Crawford F. GrayThomas W. RedpathFrancis W. SmithRoger T. Staff

Authors’ affiliations:Crawford F. Gray, Thomas W. Redpath,Roger T. Staff,Department of Bio-Medical Physics andBio-Engineering, University of Aberdeen,Foresterhill, Francis W. Smith, MRI Centre,Woodend Hospital, Aberdeen, Scotland

Correspondence to:Thomas W. RedpathDepartment of Bio-Medical Physics andBio-EngineeringUniversity of AberdeenForesterhillAberdeen AB25 2ZDUKTel: π44 1224 681818 ext. 52263Fax: π44 1224 685645e-mail: twr/biomed.abdn.ac.uk

Date:Accepted 12 November 2001

To cite this article:Gray CF, Redpath TW, Smith FW, Staff RT. Advancedimaging: Magnetic resonance imaging in implantdentistryClin. Oral Impl. Res, 14, 2003; 18–27

Copyright C Blackwell Munksgaard 2003

ISSN 0905-7161

18

Advanced imaging: Magneticresonance imaging in implant dentistryA review

Key words: dental implants, magnetic resonance imaging, radiology, sinus floor elevation,tomography.

Abstract: For accurate and safe placement of dental implants, and planning of associatedsurgery, a full assessment of the surgical anatomy of the site is necessary. Thus, it is highlydesirable to have tomographic, sectional information available, to permit the implant to bealigned correctly. In recent years, X-ray computed tomography (CT) has become accepted asthe gold standard in assessment, but the exposure to ionising radiation can be substantial.Artefacts due to dental restorations can also be significant, and some doubts may exist overthe accuracy of reformatted CT. Magnetic resonance imaging (MRI) entails no exposure toionising radiation, and allows direct acquisition of tomographic information in any desiredplane. Sequential studies may be safely performed, allowing us a valuable insight into bonegraft behaviour. Other than in a small number of cases, MRI may be safely used for presurgicalassessments. Artefacts are few and in most cases localised. The surgical confidence from thesectional information gained is a significant step forward in the safe placement of dentalimplants.

Since the pioneering work by Branemarket al. (1969) and Schroeder et al. (1976), os-seo-integration of dental implants has be-come predictable (Buser et al. 1997). Thepossibilities for implant placement intosites of insufficient bone height or volumehave become reality with bone grafting,such as with the maxillary sinus lift (Boy-ne & James 1980) and onlay grafting(Smiler 1994).

For safe implant placement, full knowl-edge of the shape and quality of the im-plant site is a prerequisite. Incorrect presur-gical assessment can at best lead to im-plant failure, and at worst to damage tonerves (Berberi et al. 1993; Ellies &Hawker 1993) and blood vessels (Ten Brug-genkate et al. 1993), perforation of themaxillary sinus (Regev et al. 1995) and

other sequelae. A number of methods forassessment have been developed, bothphysical and radiological.

Physical assessment

Physical assessment may be performed bythe technique of ridge mapping (Wilson1989), where the bone architecture is esti-mated by probing through the oral mucosato the bone, and relating these measure-ments to a cast stone dental model. Thistechnique can give an indication of labialcontour, but gives no information on bonequality or the position of vital structures. Ina recent study (Allen & Smith 2000), thelack of accuracy of this method is high-lighted.

Gray et al . MRI in implant dentistry

Assessment using X-rays

Pre-implant radiological examination maybe made in a number of ways, as will bediscussed below. When considering the useof ionising radiation, the following con-ditions should be met

O the image projection should be in aplane that allows accurate measure-ments of anatomical features to bemade;

O all relevant features such as vital struc-tures, anatomical boundaries and bonestructure, should be clearly visible;

O if possible, sectional information of theimplant site should be available;

O artefacts should be minimal and not af-fect the accuracy or clarity of the area ofinvestigation;

O radiation dosage should be kept to aminimum, and the quality of the filmsshould justify the exposure.

Periapical radiographs (using an appropriateparalleling technique) can give us goodtwo-dimensional views of implant sites,with clear delineation of cortical and can-cellous bone, bone trabeculae and vitalstructures. The main limitation, is thateven when supplemented with occlusalviews, sectional information such as thebucco-lingual position of the inferior den-tal canal is not accurately available.

