**2.3 CT vs. tomosynthesis**

Imaging by X-ray CT has improved over the last three decades and is now a powerful tool in medical diagnostics. CT imaging has become an essential noninvasive imaging technique since the advent of spiral CT imaging in the 1990s, which led to shorter scan times and improved 3D spatial resolution. CT provides high resolution in the tomographic plane but limited resolution in the axial direction. However, the quality of images generated by a CT scanner can still be reduced due to the presence of metal objects in the field of view. Imaging of patients with metal implants, such as marker pins, dental fillings, or hip prostheses, is susceptible to artifacts generally in the form of bright and dark streaks, cupping and capping, etc. This artifact susceptibility is mostly due to quantum noise, scattered radiation, and beam hardening (Hsieh 1995). Metal artifacts influence image quality by reducing contrast and by obscuring details, thus impairing the ability to detect structures of interest and possibly leading to misdiagnosis. In addition, CT values are impaired, which can lead to errors when using these data e.g., for attenuation correction in positron emission tomography (PET)/CT imaging (Kamel et al 2003). The metallic components of arthroplasty devices are high-contrast objects that generate artifacts when imaged using CT scans. These artifacts can make it extremely difficult or impossible to interpret images obtained by devices. The presence of artifacts along with partial volume effect (PVE) severely limits the potential for objective quantification of total joint replacement with CT.

Fig. 4. Patient (52-year-old woman; total hip arthroplasty; THA). Anteroposterior (AP) radiographs of the hip joint prostheses are demonstrated.

Imaging by X-ray CT has improved over the last three decades and is now a powerful tool in medical diagnostics. CT imaging has become an essential noninvasive imaging technique since the advent of spiral CT imaging in the 1990s, which led to shorter scan times and improved 3D spatial resolution. CT provides high resolution in the tomographic plane but limited resolution in the axial direction. However, the quality of images generated by a CT scanner can still be reduced due to the presence of metal objects in the field of view. Imaging of patients with metal implants, such as marker pins, dental fillings, or hip prostheses, is susceptible to artifacts generally in the form of bright and dark streaks, cupping and capping, etc. This artifact susceptibility is mostly due to quantum noise, scattered radiation, and beam hardening (Hsieh 1995). Metal artifacts influence image quality by reducing contrast and by obscuring details, thus impairing the ability to detect structures of interest and possibly leading to misdiagnosis. In addition, CT values are impaired, which can lead to errors when using these data e.g., for attenuation correction in positron emission tomography (PET)/CT imaging (Kamel et al 2003). The metallic components of arthroplasty devices are high-contrast objects that generate artifacts when imaged using CT scans. These artifacts can make it extremely difficult or impossible to interpret images obtained by devices. The presence of artifacts along with partial volume effect (PVE) severely limits the

potential for objective quantification of total joint replacement with CT.

Fig. 4. Patient (52-year-old woman; total hip arthroplasty; THA). Anteroposterior (AP)

radiographs of the hip joint prostheses are demonstrated.

**2.3 CT vs. tomosynthesis** 

Methods for reduction of metal artifacts aim to improve the quality of images affected by these artifacts. In recent years, modified iterative (Wang et al 1996 , 1999 , 2000 , Man et al 2000) or wavelet reconstruction techniques have produced promising results. However, these methods cannot be combined with the fast and robust FBP algorithm, which is the standard reconstruction technique (Robertson et al 1997) implemented in modern CT scanners.

Digital linear tomosynthesis using the FBP algorithm shows adequate overall performance, but its effectiveness depends strongly on the region of the image. Digital linear tomosynthesis using FBP algorithm images gives good results independent of the type of metal present in the patient and shows good results for the removal of noise artifacts, especially at greater distances from metal objects. Application of digital linear tomosynthesis to the imaging of hip prostheses appears promising. In addition, flexibility in the choice of digital linear tomosynthesis imaging parameters based on the desired final images and generation of high quality images may be beneficial.

In Fig.4, Coronal slice images (multi-planar reconstruction; MPR) of the hip prosthesis at center heights on metal artifact reduction (MAR) CT (MAR-CT) and non MAR-CT scans at approximately the same level. Remarkable metal artifacts can be seen occurring in the neighborhood of the hip prosthesis. However, MAR-CT processing reduced the metal artifacts. Tomosynthesis images of the prostheses at center heights at the same level. The new diagnostic information that could not be acquired from CT images is provided. Reduction in metal artifacts was obtained in the images as shown here. The use of tomosynthesis allowed better visualization of the prosthesis caused by the blurring of anatomic structures above and below the visualized planes.

