I was speaking with a dental imaging expert who I highly respect for his depth of knowledge and expertise regarding dental imaging. He is a dentist, radiologist, and programmer and widely recognized as one of the foremost experts in dental imaging technology.
I wanted to pick his brain about "modulation transfer function" and how it is measured. In layman's terms MTF is the measurement of the difference between reality and the image. So in the case of a CBCT volume, how accurate of a reproduction is the 3D volume. My friend the expert told me that MTF is the true measurement of quality in all imaging from photography to CBCT. He said that MTF can actually be measured at the pixel or voxel level electronically which started to get over my head. He then said that he believes that this measurement should be required from every CBCT system manufactured. This would provide a true ranking of image quality among competitive CBCT machines. In this way the dentist could rely on science rather than anecdotal evidence to determine how image quality compares when shopping. If I want to buy a fast car, I rely on tested and published 0-60 times, 1/4 mile times, and 0-100 time as well as top speed. I do not take the sales person's word that his model is the fastest. I also do not look at one factor such as horsepower to make my decision because I know that power to weight ratio is more important than either of those variables alone.
Unfortunately, in dental imaging there are not resources to compare this measurement. Instead many units are sold by the sales person's comments about their system's superiority. For example " our unit goes down to a 75 micron voxel and therefore it gives the finest image quality." If the rotation time is 30 seconds for that micron size, the resulting MTF will be greater than a larger micron voxel with a faster scan time. Additionally, as pixels and voxels are reduced in size more noise is introduced into the image and more radiation is required to take the image. Thus, just like a car's performance, a CBCT's ultimate image quality will be determined by a combination of factors (time of rotation to take the image, voxel size, harmonic vibration, sensor quality, scintillator quality, and processing algorithms to name a few) It can all get very confusing! That is why there needs to be a standard meaningful measurement of MTF.
To my knowledge, J.Morita is the only manufacturer that has measured MTF and resulting LPPMM for the CBCT systems that they produce. The measurement is in the brochure for the Morita R100. I think it's great that the J.Morita is trying to bring truth to digital imaging but it is all meaningless if other manufacturers do not provide this measurement for comparative purposes. The December 2011 issue of The Journal of Oral Science Volume 53, #4 rated two of Morita's products as having the top image quality among 8 different units evaluated. However, this was a subjective study done by 5 dentist observers so opinions varied. Morita was rated #1 in virtually every assessment. It would be interesting to have true MTF figures and see if the measurements matched the subjective ratings. The images in this study represented a HUGE difference in subjective image quality.
It is not surprising that J. Morita, who is internationally recognized as a leader in CBCT image quality, is leading the way by providing MTF measurements for all of their units. If other manufacturers would agree to use the same universally recognized measure of image quality, it would be much easier for dentists to know how their chosen unit stacks up against other options.
CBCT is a big investment for your practice and there's a lot to know before you make a purchase decision. So if you're thinking about investing in CBCT make sure you ask the right questions and download my free purchase guide below!
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