Panoramic radiographs are most helpfulin the initial assessment of the region ofinterest, but again, are only two dimen-sional, and are subject to magnificationerrors (Bolin et al. 1996), which may varyat different points in the dental arch (Grayet al. 2001). Oblique lateral cephalometricradiographs have been shown to give repro-ducible height measurements in the man-dible, but the information is still two di-mensional, and care must be taken toavoid positioning errors due to angulationof the beam (Verhoeven & Cune 2000).

Sectional information in assessment ishighly desirable, as conventional radio-graphs often give an erroneous picture ofthe site (Gray et al. 1998a). To gain sec-tional information of potential implantsites, we need to use a tomographic tech-nique. Conventional X-ray tomography(Eckerdal & Kvint 1986; Gröndahl et al.1991) has been recommended for the max-illa and posterior mandible, but is still sub-ject to positioning and reproducibilityerrors (Fredholm et al. 1993). Conventional

19 | Clin. Oral Impl. Res. 14, 2003 / 18–27

tomography has recently made a re-surgence with modern machines such asthe ScanoraA (Tammisalo et al. 1992) andthe Cranex TOMEA (Bou Serhal et al.2000). Whilst these machines are useful forsingle implant sites, they become cumber-some in complex cases. Dula et al. (1997)reported significant exposure to ionisingradiation with the Scanora in large exami-nations, but significant dose reductions onsingle site studies. Modern tomograms cangive good accuracy (Kim & Park 1997), butcare has to be taken in interpretation of theimages (Kaeppler 2000). Blurring has beennoted in the posterior maxilla, due to over-lying bony structures (Bou Serhal et al.2000), and posterior mandible (Kim & Park1997).

For dealing with more complex cases,the technique of X-ray computed tomo-graphy (CT) has been successfully used forsome time (Schwarz et al. 1989; Jameset al. 1991).This X-ray modality allows sec-tional imaging of the jaws, and either thewhole mandible and maxilla or a restrictedregion of interest may be imaged (Dunn &van der Stelt 1995). Direct CT at rightangles to the contour of the jaw gives ex-cellent images. Although some radiologistsstill image in this way (Ekestubbe et al.1997), the radiation exposure to the eyesand thyroid gland make this technique un-acceptable. This problem has been partlyaddressed by using reformatted axial CT(Imhof 1992), but the radiation dose is stillsubstantial, and the justification for usingCT when other modalities exist is debat-able (NRPB 1992; Dula et al. 1996). Sophis-ticated software has been developed to as-sist implant planning using CT data (Cas-selman et al. 1990; Dandrau et al. 1992).Although thought of as the ‘gold standard’,CT has been shown to have errors (Markie-wicz et al. 2000a). Some dimensionalerrors in reformatted CT may be due to pa-tient positioning (Kohavi et al. 1997). Ingeneral, severe image degradation andartefacts can result from metal dental res-torations, making interpretation difficult(Svendsen et al. 1980; Lufkin et al. 1988).‘Bone quality’ is a rather nebulous termused by clinicians to describe various fea-tures of the implant site ranging from bonedensity to trabecular pattern. Certainly,CT is good for exhibiting trabecular detail,and Norton & Gamble (2001) have corre-lated bone density (measured in Houns-field units) to the subjective bone quality

classification scale of Lekholm & Zarb1985). The value of this is debatable, asbone trabecular pattern is a variable fea-ture, and changes on loading (Steflik et al.1995).

One novel method of gaining three-di-mensional information is by Tuned-Aper-ture Computed Tomography,TACT (Webber 1994; Webber et al. 1997).TACTTM has its roots in the fundamentalwork on tomography of Zeides des Plantes(1932) and the further development of to-mosynthesis by Grant (1972). The workusing TACTTM to date has been with invitro studies, including one of cross-sec-tional imaging (Liang et al. 1999). The pro-jection/imaging geometry used allows a 3Dacquisition with a radiation exposure simi-lar to that from a simple two-dimensionalradiograph.

Magnetic resonance imaging

Magnetic resonance imaging (MRI) doesnot use ionising radiation. Instead, the pa-tient is placed in a strong magnetic fieldand subjected to short pulses of radiowaves. MRI is based on the phenomenonof nuclear magnetic resonance (NMR),which was first described independently bytwo groups of workers in the USA (Blochet al. 1946; Purcell et al. 1946). In the1950s, NMR was used by chemists toexamine molecular structure.

