Spotlight on Sensors: True vs. Theoretical Resolution

As dental professionals, you already know how important resolution is when you select digital imaging products for your practice —whether it’s direct digital, phosphor plates, panoramic systems or even a new camera for your practice. However, sometimes the “advertised” resolution is not the same as the “tested “resolution, and the wording companies use to describe resolution can be confusing. For this post, I would like to take the opportunity to discuss a question that many others have asked when choosing a sensor: what’s the importance of spatial resolution and what is the difference between true and theoretical spatial resolution?

Image Resolution

First of all, image resolution is directly proportional to the detail an image holds. The term can apply to digital or film images and the higher the resolution of the image, the more detail the image holds.

Spatial Resolution and Line Pairs            

Second, spatial resolution can be measured many ways, but in dental digital imaging it’s common to measure in line pairs per millimeter. On a basic level, resolution quantifies how close lines can be to each other and still be visibly resolved. A line pair is comprised of a dark line and an adjacent light line. A resolution of 10 lines per millimeter means 5 dark lines alternating with 5 light lines, or 5 line pairs per millimeter (5 lp/mm).  In dentistry, this data is obtained by placing a line pair test gauge on an image sensor and exposing it under carefully controlled conditions.

Line pair gauge

Figure 1. Caption line pair exposure on RVG 6100 sensor courtesy Carestream Dental, Atlanta Georgia

Theoretical vs. True Resolution

When looking for a sensor for your practice, you will probably hear the terms theoretical and true/measured resolution used quite often. Because the manufacturers may advertise different types of resolution, it’s important to understand the difference to ensure that you’re using an apples-to-apples comparison and not comparing theoretical claims with measured resolution.

Also don’t take manufacturers’ claims at face value. A recent study by Dr. Wisam Al- Rawi at the University of Michigan noted a significant discrepancy between actual tested vs. claimed spatial resolution in a scientific study of the properties of dental digital sensors

Theoretical resolution is a calculation of what the sensor is capable of in an ideal world, based solely upon the number of pixels and pixel size of the CMOS sensor—but this is just one piece of the puzzle. When you add in the components of the finished product, which includes sealants, shock layers, scintillators, and protective housing, as well as detector noise and scanner vibrations, true/measured resolution—in lp/mm—is actually much different.

Please note that theoretical resolution describes complex mathematical calculations made in a laboratory while using only the sensor component—it doesn’t translate well to a clinical setting. What’s more, higher theoretical resolution/smaller pixels on the CMOS component is at some point counterproductive and can actually reduce true image quality by introducing artifacts. In fact, the higher the theoretical resolution, the higher the signal-to-noise ratio. Therefore, you would be better served to ask about true resolution when selecting a sensor. After all, better resolution:

  • leads to more confident diagnoses;
  • can reduce the number of retakes required;
  • improves patient communication and education; and
  • facilitates co-discovery and co-diagnoses.

What are your thoughts on theoretical vs. true resolution as it pertains to sensors?

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