Name: protocol for the evaluation of mri artifacts caused by metal implants to assess the suitability of implants and the vulnerability of pulse sequences
Uploaded: 2018-05-17T14:00:00
Description: Watch this Scientific Journal Video about Protocol for the Evaluation of MRI Artifacts Caused by Metal Implants to Assess the Suitability of Implants and the Vulnerability of Pulse Sequences at JoVE.c

The authors like to thank Stefanie Sauer, pharmacist at the Department of Pharmacy Heidelberg University Hospital, for her contributions to the MRI phantom. In addition, we would like to thank NORAS MRI products GmbH (H&#246;chberg, Germany) and especially Daniel Gareis for providing a prototype of the 16-channel multipurpose coil. Furthermore, we are grateful for the kind cooperation with SIEMENS Healthcare GmbH (Erlangen, Germany) and especially Mathias Nittka for their assistance in the sequence setup.

1. Phantom Preparation
<ol>
	<li>Determine the implant volume (e.g., by using the water displacement method). 
	NOTE: The volume of the CCT-T sample and the Z-T sample measured 0.65 mL and 0.73 mL, respectively.</li>
	<li>Fix the implant position in the middle of a non-ferromagnetic, plastic, waterproof box by using a thin thread. Use a box that is larger than the expected MRI artifacts. 
	NOTE: If no rough estimates of the artifact volumes of the implant and/or pulse sequence of interest are availab...

<ol>
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The number of patients with metallic implants and the number of MRI examinations is currently increasing1,2,3. In the past, MRI examinations were avoided after joint replacements. Today, however, MRI is not only requested for imaging such patients but should also allow for the evaluation of complications directly adjacent to joint arthroplasty. Thus, the MRI safety and MRI suitability of implants, as well as the robust pulse seq...

MRI has become an indispensable diagnostic tool. As a result, the number of MRI systems used in routine diagnostics is further increasing1. At the same time, the number of patients with implants is increasing as well2,3. In 2012, for instance, more than 1 million knee and joint replacements have been performed in the USA alone4. The prevalence of such implants was about 7 million in 2010, which corresponds to more than 10% of females in the age group 80-89 years5. As a result, the image quality and the diagnostic significance of MRI examinations are often impaired by artifacts due to metallic implants, resulting in a decreased diagnostic accuracy. Therefore, the MRI suitability of implants and the artifact vulnerability of pulse sequences are becoming increasingly important. Numerous approaches have been published to evaluate these characteristics. Due to strong discrepancies in the used evaluation methods, however, the respective results are hard to compare.
An evaluation of the MRI suitability of materials can be performed by calculating their magnetic susceptibility6. However, the vulnerability of different pulse sequences to artifacts cannot be compared with that approach for a given implant. Vice versa, the artifact volumes for a given pulse sequence can only be roughly estimated for different implants. In addition, the analysis is often performed with artificially shaped implants7,8. As the material volume and shape have an influence on the artifact size6, these features should be taken into account as well. As an alternative to magnetic susceptibility, the artifact size can be evaluated. Frequently, studies only rely on the qualitative evaluation of the artifact size9 or focus on the two-dimensional artifact size only covering one slice of the implant artifact10,11. Moreover, manual segmentation approaches are often used, which is not only time-consuming but also prone to intra- and inter-reader differences11. Finally, protocols often do not allow to test for non-fat-saturated and fat-saturated sequences at the same time12. This, however, would be desirable, since the applied fat suppression technique profoundly affects the artifact size.
Here, we present a protocol which allows for the reliable, semiautomatic, threshold-based, three-dimensional quantification of signal loss and pile-up artifacts of the entire implant, or all slices containing visible implant artifacts. Furthermore, it allows for testing T1- and T2-weighted images with or without fat-saturation. The protocol can be used to evaluate the MRI suitability of different implants or the vulnerability of different pulse sequences to metallic artifacts for a given implant.