MRI uses signals from hydrogen nuclei(protons) in water and fat to form cross-sec-tional images of the body. In the USA, Da-madian (1971) reported increased hydrogenNMR relaxation times in canceroustissues, allowing detection of tumours,while Lauterbur (1973) developed the im-aging principle which allows us to con-struct MR images. These initial ideas weredeveloped into a practical imaging tool inthe UK. The first clinical whole body scanwas performed in Aberdeen (Edelsteinet al. 1980; Hutchison et al. 1980), whilegroups in Nottingham and London pion-eered brain imaging (Hawkes et al. 1980;Young et al. 1981). Redpath (1997) has re-viewed the technical development of MRIwhich followed Lauterbur’s 1973 paper.MRI has been used in the head for investi-gation of temperomandibular joints (Katz-berg 1989), facial nerve studies (Worthamet al. 1989) and tumour pathology (Wong1996).

In MRI, to create images, we use a stronguniform static magnetic field, switched

Gray et al . MRI in implant dentistry

Fig.1. A 1.5tesla conventional mid-field magnetic resonance imaging scanner.

magnetic field gradients, with radiofre-quency magnetic field pulses (Stark &Bradley 1999).The molecular environmentand the proton densities influence the rela-tive intensities of the MR signal produced.The signals from the hydrogen protons inwater and fat vary depending on the natureof the tissue being examined. Most MRImagnets operate in the mid-field range (ap-proximately 0.5–1.5tesla, where 1tesla isaround 20,000 times the strength of theearth’s magnetic field). Mid-field scannersusually require the subject to be imagedwithin a tunnel (Fig.1), which may causeclaustrophobia (Medical Devices Director-ate 1993). McIsaac et al. (1998) reportedthat 25% of participants of their study ex-perienced anxiety during MRI scanning.Novel low-field scanner designs allow thesubject to be imaged with an ‘open mag-net’, which offers a less claustrophobic im-aging environment (Fig.2). Minor inter-ventional surgery may even be performedin open scanners (Jolesz et al. 2001; Park-kola et al. 2001). Spouse & Gedroyc (2000)reported a 94% success rate when scanninga group of patients in an open magnet whohad failed to accept a conventional MRIscanner. High-field scanners (above1.5tesla) which allow very high spatial res-olution have been used for dental researchon ex vivo samples (e.g. Lockhart et al.1992; Baumann & Doll 1997). Improve-

20 | Clin. Oral Impl. Res. 14, 2003 / 18–27

ments in MRI technology may allow thesetechniques to be applied clinically.

MR images are often described as beingeither T1- or T2-weighted. T1 and T2 referto the longitudinal and transverse protonrelaxation times, respectively. T1- and T2-weighted sequences are made by varyingthe timings of the radiofrequency pulses,thus altering the sequence repetition time(TR), the echo time (TE) and, hence, image

Fig.2. A 0.2tesla ‘open’ magnetic resonance imaging scanner.

contrast. In general, T1-weighted imagesare used to show normal anatomy, whileT2-weighted images are useful for detec-tion of infection, haemorrhage and tu-mours. Due to the different informationavailable from T1- and T2-weighted im-ages in neoplastic tissue, both sequencesshould be obtained when investigatingpathology. To reduce the effect of fattytissue such as cancellous bone makinginterpretation difficult, the technique of fatsaturation may be used. This techniqueutilises the small difference (3.5 parts permillion (p.p.m.)) in resonant frequency be-tween protons in water molecules, andthose in lipid molecules, to suppress thesignal from fat. A review of the principlesand clinical applications of MRI may befound in Stark & Bradley (1999).

Pre-implant imaging

MRI was not originally thought of as a use-ful modality for preimplant assessment(Taylor 1991), but the use of appropriate se-quences and parameters allows us largelyto overcome the perceived difficulties. In-deed, the potential for bone imaging is wellestablished, especially in regions where de-lineation of soft tissues is also required(Pevsner et al. 1986; Smith 1988).

