Imaging characteristics of metallic interbody spacers: in vitro score evaluation of susceptibility artifacts considering different MRI sequences

doi:10.4236/jbise.2010.32023

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J. Biomedical Science and Engineering, 2010, 3, 181-186

doi:10.4236/jbise.2010.32023 Published Online February 2010 (http://www.SciRP.org/journal/jbise/

JBiSE

Published Online February 2010 in SciRes. http://www.scirp.org/journal/jbise

Imaging characteristics of metallic interbody spacers: in vitro

score evaluation of susceptibility artifacts considering

different MRI sequences

T. Ernstberger1,2, G. Heidrich3, G. Buchhorn4

1Clinic for Spinal Surgery, Klinikum Bad Bramstedt, Bad Bramstedt, Germany;

2Department of Orthopedic Surgery, University of Goettingen, Goettingen, Germany;

3Department of Diagnostic Radiology, University of Goettingen, Goettingen, Germany;

4Biomaterials Laboratory/Department of Orthopedic Surgery, University of Goettingenm, Goettingen, Germany.

Email: [email protected], [email protected]

Received 2 December 2009; revised 10 December 2009; accepted 14 December 2009.

ABSTRACT

Aim: Intervertebral spacers for anterior spine fusion

are made of different materials, such as titanium,

carbon or cobalt-chrome, which can affect the post-

fusion MRI scans. Implant-related susceptibility ar-

tifacts can decrease the quality of MRI scans, thwar-

ting proper evaluation. This cadaver study aimed to

demonstrate the extent that implant-related MRI

artifacting affects the post-fusion evaluation of in-

tervertebral spacers. Methods: In a cadaveric por-

cine spine, we evaluated the post-implantation MRI

scans of 2 metallic intervertebral spacers (TiAL6V4,

CoCrMo) that differed in shape, material, surface

qualities and implantation technique. A spacer made

of human cortical bone was used as a control. The

median sagittal MRI slice was divided into 12 regions

of interest (ROI). Results: No significant differences

were found on 15 different MRI sequences read in-

dependently by an interobserver-validated team of

specialists (P>0.05). Artifact-affected image quality

was rated on a score of 0-1-2. A maximum score of 24

points (100%) was possible. Turbo spin echo sequences

produced the best scores for all spacers and the con-

trol. Only the control achieved a score of 100%. The

titanium and cobalt-chrome spacers scored 62.5%

and 50%, respectively. Conclusions: Our scoring

system allowed us to create an implant-related rank-

ing of MRI scan quality in reference to the control

that was independent of artifact dimensions. Even

with turbo spin echo sequences, the susceptibility

artifacts produced by the metallic spacers showed a

high degree of variability. Despite optimum sequen-

cing, implant design and material are relevant fac-

tors in MRI artifacting.

Keywords: Intervertebral Spacers; Metallic Implant

Materials; MRI; Susceptibility Artifacts

1. INTRODUCTION

In the preoperative diagnostics of spinal diseases, mag-

netic resonance imaging (MRI) is used as a standard

procedure that can visualize disc pathologies and neuro-

logical changes of the spinal canal with high precision.

When anterior spine fusion proves indicated, implanta-

tion of intervertebral spacers often represents the treat-

ment of choice. The selection of implant design, implant

material and implantation technique is dictated by the

diagnostic findings. Intervertebral spacers made of vari-

ous materials can be used as stand-alone cages for ex-

clusively anterior fusion or in combination with dorsal

instrumentation for dorsoventral fusion.

When postoperative complications arise secondary to

vertebra fusion, MRI scans are frequently necessary to

evaluate implant position and demonstrate any clinically

relevant abnormalities and to direct further surgical deci-

sion-making [1]. Depending on the material, however,

implant-related susceptibility artifacts can decrease the

quality of MRI scans, thereby thwarting proper evaluation.