<table border="0" cellpadding="0" cellspacing="0" width="638">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Aqua B. Braun Ecotainer</td>
			<td style="width:288px;">B. Braun Melsungen AG, Melsungen, Germany</td>
			<td style="width:136px;"></td>
			<td style="width:64px;"></td>
		</tr>
		<tr height="33">
			<td height="33" style="height:33px;width:151px;">Semisynthetic fat: Witepsol W25</td>
			<td style="width:288px;">Caelo Caesar &#38; Loretz GmbH, Hilder, Germany</td>
			<td style="width:136px;">4051</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Macrogol-8-stearate</td>
			<td style="width:288px;">Caelo Caesar &#38; Loretz GmbH, Hilder, Germany</td>
			<td style="width:136px;">3023</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Plastic box: not specified</td>
			<td style="width:288px;"></td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Implants: Nobel Replace</td>
			<td>Nobel Biocare, Z&#252;rich, Switzerland</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Water bath Haake S5P</td>
			<td style="width:288px;">Thermo Scientific, Waltham, MA, USA</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="52">
			<td height="52" style="height:52px;width:151px;">Measuring cylinder Blaubrand Eterna, Class A, Boro 3.3</td>
			<td style="width:288px;">BRAND GmbH + Co Kg, Wertheim, Germany</td>
			<td style="width:136px;">32708</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Coil: Variety</td>
			<td style="width:288px;">Noras MRI products GmbH, H&#246;chberg, Germany</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">MRI: Magnetom Trio</td>
			<td style="width:288px;">Siemens Healthcare GmbH, Erlangen, Germany</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:151px;">Postprocesing software: Amira 6.4</td>
			<td style="width:288px;">Thermo Scientific, Waltham, MA, USA</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
	</tbody>
</table>

protocol for the evaluation of mri artifacts caused by metal implants to assess the suitability of implants and the vulnerability of pulse sequences

As the number of magnetic resonance imaging (MRI) scanners and patients with medical implants is constantly growing, radiologists increasingly encounter metallic implant-related artifacts in MRI, resulting in reduced image quality. Therefore, the MRI suitability of implants in terms of artifact volume, as well as the development of pulse sequences to reduce image artifacts, are becoming more and more important. Here, we present a comprehensive protocol which allows for a standardized evaluation of the artifact volume of implants on MRI. Furthermore, this protocol can be used to analyze the vulnerability of different pulse sequences to artifacts. The proposed protocol can be applied to T1- and T2-weighted images with or without fat-suppression and all passive implants. Furthermore, the procedure enables the separate and three-dimensional identification of signal loss and pile-up artifacts. As previous investigations differed greatly in evaluation methods, the comparability of their results was limited. Thus, standardized measurements of MRI artifact volumes are necessary to provide better comparability. This may improve the development of the MRI suitability of implants and better pulse sequences to finally improve patient care.

With the above-mentioned protocol, we evaluated the artifact volume of 2 different dental implants made of Titanium (T; see the Table of Materials) supporting different crowns [porcelain-fused-to-metal non-precious alloy (CCT-T) and monolithic zirconia (Z-T); Figure 1b and 1c]. The CCT-T sample represents a highly paramagnetic material composition predicting large artifacts (Cobalt 61%, Chrome 21%, and Tungsten 11%; CCT). The...

Watch this Scientific Journal Video about Protocol for the Evaluation of MRI Artifacts Caused by Metal Implants to Assess the Suitability of Implants and the Vulnerability of Pulse Sequences at JoVE.c

Protocol for the Evaluation of MRI Artifacts Caused by Metal Implants to Assess the Suitability of Implants and the Vulnerability of Pulse Sequences

We describe a standardized method to evaluate magnetic resonance imaging artifacts caused by implants to estimate the suitability of the implants for magnetic resonance imaging and/or the vulnerability of different pulse sequences to metallic artifacts simultaneously.

As the number of magnetic resonance imaging (MRI) scanners and patients with medical implants is constantly growing, radiologists increasingly encounter ...

