Definition: Spatial Resolution is the ability to detect a single small structure against its background or to distinguish as separate two small structures close together.
The spatial resolution of an imaging system is tested by a “line pair phantom” which is also known as a bar test tool. This tool contains a number of equally spaced lead bars. A lead bar and the space between them is known as a line pair. The line pair phantom measures resolution in terms of line pairs per millimeter (lp/mm). The following is an example of a line pair phantom.
In this line pair phantom, you will notice that there are numbers next to the different bars. Although they are too small to see on these images. The numbers 1.6 and 2.2 represent line frequencies of 1.6lp/mm and 2.2lp/mm.
To use a line pair phantom, the phantom is radiographed and the image is zoomed or magnified to determine how many line pairs per millimeter it is able to resolve. Systems that can resolve greater numbers of lines per millimeter have better spatial resolution.
The following is an image of a line pair phantom radiographed that has just about 2.0 lp/mm.
The following is an image of a line pair phantom radiographed that has about 3.4 lp/mm.
What does this all mean to the veterinarian looking to purchase a digital radiography system????
Take home message 1: The greater the number of line pairs the better. However, there is likely a point of diminishing returns. If there was a super detector that could resolve 50 lp/mm that would be great. However, it would certainly be expensive but much of that resolution would be wasted. Iin veterinary radiography we generally don’t resolve pathology that is smaller than 1/50th of a millimeter. As a guideline, The American College of Radiology recommends that radiographic images should be digitized to a matrix corresponding to 2.5 lp/mm or greater. Bottom line: The ACR says that if your detector is greater than 2.5 lp/mm you should be OK on the spatial resolution front.
Take home message 2: There is much more that goes into a quality image than spatial resolution. Remember, traditional film has a resolution of 10-15p/mm which is greater than any digital detector sold on the veterinary market. Yet we all know that if a film is overexposed or poorly processed it does not make one hoot of a difference what the resolution of the film is - the image will not be diagnostic. Take another look at the line pair phantom images above. One image has nice contrast and the other image is grainy and murky. Contrast resolution is very important as well. Which iimage do you like better? Bottom line: LPMM specs are just one factor to consider when purchasing a digital radiography system.
Take home message 3: Vendors stretch the truth! We learned at this year’s SHOWDOWN there were vendors that embellished on their quoted LPMM resolution. In one case, a vendor was quoting 3.4LPMM when they only achieved about 2 lpmm (below the ACR standard). When approached about this, the vendor stated [my paraphrase] "Our listed LPMM number is a theoretical number. After processing the resolution goes down." Be sure that you specify the AFTER PROCESSING LPMM when purchasing a digital radiography machine.