer, and the imaging also requires more time. With conventional MR systems, most pulse sequences take a minimum of 2 minutes. Echo-planar capability has become standard on most MR systems, and this advanced technology can acquire sub-second MR scans.
Due to its high sensitivity for brain water, MR is generally more sensitive for detecting brain abnormalities during the early stages of disease. For example, in cases of cerebral infarction, brain tumors or infections, the MR scan will become positive earlier than CT. When early diagnosis is critical for favorable patient outcome, such as in suspected herpes encephalitis, MR is the imaging procedure of choice. MR is exquisitely sensitive for white matter disease, such as multiple sclerosis, progressive multifocal leukoencephalopathy, leukodystrophy, and post-infectious encephalitis. Patients with obvious white matter abnormalities on MR may have an entirely normal CT scan. Other clinical situations where MR will disclose abnormalities earlier and more definitively are temporal lobe epilepsy, nonhemorrhagic brain contusions and traumatic shear injuries.
In general, nonenhancing disease processes are much more apparent on MR than CT. When the blood-brain barrier is damaged, enhancement occurs with both gadolinium and iodinated contrast agents on MR and CT, respectively. As a rule, the degree of enhancement is greater on MR scans.
For evaluating posterior fossa disease, MR is preferable to CT. The CT images are invariable degraded by streaking artifacts from the bones at the skull base. In conjunction with gadolinium enhancement, MR can reliably detect intracanalicular acoustic neuromas and other schwannomas arising along the cranial nerves within the basal cisterns and foramina of the skull base. Similarly, MR has largely supplanted CT for imaging the sella turcica and pituitary gland.
The value of MR for defining congenital malformations is unquestioned. The multiplanar display of anatomy gives important information about the corpus callosum and posterior fossa structures. The superior gray/white contrast allows accurate assessment of myelination.
The phenomenon of flow void within arteries on spin-echo images, the high sensitivity for hemorrhage and hemosiderin deposition, and the capability of MR angiography give MR distinct advantages over CT for imaging vascular disease. Vascular stenoses or occlusions, aneurysms, and arteriovenous malformations can be imaged without intravenous contrast media. In cases of cryptic vascular malformations and cavernous angiomas, where the angiogram and CT scan are often negative, MR may reveal small deposits of hemosiderin from prior small hemorrhages. Diffusion-weighted sequences are highly sensitive for restricted diffusion and cytotoxic edema associated with acute cerebral infarction. By combining conventional MR images with diffusion and perfusion-weighted imaging and MR angiography, a complete workup of vascular disease can be accomplished.
Along with the function of MR as a primary imaging procedure, there are indications for MR as a secondary procedure after the pathology has already been demonstrated by CT. In patients with solitary lesions on CT, in whom the diagnosis of metastatic disease, abscess, or multiple sclerosis would be strengthened by finding additional lesions, MR may resolve the issue. Similarly, in a patient with brain metastases in whom none of the lesions account for the patient's signs or symptoms, MR can help evaluate the particular anatomic area of interest. A potential problem in both of these circumstances is the nonspecificity of white matter hyperintensities, and contrast MR may be necessary to clarify the situation.