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Department of Radiology, Brigham and Women's Hospital
Harvard Medical School |
II. Unsharpness
Unsharpness and resolution refer to the degree of blurring along the boundaries between different regions of the image (usually defined by different patient organs or, in nuclear medicine, different radiopharmaceutical uptake distributions):
Methods for measuring the resolution of an imaging system
Use two point sources of the isotope to be imaged:
Move the sources toward each other and measure the separation at which the images of the two points cannot be distinguished.
Note that the images will blur together completely at a separation approximately equal to the full-width at half maximum (FWHM) of the image of the point.
When the resolution is specified as a single number, that number is usually the FWHM of the point-source image.
The multidimensional image of a point source (2D for planar imaging, 3D for tomographic SPECT or PET imaging):
The 1D response of an imaging system perpendicular to an infinitely long line source (the convolution of the PSF with an infinite line):
The 1D response of an imaging system perpendicular to a straight boundary between two infinitely long regions (the convolution of the LSF with a step function):
The fidelity with which object information is transferred to the image as a function of spatial frequency in the object. Calculate the MTF as the Fourier transform of the PSF.
Or, measure the MTF by imaging bar patterns of different spatial frequencies:
1. Collimator Resolution
Collimator resolution is typically 0.8 -> 2.5 cm FWHM in a patient and worsens with distance from collimator:
2. Intrinsic Resolution
A gamma camera's intrinsic resolution is about 0.3-0.5 cm FWHM.
3. Scatter of Photons in Patient and Other Materials Before Being Detected
Scatter of photons in a patient primarily affects the shape of the long "tails" on a realistic system PSF:
Lesson Author: Stephen C Moore, PhD, scmoore@bwh.harvard.edu
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Updated May 3, 2001