For preimplant assessment, as normalanatomy is being examined, the use of T1-weighted sequences is indicated (Fig.3). InT1-weighted images, the external corticalplate appears black, unlike the normal

Gray et al . MRI in implant dentistry

Fig.3. (a) A T1-weighted high resolution cross sec-tional view showing the normal maxillary anatomy.A: cortical bone. B: Cancellous bone.C: The maxillarysinus. (b) A T1-weighted high resolution cross-sec-tional view showing the normal mandibular anatomy.A: cortical bone. B: cancellous bone. C: mandibularnerve and vessels. D: mental nerve and vessels.

radioopacity due to increased bone densityseen on radiographs. This MRI appearanceis due to the very low signal owing to theabsence of water or lipid protons. In con-trast, the more organic cancellous bone ap-pears very bright in T1-weighted images, asa result of the signal from protons in thefatty bone marrow, unlike the grey radio-logical appearance of this tissue. Neuro-vascular channels such as the inferior den-tal canal and the naso-palatine foramen are

21 | Clin. Oral Impl. Res. 14, 2003 / 18–27

identified as discrete dark structureswithin the bright cancellous bone.

In this assessment, as we can clearly de-lineate the external interface between cor-tical bone and mucosa/mucoperiosteum.Using this margin as the exterior limit ofbone, we can measure the available height,width and angulation to avoid vital struc-tures and maximise implant size. The in-terface between cancellous and corticalbone is also clearly seen, although Naselet al. (1999) found that the correlation be-tween cortical to cancellous bone was in-correct in both CT and MRI when com-pared with direct osteometry of the sites.These authors did, however, note that thedimensions of the mandibles and the posi-tions of the inferior dental canals corre-lated strongly when CT and MRI werecompared with direct osteometry. Markie-wicz et al. (2000b) show a good correlationbetween MRI and CT, that in the mandiblebeing better than in the maxilla. Theauthors thought the maxilla more difficultto assess, and that MRI was better in youngpatients. However, details of MR or CTscanning protocols are not given.

MR interpretation has an initial learningcurve, and clinicians may at first find themconfusing, as the appearance of the variouscomponents of bone is very different fromthose in conventional radiographs. The ap-pearance of soft tissues on the image mayalso be unusual to those used to radio-graphs, but this gives additional infor-mation on tissue contour and thicknessand shape of mucosa over the ridge. Softtissues appear as a grey mid-level signal inT1-weighted MRI scans. Although CT mayalso be used to show soft tissues, the algo-rithms used for this are incompatible withthose demonstrating hard tissues such asbone.

In general, MRI can be considered an ex-tremely safe imaging modality. As in allmedical examinations, a full medical his-tory should be taken before an MRI scan istaken. In general, contra-indications arefew, for example

O do not scan in the first trimester of preg-nancy;

O do not scan a patient who has a cardiacpacemaker;

O do not scan a patient who has shrapnelwounds, especially around the orbit;

O do not scan a patient with retained ferro-magnetic surgical clips in situ.

It should be remembered that, for some pa-tients, simply placing them inside thescanner can have immediate and fatalconsequences, so that a thorough screeningprocedure prior to MR scanning is essen-tial. However, in the above list, the restric-tion on scanning during pregnancy is pure-ly precautionary, as no significant hazardhas been proven (NRPB 1991).

The effect of high magnetic fields ongenetically predisposed mice embryo maylead to an increase in abnormalities (Tynd-all & Sulik 1991). Tissue heating may alsooccur as a result of the RF pulses (Kanal1992) but the risks at normal operating par-ameters are negligible (NRPB 1991).

Zabalegui et al. (1990) performed a pre-liminary study into implant assessmentusing MRI for one patient. Gray et al.(1996) used MRI assessment for 12 patientsneeding dental implants. This study wasperformed using a 1tesla mid-field MRIscanner and described the procedure forsetting up the imaging planes. A T1-weighted sequence was used, with the pa-tient being imaged within the system’sradiofrequency head coil. To facilitateidentification of the region of interest, werecommend the constructed imaging/sur-gical templates. This is especially usefulfor edentulous, or long arch span cases. Fortemplate construction, we use a clearacrylic dental registration block, or replicadenture, with 2mm diameter marker holesdrilled over potential implant sites. Theholes are filled with a solution of Magnev-ist (Schering AG, Berlin, Germany) diluted2ml to 1000ml saline, and sealed with selfcure acrylic. Magnevist, normally used asan intravenous MRI contrast agent, con-tains paramagnetic gadolinium ions whichmake it clearly visible in T1-weighted se-quences (Carr et al. 1984). After imaging,the block may be redrilled and convertedinto a surgical template.

