How to choose the right OPG for your surgery: part three

When you are thinking of buying an OPG, the question of justification is increasingly being raised. Due to the number of poor panoramic X-rays taken, the resulting lack of detail and the influx of cheap, featureless OPGs onto the market (by featureless I mean basic ethical requirements, for instance a true paediatric mode), it’s no wonder that regulatory bodies and opinion leaders can be sceptical about their true value.

The new regulations from the EU require you to justify taking your radiograph to an even greater degree. So how do you decide which OPG is right for you? Well, first of all you need to sit down and think about your business – a good quality OPG may last you for 10 years or more, so you need to think whether your practice will stay the same for the next 10 to 15 years.

Do you intend to specialise in that time? Add more surgeries? Will you bring in a specialist to fulfill a need? The machine you buy now may be a white elephant in a few years if it does not have the capability to grow with your business. Not forgetting, of course, the basic reason to purchase an OPG, which is to assist with diagnosis and planning.

Well, as with everything, you get what you pay for. The better quality the OPG, the better quality the image. Think quality, quality, quality! Below I have some of the features you should look for in your new OPG. The more boxes you tick, the better the image will be.

How does an OPG work?

Whether digital or film-based units, they all work on the same principle and all have the same factors affecting the quality of the image in terms of kV, mA and time.

A simple way to explain an OPG is as follows: a collimated X-ray source travels around the patients head, the X-rays are focused on one finite area along the jaw line, the focal trough; they are then captured at the other side by either film or a sensor.

It sounds simple but as the X-rays go through the skull they pick up images of structures that are not relevant to the area in focus. These structures are generally out of focus due to the panoramic principle and cannot be clearly viewed (although they sometimes leave artefacts on the final image).

Anything inside the focal trough will be sharply focused while anything outside the trough will be out of focus. So the secret of getting a good panoramic radiograph is to get all the relevant structures inside the focal trough. Below is an example of an acceptable radiograph. Notice the detail in the anterior region and lack of ‘spinal shadow’.

Variable focal trough

A variable focal trough means that the OPG will rotate automatically around the patient’s head in sync with the shape of the jaw. In the picture to the right, the shaded area is the focal trough. If the teeth are outside of this then they will either not be in focus, be distorted or not visible at all.

Some machines on the market have fixed rotations so no matter what size the patient’s head and dental arch shape, you only have one focal trough position that is used for everyone. This is okay if your patients have a size and shape of skull that match the fixed focal trough. However, for those patients who have features outside this zone that you cannot see clearly, you are exposing this area to radiation for no diagnostic or planning value at all.

These manufacturers work on the presumption that with good positioning technique you will get a large percentage of your patients to fall within the sharp region. But what about the rest? What should you look for? The answer is simple – look for units that automatically adapt to your patients’ anatomy.

Easy positioning

Easy and accurate positioning is one, if not the most important, aspect of achieving good OPG images. If you cannot position the patient easily every time, then any radiograph you take could be rendered useless, no matter how good a unit you have.

Ideally, what you need is a fixed bite block that locks the patients’ teeth into the narrow anterior region of the focal trough. Any unit that has a bite block that can move buccally and lingually on the horizontal plane can cause the patient to be set up incorrectly. You need simple positioning of the mid-line and Frankfurt plane (laser positioning lights are a must). As always, the less you have to do, the less that can go wrong. The patient also needs to be fixed in place, with head supports to stabilise and prevent movement, while a forehead support, in my opinion, makes positioning a whole lot easier.

Image quality

Remember, if your developer fluid is old, too hot or too cold then it does not matter how good your OPG is, the image quality will be poor. (Panoramic films are far more sensitive to these factors than intra-oral films.) But with digital sensors these pitfalls are eliminated straight away. These days, most modern OPGs have preset parameters that take the guesswork out of setting kV and mA values, and should the patient’s image appear slightly under- or over-exposed, you have the power of your imaging software to adapt the image to suit your needs.

As for that old contentious issue of resolution, theoretical resolution, actual resolution and scanned resolution, the reality is that phosphors plate systems and direct sensor systems all produce images of around 5 lp/mm.

The rule of thumb has to be don’t look at figures, look at the image. The human eye is a fantastic tool and generally what you see is what you get. It’s the fallible brain that thinks ‘those numbers are higher, so it must be better’!

Another factor that affects image quality is the screen upon which you view the resulting image. What is the point of spending thousands on your new OPG if you go and buy a cheap monitor? You need at least a 17” screen and it should be high resolution. The quality does vary and very often you leave this critical decision to your computer technician, who really doesn’t understand your needs when viewing a digital radiograph.

Engineer back up and spare parts

Anyone can sell anything. The big question should be this: is the support there from both the dealer and the manufacturer?

Will your dealer still be here in eight years’ time when the unit needs a part? Will the dealer still be working with that manufacturer? You will know of many dealers that have swapped and changed what they sell depending on how the wind blows. You also know of manufacturers that have sold their kit through one dealer, then another and then another.

There is always a reason why dealers drop a manufacturer’s product and it’s not normally good. Make sure the dealer has good, highly trained engineers. Many manufacturers have intensive training programmes for dealers to make sure their products can be maintained and serviced efficiently, which is very important to you when you are running a business.

In conclusion

The quality of the OPG that you invest in will ultimately determine the quality of your image. Look for simple positioning, a unit that will help the work flow of your practice and one that can grow with your business.

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