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.
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 13 13 9 8 18 18
T1 FLASH 2D FS
TR:275 TE:4 13 13 10 10 19 19
T2 MEDIC 2D FS
TR:2660 TE:27 11 11 4 4 18 18
T1 FLASH 3D FS
TR:60 TE:11 10 10 5 6 15 15
T2 DESS 3D
TR:23 TE:6 13 12 6 6 21 21
T2 CISS 3D
TR:10 TE:5 13 12 7 7 22 23
T1 SE
TR:1270 TE:14 13 14 12 12 22 23
T1 TSE
TR:2260 TE:14 15 15 12 12 24 24
T1 TSE
TR:600 TE:14 15 15 12 12 24 24
T1 SE
TR:600 TE:14 13 13 12 12 23 23
T1 SE FS
TR:684 TE:14 13 13 8
7 22 23
PD+ T2 TSE
TR:6110 TE:14 13 13 8 8 19 20
PD+ T2 TSE FS
TR:6760 TE:14 13 13 7 7 17 18
STIR
TR:10000 TE:38 13 13 10 10 19 19
TOF FISP 3D
TR:36 TE:4 12 12 3 4 19 18
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
ch
ne
gn
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
m
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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