For preimplant imaging, Gray et al.(1996) suggested an initial triplanar pilotscan in sagittal, coronal and axial planes,with a low-resolution gradient echo se-quence. The sagittal pilot is used to set upa series of high resolution, fast spin echoaxial slices (Fig.4). From these slices, an ap-propriate slice showing the markers is se-lected (Fig.5) and set up for a series ofcross-sectional high resolution images atright angles to the region of interest maybe made (Fig.6). After obtaining cross-sec-tional images, an appropriate slice is se-

Gray et al . MRI in implant dentistry

Fig.4. A low resolution gradient echo sagittal pilotscan used to set up the plane for the acquisition ofthe high resolution axial slices.

Fig.5. A high resolution fast spin echo T1-weightedaxial slice being used to set up a series of high resol-ution views at right angles to the potential implantsite.

lected, and a scanning plane parallel to thearea of interest is set up for a further set ofhigh resolution images (Fig.7). This planeshould be set up in the proposed line ofinsertion of the implant to prevent fore-shortening of the available bone measure-ment.

Direct imaging with MRI allows us aflexible plane of acquisition, with no needto reformat. Multiple site acquisitions maybe made, as long as the intersection of theslices is not at a region of interest (Fig.8).On compatible machines, CT software hasbeen used for reformatting MRI (Gahleitn-er et al. 1999) but this may invoke similarerrors to those postulated for reformattedCT. Markiewicz et al. (2000b) noted theirdifficulty in setting up MRI dental scans

22 | Clin. Oral Impl. Res. 14, 2003 / 18–27

Fig.6. High resolution fast spin echo T1-weightedview at right angles to the implant site being used toset up a series of high resolution views parallel to thejaw at the implant site. Note the slices are alignedin the path of implant insertion, to allow us tomeasure the actual distance in that plane.

Fig.7. A high resolution fast spin echo T1-weightedslice aligned parallel to the jaw at the implant site.

without the dedicated software found inCT. In our experience, once the operatingstaff understand what is desired of the MRIscan, setting up is rarely a problem. Tosimplify orientation, we describe cross-sec-tional views as ‘right angled views’, andviews in the line of the arch as ‘parallelviews.’ This terminology is to reduce con-fusion, as a cross-sectional view of the in-cisor region may be quasi-sagittal, whilecross-sections of the molar region arequasi-coronal.

A further study with 11 patients usinglow-field MRI has been reported (Grayet al. 1998b). This was performed using a0.2tesla ‘open scanner’ (Open Viva, Sie-mens AG, Erlangen, Germany). The openscanner has the advantage of being less

Fig.8. A high resolution fast spin echo T1-weightedaxial slice set up to acquire right angle sectional in-formation at three potential implant sites. Note thatthe cross-over points must not be over a region ofinterest.

claustrophobic for patients. The low mag-netic field may reduce the possibility ofmagnetic susceptibility artefacts at air/tissue interfaces (Sumanaweera et al. 1993;Gray et al. 1998b). Acquisition times (TA)of the sequences are, however, longer thanthose for mid-field machines, but use ofsignal averaging and better patient comfortreduces the potential of motion artefacts.

The scanning times for MRI are longerthan those for CT sequences, which maylead to claustrophobia in a small number ofpatients, but the lack of ionising radiationmore than compensates for this. Most pre-implant assessments may be completedwithin 30min (Gray et al. 1996).

Sinus lift assessment

The sinus lift operation for bone augmenta-tion of the posterior maxilla (Boyne & Jam-es 1980) has enabled implant placementinto previously untreatable regions. Anumber of variations in surgical techniquecan be applied (Smiler 1997) and variousgraft materials may be used (Jensen et al.1998).

An understanding of the three-dimen-sional shape of the sinus is desirable beforesurgery (van den Bergh et al. 2000) in termsof both anatomical form and volume. Themaxillary sinus anatomy may be furthercomplicated by transverse (Underwood’s)septa, which have been reported in 32% ofcases (Ulm et al. 1995). Failure to recognisethese structures can lead to unpredictablesurgery due to incomplete filling of thesinus.

