Key words
prostheses - imaging sequences - femur
Introduction
Total hip arthroplasty (THA) is a common treatment for osteoarthritis [1]. Some of the issues occurring in patients with total hip prostheses (THP) are osteolysis,
dislocation, granulomatous disease, loosening and infection. Nowadays, it is feasible
to use magnetic resonance imaging (MRI) to investigate these issues [2]. MRI offers the advantage of soft tissue contrast, but it is subjected to magnetic
susceptibility artifacts in the presence of metal [3].
Conventional MRI sequences can be optimized to reduce susceptibility-related image
artifacts [4] and allow hip imaging even in the presence of metal prostheses, but are still subjected
to considerable distortions, such as “pile-up” artifacts and signal voids [2]
[5]. Recently, however, novel MRI sequences have been introduced, such as view angle
tilting (VAT) [6], multiple acquisition with variable resonance image combination (MAVRIC) [7], single-point imaging [8] and slice encoding for metal artifact correction (SEMAC) sequences [9] that overcome some of these limitations [2]. VAT is used to correct for in-plane geometric distortions by repeating the slice
selection gradient during the signal readout, but suffers from geometric through-plane
distortion and blurring effects. In contrast, SEMAC is a multi-slice 2 D TSE sequence
that uses both VAT and additional variable slice encoding steps (SES) to correct through-plane
artifacts and improve the visualization of anatomical details [10]. Various studies have been performed for the evaluation of SEMAC on reducing susceptibility
artifacts in phantoms and in patients [10]
[11]
[12]
[13]
[14]
[15]. Objectively and consistently comparing the efficiency of artifact reduction is
an exacting process, since the metal-related artifact form and type can be very irregular
[10]
[14]
[15]
[16].
Generally, SEMAC offers a considerable reduction of metal-related image artifacts,
but requires a considerable increase in scan time – typically diametral to clinical
workflow optimizations. Whereas, in principle, a huge number of SES may completely
resolve the material-related artifacts issue, the resulting scan time requirements
will not be in line with clinical demands. Consequently, a simple tradeoff between
prohibitively long scan times and clinically useful images is to use the highest possible
SES depending on the predetermined available scan time within the scan protocol. However,
in practice there is a wide range of prostheses and materials related to how recent
the prosthesis model is. The overall requisite scan time is highly dependent on the
type of implant that is present and on the desired degree of image artifact reduction.
Depending on the prosthesis material, a larger or smaller number of SES would be sufficient
for SEMAC metal artifact reduction. Therefore, several studies have recently taken
into account the influence of the implant material on the efficiency of different
artifact suppression methods: a) SE, GRE and high bandwidth TSE [14]
[17], b) MAVRIC [16] and c) SEMAC [13]
[14].
Månson et al. highlighted that a proper number of SES is critical for the performance
of SEMAC and also that the material of a prosthesis is a decisive parameter affecting
the performance of an artifact reduction technique [13]. However, how the efficiency of the sequence changes for a continuous range of SES
and whether it changes for different types of image contrast (T1-w versus fat suppression) are not investigated. In this study, we evaluated residual
metal-related image artifacts for a T1-w and a short-tau inversion recovery (STIR) SEMAC protocol on five of the most common
total hip prostheses ([Fig. 1]) as a function of the number of SES and thus of the total scan time. The least distorted
image, which was acquired with a very high number of SES, was used as a control reference.
The aim of this evaluation was to propose a guideline for the minimum number of SEMAC
SES steps required to provide adequate, i. e. diagnostically relevant, image quality.
Fig. 1 Head and stem pictures of the five implants used in the present study. Prostheses
were placed on plastic stands in order to be above the bottom of the container and
surrounded by gel.
Abb. 1 Bilder des Kopfs und des Schafts der fünf Implantate, die in dieser Studie verwendet
wurden. Die Prothesen wurden auf Sockeln aus Kunststoff über dem Boden des Behälters
platziert und von Gel umgeben.
Materials and Methods
Hip implant models
Five of the most commonly used total hip prostheses (stem and respective cup, [Fig. 1]) were immersed in agar gel (1 % agar) doped with Gadolinium contrast agent to mimic
muscle tissue properties (at 1.5 T T1~ 1000 ms, T2 ~ 40 ms [18]). Special 3D-printed plastic bases were constructed to hold the material during
gelification.
The stems are listed here with decreasing magnetic susceptibility:
-
Stainless steel (magnetic susceptibility (χ) 3520 – 67 000 ppm [19])
-
Old Chanley prosthesis (Exeter stem™ Universal hip, Stryker Inc., Newbury, UK) is
made of stainless steel for cemented fixation in total hip replacement. It is highly
polished and it is an implant usually encountered in older patients.
