A real head turner

06 December, 2010 / Infocus
 

The following article is a light-hearted look at some issues around cross-sectional dental imaging with a few serious comments and warnings for the uninformed. General dental practitioners are becoming increasingly involved in implant planning, placement and in the restorative phase of this newer treatment modality.

Bright eyed and bushy tailed, we headed off to university. For some this is a fond, but distant memory. Remember struggling with the vagaries of anatomy? Then later, trying to apply it when looking at X-rays, oops… meant radiographs – “X-rays being invisible”, I was reminded often.

What memories of the X-ray department? Didn’t the staff have strange job titles such as radiographer, radiologist and dark room technician? The latter was the poor soul condemned to darkness for much of their working day – what was all that about?

Dad said to study an “ology”, so maybe that made radiology a useful topic – so we got those radiology tomes out, laid them on our chests in the park on a sunny afternoon in May just before the exams and allowed osmosis to take effect. Spotting caries was not rocket science if you could sort out the pesky “cervical burnout” lesion!

Wade through the black and white/gray stuff for exam purposes and get stuck into real dentistry – this perhaps sums up most people’s experience of radiology at undergraduate level, unless like this author you were a bit of an imaging geek.

But what if all of that radiology speak actually had a practical use? I soon became aware, after graduation in dentistry, that many clinical decisions centre on a sound understanding of radiological anatomy and the ability to recognise the “abnormal”.

How to ward off that nagging thought that occurs just as you place your head on the pillow: “what exactly was that radiographic appearance I noted today?” This is the ongoing challenge. The spectre of missing a sinister lesion can sometimes weigh heavily. Should I refer on for an opinion or will I look silly?

The steep learning curve tails off and the bump and grind of daily practice can begin to lull one into a state of professional stupor. Then along comes a new “ology”. Driven by the endless makeover programmes and the promise of restored function and eternal youth, implantology makes its entrance on to the dental high street.

This offers the opportunity for some skilled and daring souls in our profession to move out of their long-established comfort zone of routine dentistry into a brand new world of surgical “Meccano”.

Remembering dad’s advice, implantology represents a good “ology” to get involved in. Besides, there are some flashy new shiny toys which fit easily with the dental profession as we are so used to working in confined spaces and have excellent manual dexterity.

In order to avoid that nasty letter from a lawyer should the expensive implant work fail and to avoid the “welling up” of red stuff in the floor of mouth due to pranging a neurovascular canal or misaligning the implant completely, perhaps a timely bit of imaging is required.

So we dust off that radiology book and apply the knowledge. Conventional views have served for a number of years with well-placed per-apicals (paralleling technique) and the trusty panoramic, but the illusive third dimension is missing.

There are those at conferences who will sell the line that they can eyeball an implant into the mandible from a thousand yards, using skills honed over a thousand cases. The realities may be quite different to the novice operator on the implant scene.

Alas, 3D imaging systems can offer some peace of mind. Visualisation of the bucco-lingual dimension is now possible – seen in the coronal plane, remember. Sagittal, coronal and axial, the orthogonal planes used by anatomists and radiologists to describe sections. The fog of the undergraduate memory clears.

The cone beam CT scanner

Since units have become available on the high street, dentistry has been quick to appreciate the benefits of an office-based (how American), low-dose (compared to medical CT), 3D imaging device, the cone beam CT scanner. CBCT can address many of the problem solving aspects of diagnosis in dentistry.

With regard to implants, knowing where those pesky nerves are is all-important in the quest to give a predictable result and avoid operator hypertension. A bit larger than an OPG machine, obtaining its images by circling the head in anything between 10 and 40 seconds, the CBCT produces cross-sectional images that can be viewed from three plains: axial, coronal and sagittal. Variable thicknesses in section can be shown, down to 0.25mm.

In addition, using a software package, the data can be made so that the acquired volume (field of view) can be viewed like the trusty old friend “the dental panoramic”. 3D reconstructed images of the jaw are possible and can be manipulated at the chairside, sending the patient into raptures of delight as they see their inner-self revealed.

So, CBCT imaging – can’t be that difficult, can it?

Perhaps I should get one for the practice?1 CBCT comes in various shapes and sizes and are classified as large (field of view) FOV and small FOV. The smaller field size can offer dose reduction and limited viewing of teeth and immediate surrounding structures.

Super-crisp images in the brochures tempt one with resolution “to die for”. Was the picture produced on a patient though, I hear you say?

What is not immediately apparent is that there are nuances to the CBCT systems. For example, the 0.4mm voxel setting gives better pictures than the 0.2 mm voxel setting on iCat. This comes down to the maths involved in the imaging algorithm. The system utilises a limited tube current in order to keep dose down. No more, you are losing me, you might say… Put simply, this is essentially good news for the patient as it cuts down the rays.

Having spent a serious amount of cash acquiring the impressive looking “Daddy of all dental imagers”, the question arises as to who is going to take the scan? In the opinion of this author, it should be a medical/ dental professional with appropriate training. This may include a radiographer, or a dentist. One should appreciate that the use of dental nurses or hygienists even with a dental radiography qualification is contentious in the acquisition of CBCT images at this time.

