Ever since CT came about, there have been attempts to lower the dosage of X-rays required to get a decent picture. In the last year or so, perhaps due to scary articles like THIS , dose reduction has become quite in vogue. I don't mean to be flippant, as radiation is something that deserves respect and careful handling, much like electricity, but I just have to laugh a bit about the mania that has taken over. The potential dangers are nothing new, and we really don't need to panic. We've been dealing with this for quite a while.
The main principle of handling radiation in the imaging world is called ALARA: As Low As Reasonably Achievable. That doesn't mean we avoid necessary studies, but we are simply attempting to do what we can to lower the dose, and still answer the question that prompted the exam in the first place. Working within this framework gives us some direction.
So...in very basic terms, we can reduce the dose by decreasing the radiation passed through the patient. We can use higher energy beams which do indeed pass through the patient more cleanly, if you will, stopped by less tissue, and therefore depositing less energy, but, alas, we reach a point where we don't get much detail. We can send fewer X-rays through the patient, but too few and we don't see, ummm, stuff. OR, we can increase the sensitivity of the detector, so we don't need so many X-rays. OR.... we can use mathematics to recover information from a crappy image, thus lowering the dose and "rescuing" the picture later.
You REALLY don't want the details of the mathematics involved here, but the newest scanners use something called iterative reconstruction to this end. Let me just borrow the definition from the Wikipedia
The reconstruction of an image from the acquired data is an inverse problem . Often, it is not possible to exactly solve the inverse problem directly. In this case, a direct algorithm has to approximate the solution, which might cause visible reconstruction artifacts in the image. Iterative algorithms approach the correct solution using multiple iteration steps, which allows to obtain a better reconstruction at the cost of a higher computation time.Aren't you glad you asked? Basically, you keep plugging your image back into the computer until it looks good. The major vendors all have iterative recon in one form or another. Now, I must give credit to GE, whose new VEO system (which just won FDA approval) goes this one better, and I'll let my friends from Medgadget tell you how
For decades, the standard CT image reconstruction algorithm has been filtered back projection, which uses mathematical methods to reconstruct tomographic images from the projections that are obtained by the circling detectors. More recently, a new reconstruction algorithm, adaptive statistical iterative reconstruction (ASIR), has been introduced that performs modeling of the noise distribution, cutting radiation dose by up to 80% for many applications.This is really, really clever. The resulting image (bottom pane) looks pretty good as compared to standard reconstruction (top pane)
I'm assuming the modeling has to be done for each individual machine, because there are variances in even the most precisely-made product. No doubt there is scanning of some some standard phantom followed by back-tracking to form a mathematical version of what the scanner looks like to the average photon. Keep in mind, though, this is all done in software, not in hardware, and software can be reverse-engineered. Thus, I doubt GE is going to have this exclusively for much longer. Still, credit where credit is due. This was a rather brilliant innovation. It does seem to take a LOT of computing power to run these numbers, however, and the reconstruction is far from instant. I'm thinking GE needs to set up something like the old SETI desktop program wherein concerned individuals could donate their computers' idle time to the processing of medical images.
No one has mentioned applying the above techniques to old scanners, but there are a lot of them out there, and they need some low-dose love, too. I've encountered two vendors who promise to provide that love. For a price, that is, and substantially more than what love goes for on the street corner not far from one of our hospitals.
The good folks from Sapheneia in Sweden are very anxious to sell you this
The Sapheneia product Clarity Server is a software providing image quality enhancement optimized for greater diagnostic confidence. Clarity image processing algorithms enable radiologists to lower radiation dose exposures during image acquisition.To be characteristically blunt and pugilistic, the Clarity Server is a computer that sits between the CT and the PACS, massaging the data, and prettifying the images that you have deliberately scanned at suboptimal parameters, in hopes of recovery to robust diagnostic status. I'm not so sure about this approach. (Could you tell?) While I haven't confirmed it, the third paragraph from Sapheneia suggests to me that all they are doing is simple digital filtration of the images, smoothing (Gaussian noise reduction) and then edge-enhancing, as you can do with the free Photoshop clone, Gimp2 . This should be something that is built into your PACS viewer, and so it has been, at least partially, in AMICAS PACS since version 3.x. OK, AMICAS just gives you a few steps of edge enhancement, but it proves the concept. One press of the "S" key sharpens every CT slice in your study.
Here's a single slice with progressing grades of edge enhancement
The real question we have to ask is whether or not data gets lost in the process. Sapheneia shows charts and so on that show various parameters are improved, mainly SNR and CNR (signal and contrast to noise ratio, respectively) but that doesn't necessarily mean that some details aren't smoothed out in the process. I haven't yet found a paper that proves or disproves my paranoia, but I'm going to keep looking. In the meantime, Sepheneia has apparently jacked up the price on the magic box considerably in light of the dose mania over here in the States.
Another vendor offers similar denoising as part of their advanced imaging suite. Vital Images, now owned by Toshiba, has made it through my door in spite of misgivings from way back, not to mention the fact that Larry D. still works for them, and the new Vitrea looks pretty good. It includes a denoisify function which applies similar filtering to legacy CT images. It can be toggled on and off, something which I'm not sure the Sapheneia box can do. From Vital
The Noise Reduction menuSure sounds like simple image filtering to me. These can be applied to 3D renderings, a nice touch.
Now, there are far more advanced methods to denoisify than simple filters, and if you really want to punish yourself, have a look at THIS thesis about using wavelets (the same stuff used in teleradiology compression) to do the job. I'm not going there, but you'll be glad to know that there is a GIMP denoisifying wavelet filter app for that.
In the end, we have a few choices to achieve dose reduction. We can all go out an buy new scanners with the most efficient X-ray tubes and detectors, and the best iterative reconstruction. These babies bring in the dose for a cardiac CT, for example, below 1mSV, which is very, vey low indeed. But if you don't have a couple of million lying around, should you invest in one of the after-the-fact denoisers? I'm not yet certain. They can definately give you a prettier picture, but will they obscure important findings in the process? That is indeed the Million Dollar question.
Of course, the most-overlooked path to dose reduction is, like the best but least-respected contraceptive, is abstinence. Think before you order a scan.
Nahhhhhh. Never mind.