Depending on the problem to be clarified, consideration

must be given to whether the MRI sequence selected will

ensure the most artifact-free visualization and enable

proper evaluation of implant positioning and/or patho-

logical processes like tumorous growth or infection.

Recent studies have shown that artifacting, particularly

caused by metallic implants, can also be minimized

through modification of routine MRI sequences [2,3,4].

This cadaveric porcine study was conducted to determine

the extent to which implant-related MRI artifacting affects

the evaluation of intervertebral spacers. A scoring system

(0-1-2) was developed to rank the artifacting produced by

different intervertebral spacer designs compared with a

human cortical bone control. Scans taken with 15 different

MRI sequences were read independently by an inter-

observer-validated team of specialists who ranked image

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Figure 1. Intervertebrals disc spacers.

Figure 2. Human cortical bone.

quality of the implant, paying special attention to

neighboring structures. The scores are presented in ta-

bles and possible implant-related factors discussed.

2. MATERIAL AND METHODS

In this study, we performed MRI on 2 implanted metallic

intervertebral spacers (TiAl6V4, CoCrMo) (Figures 1(A)

and 1(B)) that differed in shape, material, surface quali-

ties and implantation technique. The spinal column of a

domestic pig killed for commercial human consumption

purchased from a slaughterhouse served as our experi-

mental cadaveric model. The 2 spacers were implanted

in the distal third of the thoracic spine and in the entire

lumbar spine. Additionally, a piece of human cortical

bone was implanted as a control (Figure 2).

2.1. Spacers and Control

The Intervertebral Body Spacer (IBS), manufactured by

Peter Brehm GmbH, Chirurgie Mechanik, Weisendorf,

Germany, is made of a titanium aluminum vanadium

alloy. This square implant has an evenly ribbed structure

on its upper and lower faces and an edge length of 25x25

mm. The implant used in this study had a maximum

height of 10 mm in the anterior segment with a dorsal

inclination of 7 degrees.

The cylindrically shaped intervertebral disc dowel

(IDD), manufactured by ESKA Implants GmbH & Co.,

Luebeck, Germany, is made of a cobalt chrome molyb-

denum alloy and its surface has a three-dimensional tri-

podal webbed structure. The size of the IDD used in this

study measured 35 mm in length and 15 mm in diameter.

The German trade name of this implant is Band-

scheibendübel.

2.2. Implantation

Like in the human spine, the size of the vertebrae in the

porcine spine increases in the craniocaudal direction,

with the lower lumbar vertebrae extending to the maxi-

mum dimensions of 25 mm in height, 25 in width and 20

mm in depth. The dimensions of the 3 study spacers and

the control were selected to be oversized compared to

the intervertebral disc space. The two devices were im-

planted as stand-alone cages. We refrained from the use

of dorsally implanted pedicle screws so as to avoid any

potential summation effects on artifact scoring caused by

additional materials.

A purely spinal model was chosen instead of a whole

pig cadaver, since the size of the clinical field of view

routinely focuses on the spine and cuts out any thoracic

or abdominal organ structures. During dissection, the

paravertebral muscles including the surrounding skin

and the psoas muscles of the spine were retained. Special

care was taken to ensure that the neurological structures

of the spinal canal remained intact.

To determine the distance at which the spacers should

be placed and to avoid artifact overlapping of spacers

implanted in a single spine, we conducted a preliminary

trial that involved embedding the cobalt-chrome

IDD–the spacer with the highest magnetization–in a

homogenous tissue mass and then performing MR im-

aging of the tissue-embedded spacer. The measurements

showed that a width of 6.5 cm had to be maintained be-

tween spacers to avoid the artifact overlapping.

Accordingly, the lumbar and thoracic disc spaces were

dissected to achieve a median positioning of the implants.

