SPATIAL RESOLUTION

Spatial Resolution refers to the ability of the camera to image two separate objects as they are moved closer and closer to each other in space.

One measure of spatial resolution is known as the Full-Width at Half Max (FWHM) resolution, and is probably the correct method to accurately determine the resolving Power of the camera.

If the two objects must be 2 mm apart to be seen as two distinct objects, (as opposed to one single blurred object), the spatial resolution is said to be 2 mm.

• Use two 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.

Spatial resolution is, usually, measured by a bar phantom, which is simply a series of lead bars of a given width and thickness, separated by spaces the same width as the bars.  More common bar phantom is the "four-quadrant" bar phantom.  It is constructed such that each quadrant of the phantom has different width bars and spaces.

TYPICAL BAR PHANTOM IMAGE

Typical bar phantoms will have bars 2 mm wide with 2 mm spacing, bars 2.5 mm wide with 2.5 mm spacing, bars 3.0 mm wide with 3.0 mm spacing, and bars 3.5 mm wide with 3.5 mm spacing.

When looking at an image of a bar phantom, it is important to know the bar spacing, because not all bar phantoms are alike.

The bar phantom resolution can be calculated by dividing the FWHM by 1.75. This means a camera with a FWHM of 4.0 could resolve bars that are approximately 2.3mm wide, with 2.3mm spacing.

To determine the spatial resolution, the bar phantom is placed on the detector surface, and a uniform source of radiation is placed in front of the bar phantom.  An image is acquired, and evaluated to determine the smallest bars, which are visible.

If the image is taken without a collimator, it is said to be an intrinsic bar phantom image, and if it is taken with a collimator in place, it is said to be an extrinsic or overall system bar phantom image. Typical gamma cameras will resolve 2.5 mm bars intrinsically (without the collimator), and 3.0 mm bars with the collimator. Extrinsic or collimated resolution is primarily a function of the collimator in use.