-
MS-30 stem (Zimmer®) is a stainless steel, highly polished, straight, three-dimensionally tapered, collarless
implant for cemented fixation in total hip replacement. It is typically combined with
a titanium acetabular cup.
-
Cobalt-chromium alloys (χ = 900 – 1370 ppm [19])
-
Müller type- CoNiCrMo alloy (Protasul®-10) has a flat profile cap and is used with a cemented fixation.
-
Titanium (χ = ~182 ppm [19])
-
A CLS® Spotorno® socket (Zimmer®) is combined with a fitmore stem (a grit-blasted titanium alloy with wedge shape).
It is a cementless stem with a three-dimensional wedge shape and sharpened ribs in
the proximal region. It is typically combined with a titanium acetabular cup.
-
S-Rom femoral component is a proximally modular revision prosthesis (DePuy Johnson
& Johnson). The S-Rom is a cementless femoral prosthesis consisting of a titanium
stem. It is typically combined with a titanium acetabular cup.
MR imaging
MRI was performed on a 1.5 T clinical scanner (Magnetom Avanto, Siemens Medical Solutions,
Erlangen, Germany). SEMAC (WARP TSE; Siemens Healthcare) was combined with high bandwidth
radiofrequency pulses and increased readout bandwidth was used for the experiments.
SEMAC also imbeds VAT for correction of in-plane distortion and additional ‘slice’
phase encoding steps (SES) in the slice direction for the correction of through-plane
distortion. A six element body array was used for scanning. Coronal planes were acquired
(i. e., parallel to the longest axis of the stem) since they include both the stem
and the head of the implant, giving a more complete image of the prosthesis surroundings
and presenting higher clinical interest. Moreover, coronal slices require a large
field-of-view and are thus more time-demanding.
Scout images were acquired with the SEMAC sequence with 4 SES, resolution = 2.2 × 2.2 × 3.0 mm3, FOV = 223 × 280 mm2, flip angle = 140°, repetition time (TR) of 700 ms, echo time (TE) of 4.7 ms, parallel
imaging factor (iPat) = 3, scan time = 1.05 min.
Coronal T1-w and fluid-sensitive STIR-SEMAC images were acquired with a variable number of SES. For
the SEMAC T1-w protocol, the following parameters were used: TR = 700 ms, TE = 4.9 ms, flip angle = 130°,
in-plane resolution = 0.88 mm, slice thickness = 3.5 mm, matrix size = 224 × 280,
bandwidth per pixel = 781 Hz, phase oversampling of 80 %, echo train length (ETL) = 23,
turbo factor (TF) = 7. Parallel imaging (GRAPPA with iPAT = 3) was used to speed up
the acquisition for SEMAC. For STIR-SEMAC imaging, the parameters were: TR = 5000 ms,
TE = 35 ms, flip angle = 150°, in-plane resolution = 0.94 mm, slice thickness = 3 mm,
matrix size = 224 × 302. A number of SES from 2 to 23 was investigated depending on
the susceptibility of every material (i. e., higher SES numbers for materials with
stronger susceptibility, [Table 1]). The range of SES was defined based on the results presented by Månson et al. [13]. For every implant an SES was selected as the minimum number and an optimal number
of SES was defined from the analysis. The minimum number of SES was the minimum number
of SES used for the measurements of a certain implant.
Table 1
Total scan times for the protocols used in the study for the individual materials.
Tab. 1 Messzeit der in der Studie für die einzelnen Materialien verwendeten Protokolle.
|
|
T1-w SEMAC
|
STIR SEMAC
|
|
SES
|
scan time (min)
|
scan time (min)
|
|
minimum
|
maximum
|
minimum
|
maximum
|
|
1 – Exeter
2 – MS-30
3 – Müller
4 – Fitmore
5 – revision
|
10 – 17
6 – 16
7 – 14
3 – 8
2 – 6
|
5.23
3.15
3.47
1.38
1.06
|
9.09
8.37
7.32
4.19
3.15
|
5.46
3.25
4.01
1.40
1.35
|
9.52
9.17
8.07
4.36
3.25
|
|
reference
|
23
|
8.46
|
11.27
|
Quantitative analysis
In order to estimate the amount of artifact reduction as a function of SES, reference
SEMAC images (Iref) were acquired for every prosthesis material in the limit of a large number of SES
(from scan time restrictions, 23 SES were considered as a reference SES). Then a series
of SEMAC scans with various numbers of SES was performed (ISES). From this, relative signal difference images, SSES, were calculated pixel-wise according to
The amount of hyperintense pixels M in the difference image (i. e., pixels with intensity approximately 5 times above
the noise) was chosen as an indicator for the quality of artifact suppression:
M:={(x, y)∈I|SSES>thres}(2)
Finally, the volume of the distortion was calculated from
ΔVol:=ord(M)×volpixel(3)
The SES that was required to achieve a distortion below a certain threshold (i. e.,
ΔVol below 300 mm3) was defined as the optimal number of SES.