The paravertebral muscles were left intact along with the

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skin and psoas muscles. Intervertebral discectomy was

performed, and the three spacers and the control were im-

planted intervertebrally apart at a distance of 6.5 cm. Maxi-

mum implantation depth was reached when the implant

was aligned with the anterior vertebra face. After implanta-

tion was completed, additional tissue mass was padded

around the spine to optimize contrast and image quality.

3. MAGNETIC RESONANCE IMAGING

MRI was performed with a 1.5T MRI (Magnetom Sym-

phony, Siemens AG Medical Solutions, Erlangen, Ger-

many). Table 1 presents the MRI data. The median sag-

ittal MRI slice encompassing all relevant structures, im-

plants and control was evaluated according to an interob-

server-validated scoring system.

3.1. Scoring System

A 0-1-2 scoring system was established to rank the MRI

scans. An evaluation unit was defined as 2 adjacent ver-

tebrae encompassing the intervertebral disc space. 12

regions of interest (ROI) were demarcated (Figure 3).

Every ROI could achieve a maximum score of 2 points.

A total score of 24 points was equivalent to a score of

100%. Two board-certified specialists (one radiologist

(GH) and one spinal surgeon (TE)) experienced in read-

ing spinal MRI evaluated the scans independently of

each other. The evaluators scored regions as 0=not dis-

tinguishable, 1=partly distinguishable and 2=completely

distinguishable. The interobserver validation of the scor-

ing system across all 15 sequences was tested for statis-

tical significance using a t test with a significance level

of P>0.05 (Table 2).

4. RESULTS

Table 3 presents the total points scored for each implant

in each of the 15 sequences. Figures 4 I, II, III and IV

depict the artifact range in a selection of 4 MRI sequences.

Table 1. MRI sequence data.

Sequences FA TR TE ST BW FOV Number of slices Matrix

T1 FLASH 2D 70 181 4.8 5.5 260 500 19 256 x 256

T1 FLASH 2D FS 70 275 4.76 5.5 260 500 19 256 x 256

T2 MEDIC 2D FS 40 2660 27 3.0 70 500 40 256 x 256

T1 FLASH 3D 60 60 11 3.0 70 500 40 256 x 256

T2 DESS 3D 25 23.68 6.63 1.5 130 500 64 256 x 256

TOF FISP 3D 25 36 4.59 3.0 130 500 32 384 x 384

T2 CISS 3D 70 10.16 5.08 3.0 130 500 64 256 x 256

T1 TSE 150 2260 14 3.0 150 500 40 512 x 512

T1 TSE var 150 600 14 3.0 150 500 40 512 x 512

T1 SE 90 1270 14 3.0 90 500 40 512 x 512

T1 SE var 90 600 14 3.0 90 500 40 512 x 512

T1 SE FS var 90 684 14 3.0 90 500 40 512 x 512

T2 TSE/PD 150 6110 14 3.0 130 500 40 256 x 256

T2 TSE/PD FS 150 6760 14 3.0 130 500 40 256 x 256

STIR 180 10000 38 3.0 130 500 40 256 x 256

Key: FLASH=Fast Low Angle Shot, MEDIC=Multi Echo Data Image Combination, DESS=Dual Echo Steady State, FS=Fat

Saturated, FISP=Fast Imaging with Steady Precession, CISS=Constructive Interference in Steady State, SE=Spin Echo,

TSE=Turbo Spin Echo, PD=Proton density, STIR=Short Tau Inversion Recovery, TOF=Time of Flight, TR=Time of Repetition,

TE=Time of Echo, FA=Flip Angle, ST=Slab Thickness, BW=Band Width, FOV=Field of View, var=varied.

Interobserver validation across all 15 sequences.

Figure 3. MRI

Mean score value Standard deviation

Spinal

surgeon Radiologist Spinal

surgeon Radiologist

P value

IBS 12.87 12.8 +1.25 +1.32 P = 0.58

IDD 8.33 8.33 +3.02 +2.85 P = 1.0

Control 20.13 20.4 +2.67 +2.79 P = 0.1

Table 2.

evaluation unit with ROI.

