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increasing the matrix size. Through-plane (sagittal and coronal plane) resolution
may be increased by decreasing the section thickness. In MR imaging, the matrix
size and the
field of view also affect spatial resolution, in the same manner as with
CT imaging. Additionally, increasing the number of phase encoding steps and
applying stronger gradients can increase spatial resolution.
The contrast resolution and spatial resolution of an image are linked to the
signal-to-noise ratio. A contrast oriented quantity related to the SNR is the contrast-
to-noise ratio (CNR). The CNR is determined by the difference between the signal
of interest (foreground) and intensity of the surrounding structures (background),
divided by the noise power [
1
,
25
]. The contrast-to-noise ratio gives a measure of
the ability to distinguish a structure of interest from surrounding structures.
Finally, image quality may be degraded by various modality based artifacts. As
noted in prior sections, common to both CT and MR imaging, metallic surgical
hardware can cause artifact, termed susceptibility artifact on MRI and streak artifact
on CT. Other types of artifact on CT include partial volume artifact, caused by the
finite spatial size of voxels in tissue with high spatial frequency variation in X-ray
beam attenuation properties, and patient motion artifact. MR imaging is more
sensitive to motion given the longer time interval for scanning of most MRI
sequences relative to CT, and results in ghost images if the patient moves. Addi-
tionally, motion artifact may result from internal or physiologic motion, such as
cerebrospinal fluid (CSF) pulsation flow artifact in the spinal canal due to flow of
the CSF, and respiratory motion artifact. If the imaging
field of view is smaller than
the structure being imaged, aliasing artifact may occur, with portions of the
structure anatomy outside the
field of view are mapped to the opposite side of the
image.
11 Conclusion
Medical imaging is a key component of many areas of modern clinical medical
practice and biomedical research. Physicians are able to diagnose and form treat-
ment plans for numerous pathologies on the basis of data provided by radiographic,
CT, MRI, molecular imaging, and ultrasound studies. Additionally, new areas of
research are emerging based on data provided by imaging, and conversely, imaging
is used as an investigative tool in a variety research areas. The medical imaging
field has been and is continuing to make great progress both in expanding clinical
medical applications and at the frontiers of research. Linked with advanced image
processing and visualization techniques, it has the potential to continue to open
frontiers in the development new and advanced diagnostic and treatment models.