Gray et al . MRI in implant dentistry

Conventional radiographs give verylimited presurgical information to the cli-nician. However, with sectional imaging, itis possible for the clinician to be able toestimate the volume of graft necessary tosuccessfully gain the appropriate boneheight for implant placement. For dimen-sional assessment of the graft, T1-weightedsequences are appropriate (Fig.9) and thedelineation of the sinus and oral mucosa(i.e. the boundaries of the available bone)may be enhanced by the use of intravenousMagnevist (Gray et al. 1999). This is espe-cially important when autogenous bone isbeing harvested from regions such as theiliac crest, to minimise surgical trauma.After graft placement, it is necessary toevaluate the success of the operation, asgrafts may alter considerably in shape andvolume on maturation (Gray et al. 2000).While the maxillary sinus volume may becalculated using CT (Ariji et al. 1996). Asequential CT volumetric study has re-cently been reported (Johansson et al.2001) but the dose of ionising radiation em-ployed for sequential examinations wouldnot be acceptable by UK standards (NRPB1994).

As no ionising radiation is used, MRI canbe safely used to give sequential imagesshowing the healing and maturation pro-cesses of bone grafting. Using a T1-weighted 3D sequence, it is possible to de-lineate implanted bone from tissue fluidwithin the healing matrix (Gray et al.2000) (Fig.10). Healing blood clots show adifferent picture on T2-weighted sequencesas the clot matures, due to the changes inthe haemoglobin component (Carollo &

Fig.9. A high resolution fast spin echo T1-weightedpresinus lift axial scan clearly demonstrating an Un-derwood’s septum.

23 | Clin. Oral Impl. Res. 14, 2003 / 18–27

Runge 1990). By matching the T2-weightedslices through the regions of interest withthe corresponding T1-weighted slices, it ispossible to identify the changes in theblood clot and tissue fluid components atdifferent times during the healing of thegraft. Through the wide range of sub-stances used as graft materials, it appearsthat blood plays an important role in theprocess (Gray et al. 2001), being directly in-fluenced by the periosteum (Engdahl 1972).Careful interpretation of the healing graftmay allow us to gain a broader understand-ing of the physiology of graft maturation,and the importance of the various compo-nents.

To date, the evidence of sinus lift vol-ume changes of graft material is largely an-ecdotal. The outcome of the operation de-pends on many factors (e.g. composition ofgraft; bone or substitute, volume im-planted, use of membranes, presence of in-fection, etc.). Any longitudinal study willhave to be with standardised conditions.Certainly, it is very important that we areable to achieve stable, predictable resultswhen autogenous bone is being used, toprevent the morbidity of repeated two siteoperations.

Artefacts in MRI

Although MR image quality is generallygood, as for all imaging modalities,artefacts do occur and should be under-stood to prevent errors in assessment. Indental MRI, two main problems arise, thefirst due to patient motion, the second dueto inhomogeneities in the magnetic fieldcaused by magnetic susceptibility effects.

Fig.10. A 3D MPRAGE MRI scan, taken 10days aftera sinus lift operation (using autogenous bone), show-ing regions of (A) bone and (B) tissue fluid.

Motion artefacts occur due to move-ment by the patient during the scanningprocedure. The acquisition time (TA) of thesequence should be kept as short as poss-ible, and signal averaging may be used tominimise artefact. In practice, with goodpatient information and instruction, andcomfortable positioning within the headcoil, this is seldom a problem.

With susceptibility artefacts, two mainproblems may occur. Geometric distortionmay be caused by air/tissue or bone/tissueinterfaces, and by the effects of ferromag-netic metals. Theoretically, distortion atair/tissue interfaces may occur, but for pre-implant assessment, as mucosa covers thebone, this effect is minimal. The main fieldinhomogeneity introduced by a bone/tissue interface is 0.2p.p.m., while at theair/tissue interface it is 9p.p.m. (Suman-aweera et al. 1993). For the protocol usedby Gray et al. (1998b) at 0.2tesla, this re-sulted in a maximum possible misregis-tration error of 0.5mm at the air/tissue in-terface, with the effect at the bone/tissueinterface being negligible. The amount ofdistortion depends on the protocol used,with bandwidth, voxel size and fieldstrength being the critical factors. The pro-tocol and imaging sequence should bechosen to minimise the effect. The distor-tion produced at air/tissue interfaces, for aparticular imaging sequence, is directlyproportional to the magnetic field strengthand may be significant in high-field (aboveabout 1.5tesla) MRI scanners.

Artefacts due to metals are well docu-mented, and usually lead to areas of signalblackout, with rims of high signal strength

Fig.11. A localised MRI artefact around a dental res-toration, originating from a steel dentine pin.

Gray et al . MRI in implant dentistry

Fig.12. A severe MRI distortion caused by a steel or-thodontic band.

