SPATIAL AND TEMPORAL RESOLUTION OF MEDICAL IMAGES
The
spatial resolution of a medical
image describes the size of the smallest anatomical structures that can be
represented independently in the image. The maximum spatial resolution of a
digital imaging system is limited by the number and spatial distribution of the
sensor elements and the geometry of the imaging system. However, the actual
spatial resolution depends on the inherent contrast of an object and the sum of
the blurring effects of all the elements in the imaging system including the
imaged object itself.
The
idea of ‘sensor elements’ and system geometry is somewhat different for MRI.
In MRI the spatial position of the detected electromagnetic energy does not
directly indicate the source or trajectory of the energy as it does in
transmission and emission imaging and in sonography. The wavelength of an MRI
photon is, in fact, about as long as a patient's. The reason we can make a high-resolution
image with such a poorly localized signal is that we manipulate the local
environment with well-defined magnetic field gradients so that the source of
the detected energy is encoded in its frequency and phase. The spatial
resolution of MRI depends on the strength of the encoding field gradients and
the rate of sampling (temporal frequency resolution) of the measured signal –
this is, effectively, the spatial frequency equivalent of the number and
spatial distribution of sensor elements mentioned above.
The
temporal resolution describes the
time required for a single image measurement and the time interval between
successive image measurements. For any single image, it is usually desirable to
acquire the necessary raw data in as short a time as possible. This will reduce
the uncertainty in spatial information due to changes in the object that occur
during the measurement of the image data. The most obvious example is the movement
of a patient during imaging, but also important is the involuntary movement of
internal organs. In functional and dynamic imaging studies multiple images are
used to show the time course of spatial changes in the subject – the beating of
a heart, for example, or the vascular flow and tissue perfusion of a contrast
agent. The actual temporal resolution required depends on the dynamics of the
body system studied.