Results
Coronal T1-w and STIR SEMAC images ISES were acquired for every implant, with a different initial SES depending on the material,
until a stable artifact reduction level was achieved. The scan time increase versus
the number of SES is depicted in [Fig. 2]. Reference images Iref were also acquired from every phantom ([Fig. 3]). Initially, the T1-w SEMAC images and subsequently the STIR images were analyzed in order to determine
a recommended number of SES for every material.
Fig. 2 Total scan time of the T1-w (dotted) and STIR (solid) SEMAC protocols used in the study as a function of the
number of slice encoding steps (SES).
Abb. 2 Messzeit der in der Studie verwendeten T1-w (gepunktete Linie) und STIR (durchgezogene Linie) SEMAC-Protokolle als Funktion der
Anzahl an Schichtkodierungsschritten (SES).
Fig. 3 Magnitude A T1-w and B STIR SEMAC images acquired with the reference number of steps (SES = 23) for THP1 – 5.
Abb. 3 Magnitude-Bilder der A T1-w und B STIR SEMAC-Messungen mit der Referenzanzahl an Schichtkodierungsschritten (SES = 23)
für THP1 – 5.
SEMAC: T1-w images
Distortion volumes for all five sets of prostheses as a function of SES are given
in [Fig. 4]. As a result of the graphs, the optimal number of SES was established. The decreasing
magnetic susceptibility of the materials (i. e., THP1 has the highest magnetic susceptibility
and THP5 the lowest) is reflected in two observations: a) the shift of the curves
towards the left, b) the lower optimal number of SES (i. e., SES required for decrease
of the distortion volume below the threshold). The comparison of different protocols
showed that for every material there is a point at which the volume of distortion
decreases below the threshold of 300 mm3 (i. e., reflecting that the distortion in a certain image becomes the same as in
the reference image) and the use of more slice encoding steps brings no considerable
reduction of the signal void.
Fig. 4 (left) T1-w SEMAC distortion volume (in mm3) as a function of the number of slice encoding steps (SES) for the five measured
prostheses. (right) Relative difference images (SESi-SESref) / SESref acquired with (i) minimum SES that was measured (red circle) and (ii) suggested SES
that reduces the artifact below 300 mm3 (green circle).
Abb. 4 (links) T1-w SEMAC-Verzerrungsvolumen (in mm3) in Abhängigkeit von der Anzahl der Schichtkodierungsschritte (SES) für die fünf
gemessenen Prothesen. (rechts) Relative Differenzbilder (SESi-SESref) / SESref berechnet für (i) die minimale gemessene Anzahl an SES (roter Kreis) und (ii) die
empfohlene Anzahl an SES, welche die Artefakte auf unter 300 mm3 (grüner Kreis) reduziert.
a) Stainless steel-based implants (THP1 & THP2)
For THP1 (Exeter), which shows the most prominent susceptibility artifacts, the volume
of the distortion is decreased below 300 mm3 for SES above 13. For THP2, which is also made of stainless steel, the volume decreases
below the threshold for a lower SES [11] and the curve is slightly shifted to the left. Therefore, it is concluded that depending
on the alloy, stainless steel prostheses can have a slightly different behavior in
the presence of a magnetic field. A number of SES of at least 11 and preferably higher
than 13 is recommended in both cases. The respective T1-w images for the minimum and optimal number of SES are presented on [Fig. 4]. For a very small number of SES (10 for THP1 and 6 THP2, [Fig. 4]), there is important signal distortion that is mainly centered in the area of the
neck of the stem. For the optimal number of SES (i. e., 13 for THP1 and 11 for THP2),
it can be observed that in both cases the relative difference images have minimum
hyperintense regions and therefore above this level there is only a very small gain
using a higher number of SES.
b) Cobalt-chromium-based implant (THP3)
In this case the artifact reduces considerably in images acquired with a number of
SES greater than or equal to 9 ([Fig. 4]). For an SES number above 9, the curve is practically flat and the distortion is
almost identical to the distortion in the reference image. From a qualitative evaluation
of the images, it can be seen that the relative difference image has only a very small
hyperintense region close to the top of the stem and there is no considerable gain
in increasing the number of SES above this limit.
c) Titanium-based implants (THP4 & THP5)
For THP4 and THP5 (titanium stems), T1-w SEMAC images were acquired with a lower number of steps. For the THP4 and THP5, both
from a quantitative and qualitative evaluation of the images ([Fig. 4]), it can be observed that the geometric distortion is already very restricted, when
using 3 and 5 SES, respectively. Therefore, for titanium-based alloys it can be considered
that it is not profitable to acquire images with more than 5 SES.