Fig.13. Significant artefact production in a CT scan,due to dental amalgam restorations.

around the offending object (Fig.11). Mostof these artefacts are very localised, withlittle geometric distortion, but some ma-terials such as orthodontic bands may givesevere effects, making reliable interpreta-tion impossible (Fig.12). Signal blackout isoften sequence dependent, and may be re-duced by using spin-echo rather than gradi-ent-echo sequences. Unlike the artefactsfound with CT (Fig.13), dental amalgamhas been shown to have little influence indental MRI, while gold alloy showed sig-nificant distortion (Abbaszadeh et al.2000). Unfortunately, the authors do notdescribe in detail the MR sequence used,and some of their test objects are of varyingshape. Other authors (Behr et al. 1996)found that their study showed precious al-loys, being diamagnetic, did not causeartefacts, while non precious alloys, whichare often paramagnetic, may cause clin-ically relevant artefacts. Clearly, with the

24 | Clin. Oral Impl. Res. 14, 2003 / 18–27

Fig.14. A SPAMM sequence superimposed on a den-tal MRI scan, showing distortion around a steel or-thodontic bracket.

vast range of compositions of dental alloys,this potential problem must be kept inmind. As dental implant assessment is nor-mally performed in edentulous regions,most of these localised artefacts are not ofsignificance. Artefacts due to titanium im-plants have been reported as minor prob-lems (Devge et al. 1997) and, in our experi-ence, are of little significance. Indeed, theymay be due to minute fragments of dentalburs left during the preparation (Kanedaet al. 1998). Posterior composite fillingscontaining radiopaque materials have beenshown to exhibit magnetic susceptibility(Vikhoff et al. 1995). It is possible to studythe effect of materials on geometric distor-tion by using the spatial modulation ofmagnetisation (SPAMM) technique(Axel & Dougherty 1989). SPAMM createsa series of dark lines across the imagingplane (Fig.14), the distortion of which canbe used to show displacement in regionsof magnetic field inhomogeneity (Bridcutet al. 2001).

Unlike the susceptibility effect at an airinterface, the magnitude of which increasesin proportion to field strength, ferromag-netic effects are likely to be constant, as thematerials will be magnetically saturated atthe field strengths used in most scanners.Researchers need to be aware of the differ-ent field strength dependencies of the vari-ous mechanisms which give rise to artefactsand geometric distortion in MRI, and alsothat good protocol design and MR imagingsequence choice can significantly reducethese effects.

In addition to artefacts associated withthe patient, artefacts also arise because of

design shortcomings of the MR scannerhardware. For convenience, let us callthese machine artefacts. For instance, limi-tations in magnet design cause an inherentinhomogeneity in the main static mag-netic field. Furthermore, limitations in themagnetic field gradient coil design give riseto small variations in the gradient strengthacross the patient. Both these effects causegeometric distortion of the images pro-duced. Machine artefacts become morewith increasing distance from the centre ofthe field in preimplant imaging, as the re-gion of interest is normally close to thecentre of the field, so that any potential ef-fects are minimised.

The future

With ever greater accessibility to MRIscanners, reduction in operating costs, andan increased awareness of the advantagesof ionising radiation free sectional imaging,the use of MRI in the field of implant den-tistry should become established. In manyother medical examinations this is indeedthe case. Increased research into magnettechnology will in time allow us to developmore sophisticated systems, which will inturn allow faster and even more accuratescanners. Future interventional scannersmay allow us image guided implantsurgery. Sequential 3D scanning opens upthe use of MRI as a research tool, allowingus to gain a unique insight into the field ofbone and bone graft behaviour.

Conclusions

For safe and accurate placement of dentalimplants and associated surgical pro-cedures, accurate planning is essential. Sec-tional imaging is highly desirable, as evensimple implant placement after standardtwo-dimensional radiographic assessmentcan lead to a significant lack of understand-ing of the morphology of the site and as-sociated structures.