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Table 3. Total scores.

MRI Sequence IBS IDD Control

1 2 1 2 1 2

T1 FLASH 2D

TR:181 TE:4

T1 FLASH 2D FS

TR:275 TE:4

T2 MEDIC 2D FS

TR:2660 TE:27

T1 FLASH 3D FS

TR:60 TE:11

T2 DESS 3D

TR:23 TE:6

T2 CISS 3D

TR:10 TE:5

T1 SE

TR:1270 TE:14

T1 TSE

TR:2260 TE:14

T1 TSE

TR:600 TE:14

T1 SE

TR:600 TE:14

T1 SE FS

TR:684 TE:14

PD+ T2 TSE

TR:6110 TE:14

PD+ T2 TSE FS

TR:6760 TE:14

STIR

TR:10000 TE:38

TOF FISP 3D

TR:36 TE:4

Key: 1) spinal surgeon; 2) radiologist.

Figure 4. I-IV: Artifact range of the different MRI sequences [I: T1 TSE (TR: 2260, TE: 14); II: T2 DESS 3D

(TR: 23, TE: 6); III: T1 FLASH 2D (TR: 181, TE: 4); IV: TOF FISP 3D (TR: 36, TE: 4)].

The results showed that the T1-TSE sequences produced

the best imaging scores for all implants. In these se-

quences, the human cortical bone control achieved the

maximum possible score of 100%, i.e. was completely

distinguishable (Figure 5, Table 3). Therefore, we used

these two sequences as a basis for following comparison

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of the imaging quality of the study implants.

4.1. IBS

In the T1-TSE sequences, the titanium IBS implant

achieved an imaging score of 62.5% compared to the con-

trol (Figure 4(IA), Table 3). The susceptibility artifact

border was clearly distinguishable from its surroundings.

As a result of artifact extension, the implant-bone contact

area was not distinguishable. The image quality was not

sufficient to determine exact implant position.

4.2. IDD

In the T1-TSE sequences, the IDD implant achieved an

imaging score of 50 % compared to the control (Figure 4

(IB), Table 3). As a result of artifacting, the implant-bone

contact area was not distinguishable. The distant vertebral

end plates were completely distinguishable; the anterior

edge of the lower vertebra and the spinous process were

partly distinguishable. As with the titanium IBS implant,

the image quality was not sufficient to determine exact

implant position in relation to the spinal canal.

4.3. Interobserver Validation

The results of the interobserver validation are listed in

Table 3. There was no statistical significance between the

evaluators with respect to t-test correlations (P>0.05).

5. DISCUSSIONS

The disadvantages associated with bone grafting alone

has led to the development of intervertebral spacers to

enhance anterior spinal fusion [5,6,7]. The use of in-

tervertebral spacers of different designs and materials has

thus become increasingly widespread in clinical routine

because they offering immediate load transmission with

direct primary stability. Post-fusion MRI scans are used

for further diagnostics to demonstrate any progressive

degenerative changes, infections, fractures and/or tumors.

However, implant-related susceptibility artifacts can

negatively impact the complex post-fusion evaluation

Figure 5. MRI slice human cortical bone (T1 TSE TR:2260

TE:14).

field gradient of varying susceptibility results in

of MRI scans. Depending on the spacer material, a local

magnetic

the area between structures. In these border areas, the re-

spective spins gyrate with different frequencies and cause

image distortions and susceptibility artifacts [8,9,10].

Our image quality scoring system gave special con-

sideration to the following material-related implant

aracteristics:

1) Distinguishability of implant shape and position;

2) Distinguishability of implant from anatomically

ighboring structures;

3) The extent of image distortions and susceptibility

artifacts.