SEMAC: STIR images
The results are almost identical ([Fig. 5]) for the STIR SEMAC images with very small differences. The curves show similar
steepness and limits of SES that effectively minimize the volume of the distortion.
The volume of distortion is higher in the T1-w images only for THP2. However, the steepness of the curves is comparable and in both
cases the distortion volume for a number of SES above 10 decreases below 300 mm3. The recommendations for SES therefore can be considered the same.
Fig. 5 (left) STIR SEMAC distortion volume (in mm3) as a function of the number of slice encoding steps (SES) for the five measured
prostheses. (right). Relative difference images (SESi-SESref) / SESref acquired with (i) minimum SES that was measured (red circle) and (ii) suggested SES
that reduces the artifact below 300 mm3 (green circle).
Abb. 5 (links) STIR SEMAC-Verzerrungsvolumen (in mm3) in Abhängigkeit von der Anzahl der Schichtkodierungsschritte (SES) für die fünf
gemessenen Prothesen. (rechts) Relative Differenzbilder (SESi-SESref) / SESref berechnet für (i) die minimale gemessene Anzahl an SES (roter Kreis) und (ii) die
empfohlene Anzahl an SES, welche die Artefakte auf unter 300 mm3 (grüner Kreis) reduziert.
Discussion
THA is performed as a treatment for degenerative joint disease with many possible
complications, such as loosening and infection [1]. MRI is an important tool for the diagnosis of these complications, but suffers
from pronounced magnetic susceptibility artifacts in the presence of metal. Recent
advances in sequence development, however, can provide diagnostic images even in the
presence of hip prostheses [2]
[6]
[7]
[9]
[11]
[20]. These techniques allow improved image quality by a considerable reduction of the
artifact volume at the expense of prolonged scan times. The range of materials for
hip prostheses is very wide, depending also on the age of the patient. As a result,
the implant material is usually unknown, the imaging protocols are not optimized based
on the material and clinical studies do not differentiate results based on prostheses
models.
Recently, there has been an increased interest in studying the influence of implant
material on the performance of an artifact reduction sequence [13]
[14]
[16]
[17]. Månson et al. presented a method for evaluating the performance of different techniques
using SEMAC, VAT and T1-w TSE sequences with different prostheses. They studied the performance of TSE, VAT
and SEMAC sequences with three different prostheses. We extended the results of this
study to include an evaluation of a continuous range of SES and two different types
of contrast for the SEMAC protocols (T1-w and STIR). The contrast type did not have a pronounced effect on the optimal number
of SES. In addition, in this study five different implants were investigated.
The materials of the implants could be categorized into some general groups (i. e.,
titanium-based, stainless steel-based, etc.). Based on the extent of image distortions
for the five examined implants, three different groups of implant categories were
identified:
-
Strong susceptibility materials (e. g. stainless steel): They create very strong susceptibility artifacts and can often be encountered in
older patients. On SEMAC images, these implants show a considerable artifact around
the area of the neck of the stem, which extends far beyond the vicinity of the implant,
and it is recommended to use an SES number equal to 13.
-
Medium susceptibility materials (e. g. CoNiCrMo): In the SEMAC images, geometric distortion is effectively minimized for an SES number
equal to 9.
-
Low magnetic susceptibility (e. g. titanium): They are more modern or often replacement prostheses. These implants can in principle
already be visualized with optimized high bandwidth TSE protocols and it is sufficient
to use 5 SES.
Although no external volume reference such as X-ray [13] was used at this point, a comparison with a protocol using a very high SES number
gave a clear evaluation of the protocol efficiency. At present, only in vitro experiments
were performed and our results need to be validated in vivo. Another limitation might
arise from the image orientation. Here, we focused on coronal protocols (i. e., parallel
to the longest axis of the stem), which is the plane with the highest clinical interest.
Nevertheless, it would be important to confirm whether the same guidelines are true
for different image planes in order to establish comprehensive clinical recommendations.
Conclusion
In a standard clinical setting the clinician may not know the exact implant of a patient
at the time of examination. However, optimization of the metal artifact reduction
protocol based on the implant material can lead to a more time-efficient scan with
better image quality. In this study, we established scan time recommended SES for
five different total hip prostheses.
Clinical Relevance of the Study
-
The results could serve as a guideline in order to trade acquisition time against
diagnostic image quality depending on the material that is present.
-
A minimum number of slice encoding steps (5 SES) allows adequate image quality of
titanium prostheses.
-
For non-titanium-based implants that cause medium distortion such as cobalt-chromium-based
alloys, more SES are needed (9 SES) to minimize metal artifacts.
-
For older prostheses made out of stronger metals, a higher number of SES (13 SES)
is beneficial in order to effectively reduce the area of the artifacts.