Whilst ionising radiation dose may besignificantly reduced with careful use ofCT and other X-ray tomograms, the totalabsence of radiation is a significant advan-tage of MRI. This, coupled with the flexi-bility of plane of acquisition, good softtissue detail, and the low level of imagingartefacts, mean that MRI should be con-sidered as a first choice for preimplant im-aging assessment. Furthermore, the ab-

Gray et al . MRI in implant dentistry

grafıa computarizada (CT) ha sido aceptada como el es-sence of ionising radiation allows MRI tobeZusammenfassung tandar de oro en la valoracion, pero la exposicion a radia-used for sequential postoperative research

ciones ionizantes puede ser sustancial. Los artefactos de-studies without exposing the patient to a bidos a restauraciones dentales pueden ser tambien signi-

Für eine gute und sichere Plazierung von dentalen Im-radiation hazard. ficativos y pueden existir algunas dudas acerca de laplantaten und für die Planung des dafür nötigen chirurgi-

exactitud de la CT-reformada. Las imagenes de resonan-schen Eingriffes ist eine komplette Auswertung der chir-

cia magnetica (MRI) no tienen exposicion a la radiacionAcknowledgements: Dr Gray would like urgisch relevanten Anatomie der prospektiven Implantat-ionizante y permite la adquisicion directa de informacion

stelle notwendig. Es ist daher höchst wünschenswert,to thank the Grampian Primary Care NHStomografica en cualquier plano deseado. Los estudios se-

Informationen von tomographischen SchnittaufnahmenTrust for financial support towards the cuenciales pueden ser llevados a cabo con seguridad, per-zur Verfügung zu haben, damit das Implantat in einer

writing of this paper. The authors would mitiendonos una valiosa vision al interior del comporta-korrekten Achsenrichtung gesetzt werden kann. In den

miento del injerto oseo. A parte de en un pequeno nume-like to thank the staff of the MRI units at vergangenen Jahren wurde die radiologische Computerto-ro de casos, la MRI puede ser usada con seguridad paraWoodend Hospital and Aberdeen Royal mographie als Goldstandard bei der Auswertung akzep-valoraciones prequirurgicas. Los artefactos son escasos y

tiert, aber die Exposition mit ionisierenden Strahlen kannInfirmary for their assistance. localizados en la mayorıa de los casos. La confianza qui-beträchtlich sein. Artefakte aufgrund von dentalen Re-

rurgica ganada es un paso significativo hacia delante enstaurationen können ebenfalls bedeutend sein und bezüg-

la colocacion segura de implantes dentales.lich der Genauigkeit der reformatierten CT können ge-Resumewisse Zweifel bestehen. Magnetische Resonanzbildge-bung (MRI) hat keine Exposition mit ionisierenden

Pour un placement sur et precis des implants dentaires, et Strahlen zur Folge und erlaubt die direkte Erlangung vonpour la planification de la chirurgie une connaissance pro- tomographischer Information in jeder gewünschten Ebe-fonde de l’anatomie chirurgicale complete du site s’avere ne. Sequentielle Studien können gefahrlos durchgeführtnecessaire. Il est donc tout a fait utile d’obtenir une infor- werden. Diese erlauben wertvolle Einsicht in das Verhal-mation par coupe tomographique pour permettre l’aligne- ten von Knochentransplantaten. Die MRI kann generellment parfait des implants. Depuis quelques annees, la to- gefahrlos für die prächirurgische Abklärung verwendetmographie par ordinateur aux RX (CT) reste la meilleure werden. Artefakte sind selten und in den meisten Fällenmethode pour evaluer la situation preoperatoire mais l’ex- lokalisiert. Die chirurgische Sicherheit durch die vonposition aux radiations peut atre importante, les artefacts Schnittbildern gewonnenen Informationen stellt einendus aux restaurations dentaires peuvent atre egalement bedeutenden Schritt vorwärts bei der sicheren Plazierungimportants et quelques doutes peuvent exister concernant von dentalen Implantaten dar.la precision de ces CT. L’imagerie par resonnance magneti-que (MRI) n’expose pas aux radiations ioniques et permetl’acquisition directe de l’information tomographique dans Resumentout plan. Des etudes sequentiels peuvent atre effectueessans problemes permettant ainsi de comprendre le com-portement osseux. Plus que dans un petit nombre de cas, Para una exacta y segura colocacion de implantes denta-MRI peut atre utilisee sans danger pour les etudes pre-chi- les y planificacion de la cirugıa asociada, se necesita unarurgicales. Les artefacts sont peu nombreux et dans la plu- completa valoracion de la anatomıa quirurgica del lugar.part des cas localises. La confiance chirurgicale de l’infor- Por ello, es altamente deseable disponer de informacionmation par section represente un pas significatif dans la se- tomografica seccional disponible para permitir la alinea-curite du placement des implants dentaires. cion correcta del implante. En los anos recientes, la tomo-

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