Optimum MRI visualization of the different interver-

tebral spacers depends on the aim of diagnosis. MRI dia-

ostics are insofar subject to different requirements de-

pending on the various postoperative pathologies in rela-

tion to the implant situation. Optimum MRI image dis-

tinguishability of the intervertebral spacers and the

equivalent control was achieved using T1 TSE sequences.

The imaging quality of the human cortical bone used as a

control scored 100% according to the study scoring sys-

tem and was therefore used as a basis to rank the in-

tervertebral spacers examined. The scores were stated as

a percentage compared to the control. Our interobserver-

validated scoring system allowed us to create a unique

implant-related ranking of MRI scan quality in reference

to a control that was independent of artifact dimensions.

The MRI imaging behavior of metallic spinal implants is

well documented in the literature [9,10,11,12,13,14,15].

However, the aims of the published studies differed in

that most focused on determining sequence-related arti-

fact size.

According to our study results the implant position in

relation to the spinal canal was best visualized using T1

TSE sequences. In studies by Rudisch et al. [16] and

Thomsen et al. [17], titanium materials showed a lower

artifact range than cobalt chrome. Consistent with our

results, the best MRI quality was achieved for both me-

tallic spacers with T1 TSE sequences. The other MRI

sequences produced no further advantages.

Studies on metallic artifacts in MRI of the anterior

spine have been conducted by Vaccaro et al. [15] and

Wang et al. [18]. In one cadaveric study, Vaccaro et al.

[15] examined the MRI artifact rates of different metal

particles introduced in predefined intervertebral drill

holes and subsequently embedded in paraffin. Vacarro

could not demonstrate any significant artifacts in T1- or

T2 SE sequences, probably due to the fact that the parti-

cle density was lower than that produced by metal im-

plants commonly used in clinical practice. The metallic

artifacts appearing in the gradient echo sequences

proved a connection between artifact size and nickel

content of the alloys examined. An increasing nickel

content reduced susceptibility artifacting.

T. Ernstberger et al. / J. Biomedical Science and Engineering 3 (2010) 181-186

186

tebral spacer

s of the intervertebral spacers

spine fusion cause susceptibil-

., Parizel, P.M. and Jinkins, J.R. (2002)

RI of the postoperative lumbar s

rative study of MR imaging profile of titanium

rs, B.N. and Eisenstein, S.M. (1989) Donor site

) A carbon fiber

. and

eduction

mpp, S., Breitenseher, M.,

, Ebraheim, N.A., Savolaine, E.R. and Jackson,

, Lewin, J.S., Duerk, J.L., Yoo, J.U. and

hew, J.T. and

, A.R., Chesnut, R.M., Scuderi, G., Healy, J.F.,

neider, U., Breusch, S.J., Hansmann, J.

In another cadaveric artifact study, Wang et al. [18]

described the MRI behavior of an interver

JBiSE

ade of titanium. Using T1 SE sequences, the implant-

related artifact rate of the titanium spacer was primarily

limited to the implant’s direct surroundings and anat-

omic neighboring structures were clearly distinguisha-

bility. In our study, when T1 TSE sequences were used

to image both metallic spacers, neither implant shape nor

implant position could be distinguished with certainty. In

a phantom study by Rudisch et al. [16], the relevance of

metallic artifacts and implant-related characteristics,

such as implant material, shape and position, was dem-

onstrated in addition to an impact by the selected MRI

sequence. In spite of the use of optimum MRI sequences,

variability in the amount of susceptibility artifacts must

be accounted for when evaluating MRI scans of metallic

spine implants.

6. CONCLUSIONS

of a

The designs and material

currently used in anterior

ity artifacts that can be rated by validated scoring sys-

tems. Of 15 sequences tested, T1 TSE sequences pro-

duced the best spacer imaging for both metallic implants

tested. An interobserver-validated scoring system proved

effective in ranking the relevance of spacer material on

MRI imaging quality. Studies are ongoing to further de-

velop MRI scoring systems and establish optimum im-

aging sequences for post-fusion diagnostics.

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