|Generally for brain tumors an MRI is
more sensitive than a CT scan. Some patients with implanted medical devices (e.g. a
pacemaker) cannot safely have an MRI performed. Generally on MRI it's easy
to tell the difference between a brain metastases and a primary brain tumor
MRI vs. CT
A Computed Tomography (CT) scanner uses ionizing radiation, X-rays, to acquire its images, making it a good tool for dense tissue (bone) exams. MRI, on the other hand, uses non-ionizing, radio frequency signals to acquire its images. MRI is best suited for soft (non-calcified) tissue exams.
Both CT and MRI scanners can generate multiple two-dimensional cross-sections or slices of tissue and three-dimensional reconstructions. Unlike CT, which uses only X-ray attenuation to generate image contrast, MRI has a long list of properties that may be used to generate image contrast. By variation of scanning parameters, tissue contrast can be altered and enhanced in various ways to demonstrate different features.
Medical MRI most frequently relies on the relaxation properties of excited hydrogen nuclei in water and fat. When the object to be imaged is placed in a powerful, uniform magnetic field the spins of the atomic nuclei with non-integer spin numbers within the tissue all align either parallel to the magnetic field or antiparallel.
As the high-energy nuclei relax and realign, they emit energy at rates which are recorded to provide information about their environment. The realignment of nuclear spins with the magnetic field is termed longitudinal relaxation and the time (typically about 1 sec) required for a certain percentage of the tissue nuclei to realign is termed "Time 1" or T1. T2-weighted imaging relies upon local dephasing of spins following the application of the transverse energy pulse; the transverse relaxation time (typically < 100 ms for tissue) is termed "Time 2" or T2.
In the brain, T1-weighting causes fiber tracts (nerve connections, i.e. white matter) to appear white, congregations of neurons (i.e. gray matter) to appear gray, and cerebrospinal fluid to appear dark. The contrast of "white matter," "gray matter'" and "cerebrospinal fluid" is reversed using T2 or T2* imaging, Gadolinium-enhanced tissues and fluids appear extremely bright on T1-weighted images. This provides high sensitivity for detection of vascular tissues (e.g. tumors) and permits assessment of brain perfusion (e.g. in stroke).
On a plain T1 image with no contrast tumors generally do not show up unless there has been a bleed. A few unusual tumors (like melanoma are 'bright' on T1). On post contrast T1 the tumors light up (enhance) reflecting a break down in the blood brain barrier (go here for pictures).We generally use the postcontrast T1 image to reflect the tumor extension for mets but for gliomas the cancer usually extends beyond that area. The T2 images will show edema so the tumor area looks quite large (larger than the extent of the tumor.) See the section on targeting tumors here.