The overall goal of this method is to evaluate the suitability of implants for magnetic resonance imaging, or estimate the vulnerability of pulse sequences to metal artifacts. This method can help to answer key questions in the radiology field, such as the evaluation of MRI suitability of implants and the estimation of vulnerability of pulse sequences from metal artifacts. The main advantage of this methodology is that it allows for a three-dimensional artifact evaluation in fat-suppressed and not fat-suppressed T1 and T2 weightings at the same time.The implications of this technique extend towards diagnosis of peri-implant diseases because it allows for optimization of MRI protocols with regard to reduction of peri-implant artifacts. Before constructing the phantom for this protocol, first determine the volume of the implant to be studied. Here, water displacement method is used, and the volume of the samples measured 0.65 milliliters and 0.73 milliliters respectively.Select a non-ferromagnetic plastic waterproof box that is larger than the expected MRI artifacts, then fix the implant position in the middle of the box using a thin thread. Next, carefully melt a mixture of semi-synthetic fat, water, and macrogol-8-stearate using a water bath at 50 degrees Celsius. Here, a 500 milliliter mixture was used for the embedding of each sample.Once the mixture becomes fluid, turn off the heat and then start stirring it slowly. While doing this, ensure that there is no separation of the fat and water phases in the mixture. Then, as soon as clotting begins, slowly pour the embedding mixture into the phantom box with the implant, being sure to pour gradually to avoid air inclusion.Finally, place the phantom into a refrigerator at four degrees Celsius overnight for desiccation, and decant any remaining residual fluid the next day. For the MRI examination, use a radiofrequency coil that allows for a homogeneous signal distribution within the imaging volume without severe and obvious signal drops. Be sure to position the phantom with the embedded implant within the radio frequency coil in the same orientation as it would be placed in vivo.Position the middle of the phantom in the iso center of the MRI. Next, set up the imaging sequences at the MRI console, making sure that the phantom box, including some air at the edges of the box, is within the imaging volume. Then acquire the images that will be evaluated for artifacts.Export the images from the MRI console without compression preferably using DICOM format. Then, import The images into post processing software that allows for placing regions of interest, or ROIs, evaluating signal intensities, threshold based segmentation, and a quantification of segmented volumes. To define the threshold for pile-up artifacts, and check for homogeneous signal distribution, first use the segmentation editor to place perpendicular lines adjacent to the outer border of the visible artifact on the slice with the maximum artifact size.Then, place a background ROI 10 millimeters in diameter outside each of the four intersection points. Next, in the project view, you use the material statistics tool to measure the mean signal intensity and standard deviation of all voxels within these four background ROI values and for each background ROI separately. ensure that the mean signal intensity of each background ROI is within 1.5 standard deviations of the mean signal of each of the other three counterparts to guarantee a homogeneous signal distribution.Then, calculate the threshold for pile-up artifacts by adding three standard deviations of the background ROI to the mean signal intensity of all voxels of the four ROI values. Next, perform a semi-automatic threshold based segmentation of pile-up artifacts. Use the masking tool to visualize the predefined signal intensity range and restrict the segmentation to it, selecting all voxels with signal intensities greater than the threshold adjacent to the signal loss artifact.To define the threshold for signal loss artifacts, first place four ROIs in air containing regions 10 millimeter in diameter at the corners of the phantom box, then measure the mean signal intensity and standard deviation of all voxels within them. Now, place an ROI in the core of the signal loss artifact defined by the largest connected area of low signal intensities. Then manually increase the size of this ROI to the largest possible size as defined on the screen here.Finally, measure the mean signal intensity and standard deviation of this core ROI. Also, calculate the signal intensity threshold for signal loss artifacts as seen here. Then perform a semi-automatic threshold based segmentation of these artifacts by selecting all voxels connected to the core ROI with signal intensities below this threshold.Use the masking tool of the segmentation editor to visualize the predefined signal intensity range and restrict the segmentation to it. If possible, use the fill function in the segmentation editor to include all voxels that are not yet selected. Subtract the physical implant volume from the calculated artifact volume to obtain the true artifact volume, and repeat this analysis at least three times.A time interval of at least two weeks should separate these multiple reads to exclude any learning bias. Typical positioning of the regions of interest for measuring the thresholds for pile-up artifacts and signal distribution in signal loss artifacts is seen here. The blue contour represents the semi-automatic segmentation for signal loss artifacts.The small red areas correspond to the result of pile-up artifacts. Dental implants supporting different single crowns used in the phantoms are seen here. These graphs show the mean three-dimensional artifact volume for each implant sample with standard deviations after subtracting the physical implant volume.Four MRI sequences were evaluated. Here, renderings of the artifact volumes for a cobalt chromium tungsten titanium implant and a zirconia titanium implant are shown. The blue areas represent signal loss artifacts and the red areas represent pile-up artifacts.Below we see the colored source images for all evaluated T-2 weighted sequences. While attempting this procedure, it's important to choose identical sequence parameters as in the Invivo scan, to allow for accurate transferability of the in vitro results. After watching this video you should have a good understanding of how to evaluate signal loss and pile-up artifacts in a standardized way.This allows you to assess the MRI suitability of implants, or vulnerability of pulse sequences to metal artifacts.

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