IRMER issues

In order to comply with the radiation regulations, a great deal of effort has to go into informing the Health and Safety Executive that you have the CBCT scanner and into documenting your IRMER 2000 and IRR1999 protocols.

CBCT room design will have to allow for radiation protection features to cope with a 120KV beam energy. This 120KV beam has increased penetrating power, compared to dental sets at approximately 65KV to 70KV. A radiation protection advisor (a medical physicist) will have to verify this has been carried out correctly in compliance with IRR1999.

All the IRMER documents must be in place, which entitle the various staff members to carry out their roles under IRMER in regard to the CBCT machine. For example, who is acting as employer, referrer, operator and practitioner.

Protocols must be in place itemising all possible CBCT imaging situations, detailing who can refer for them and under what kind of situations – that is, what “justification” is required. All must be documented and be robust enough to withstand an IRMER inspection. This gets a little complicated, but is distillable with some effort. Also, in terms of image interpretation, IRMER 2000 demands that all of the radiographic image should be assessed and an appropriate report documented.

So how do I report on the CBCT images?

Much of the software available will make dental diagnosis very user-friendly and allow planning for implant placement straightforward, for example, ICat vision software (Imaging Sciences).

There is, however, a sizeable amount of tissue data that may be missed in the FOV. This can be seen on the MPR screen on iCat vision or with better resolution on the Xoran software used with iCat. This author reports direct from the Xoran software, which is presented in the orthogonal planes.

Currently there are few if any courses being run on how to report
CBCT. Problem-solving begins when appearances are unusual. Is the appearance benign or does it represent an aggressive process? If so, is it malignant? Having knowledge of “normal” on an image section will help, but pattern recognition may be a bit scary at first.

New anatomical territory is now visualised: inner ear, base of skull, cervical spine, peri-oral soft tissues to name a few. A sound knowledge of what soft tissue outlines are supposed to resemble will give peace of mind that no tumours have been missed, for example, in the laryngo-pharynx (Fig 1 – see digital version of article).

The old imaging maxim: “Just compare left with right sides” has merit, assuming lesions are not bilateral in their presentation. Appreciation of the anatomical site, the relative density, outline of a lesion and access to any previous imaging will all help in reaching a differential diagnosis.

Getting the dissection/radiology books out from times past may be a good way to start. However, not much cross-sectional anatomy was taught at dental school.

The following examples are in no way comprehensive:

Some anatomy to know about on a CBCT sectional image

Recently, small “neurovascular canals” have come into view on CBCT that were not appreciated before2. The “lateral lingual canal” in the body of mandible is a good example. These neurovascular canals usually lie near but separate from the mental foramen. In the patient with an atrophic mandible, they can offer a potential surgical hazard for implant placement. They can contain nerve fibres from the nerve to myelohoid and so if impinged upon by an implant, pain can present post op. (Fig 2)

Some examples of lesions not to be missed from CBCT images

A keen rugby player and diver presented for a 4cm FOV pre-implant assessment of his upper centrals. Assessment of the full field of view revealed a haemangioma of CV1. (Fig 3)

Assessment of the base of skull is imperative – ear structures should be reported on

The appearance of chronic inflammatory lesions in the middle ear – an early cholesteatoma (Fig 4).

Conclusion and opinion

Having a knowledge of the appropriate “ology” is the key here, for peace of mind and ongoing success in implant planning. It is for the person functioning in the IRMER operator role to produce the image report. Confusingly, this may not be the person pressing the button to obtain the scan (see IRMER2000 regulations).

The IRMER operator (reporter) must report the CBCT scan and any other relevant images, unless that role is delegated to a suitably trained other, such as a radiologist. This author would argue that the best-placed radiologist to undertake this role should have a dental background.

This obviously has cost implications that must be passed on to the patient. However, in the long term, if cases come to light where significant medical radiological presentations are missed, there will be medico-legal ramifications.

It is also good practice to warn the patient prior to the CBCT scan that non-dental features/lesions may become apparent on the scan, which may involve ongoing referral, and potentially give rise to a period of uncertainty until a definitive diagnosis or treatment can be realised.

With respect to Ionising Radiation Regulations, if they are not properly implemented into practices with CBCTs, then the Scottish Executive may have the right under IRMER2000 to take action which could close the imaging service down.

In my opinion, the “Daddy”, in colloquial parlance, is the wise practitioner who puts the patient’s best interests first. The “Daddy” is also someone who plays to their strengths.

It may be that image acquisition is more easily carried out at another site where all the above issues have been addressed. It may be that image interpretation is best carried out by a suitably qualified other.

The fear of losing a patient to another operator who has “the big daddy” of dental imagers will be less if the CBCT is sited in a secondary referral centre/practice.

Discussions with the owning practitioner/specialist should ensure that we act as a family within the profession, with respect for each other’s role in the treatment team.

Dr Neil D C Heath, DCR, BDS, MSc, MFDS RCS, DDR RCR, Consultant and Specialist in Oral Maxillofacial Radiology, Edinburgh Dental Specialists


References

1. Heath N and Macleod R I, Dental Update 2008 (35) 353

Cone beam CT in Dental Practice

2.Trikeriotis et al 2008,Dentomaxillofac Radiol (37) 125-129.

Anterior mandible canal communications: a potential portal of entry for tumour spread.

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