The FBP tomosynthesis was compared to MAR CT, and non-MAR CT scans of a prosthesis case. The effectiveness of this method in enhancing visibility of a prosthesis case was quantified in terms of the signal-to-noise ratio (SNR), and removal of ghosting artifacts in a prosthesis case was quantified in terms of the artifact spread function (ASF). The SNR in the

prosthetic case was determined. The SNR is defined as 1 0 0 *N N* , where *N1* is the mean

pixel value in the region of interest (ROI) within the object, *N0* is the mean pixel value in the ROI in a background area, and 0 is the standard deviation of pixel values in the background ROI. Throughout these results, 0 includes structure noise that can obscure the object, in addition to photon statistics and electronic noise. Wu et al. proposed an ASF metric to quantify artifacts observed in planes outside the focus image plane (Wu et al 2003). The artifacts are generated from real features located in the focus image plane, and resemble the real feature. The artifacts exhibited in image planes are defined by the ASF as

0 0 () () () () *artifact BG artifact BG N zN z N z Nz* , where *z0* is the location of the in-focus plane of the real feature, *z* is

the location of the off-focus plane, and *Nartifact* (*z0*) and *NBG* (*z0*) are the average pixel intensities of the feature and the image background in the in-focus plane, respectively, *Nartifact*(*z*) and *NBG* (*z*) are the average pixel intensities of the artifact and the image background in the off-focus plane, respectively.

The effectiveness of this method in enhancing visibility of a prosthesis case was quantified in terms of the SNR, and removal of ghosting artifacts in a prosthesis case was quantified in terms of the ASF. In the near in-focus plane, the contrast is greater in the MAR CT or FBP tomosynthesis relative to the non-MAR CT (Fig.5).

X-Ray Digital Linear Tomosynthesis Imaging of Arthoroplasty 101

narrow angular range typically employed in tomosynthesis imaging. Gomi et al. (Gomi et al 2008 , 2009) developed artifact reduction methods based on a modified Shepp–Logan reconstruction filter kernel by taking into account the additional weight of direct current components in the frequency domain space. Processing leads to an increase in the ratio of low frequency components in an image (see appendix). Artifact reduction processing was performed with a basic and FBP algorithm. Artifact reduction processing provides a method of filtering that can be used in combination with the backprojection algorithm to yield sliced

Fig. 6. Comparison of images obtained from artifact reduction tomosynthesis (W= 0.06), FBP tomosynthesis, SAA tomosynthesis, and maximum likelihood tomosynthesis (ML, four subsets & 15 iterations) of the center plane. Artifact reduction tomosynthesis provided better visualization of the hip prosthesis by eliminating blurring and reducing artifacts above and

The quality of CT images is governed by the strength of artifacts, which depends on numerous factors such as size, shape, density, atomic number and position of metal objects, patient size, and patient's cross-section shape. For small implants manufactured from relatively light metals (e.g., titanium), the effects of beam-hardening and scattering are low. Therefore, the corrupted CT values as well as noise-induced streaking artifacts that pose a major problem to image quality can be neglected. In such cases, the digital linear tomosynthesis approach to artifact reduction processing appears to be promising for the reduction of artifacts stemming from metals with a relatively high atomic number (Fig.6). Improvement of an artificial image (part of undershooting) by an artifact reduction processing is accepted (Fig.7). The effectiveness of the artifact reduction processing method in enhancing the visibility of a prosthetic case was quantified in terms of removal of artifacts. The potential of artifact reduction processing for digital linear tomosynthesis in the evaluation of hip prostheseswas demonstrated. Artifact reduction processing tomosynthesis realize that an especially normal bone and adhesion of an artificial bone

Ideally, structures in a given plane of interest should be clearly displayed in the corresponding tomosynthesis reconstruction plane, whereas structures located outside of that plane should not be visible. Essentially, the limited angular range of the tomosynthesis image acquisition geometry dictates that the spatial resolution is limited in the dimension perpendicular to the detector plane. As a result, out-of-plane structures cannot be

images with desired properties by means of tomosynthesis.

below visualized planes.

become observable (Fig.8-9).

**2.5 Potential artifacts** 

Fig. 5. Metal artefact reduction computed tomography (MAR-CT) with in-focus plane is best for signal-to-noise ratio (SNR) optimization, whereas MAR-CT with off-focus plane is best for SNR optimization. It seems that number of the projection, total exposure dose, and pixel size of detector in MAR-CT gives better results than FBP tomosynthesis. Artifact spread function (ASF) chart demonstrates that FBP tomosynthesis results in maximum removal of metal artifacts.

### **2.4 Evaluation of artifact reduction for prosthesis imaging**

Metal artifacts influence image quality by reducing contrast and obscuring detail, thus impairing the ability to detect structures of interest and making diagnosis impossible. The objective of this report is to evaluate the clinical application of digital linear tomosynthesis in imaging a phantom and hip prosthesis using a relatively new tomosynthesis instrument and applying a selection of reconstruction algorithms. Tomosynthesis images were compared with the results from artifact reduction processing and a FBP algorithm.

Artifacts caused by high-attenuation features in hip prostheses were observed in digital linear tomosynthesis reconstruction as a result of the small number of projections and

Fig. 5. Metal artefact reduction computed tomography (MAR-CT) with in-focus plane is best for signal-to-noise ratio (SNR) optimization, whereas MAR-CT with off-focus plane is best for SNR optimization. It seems that number of the projection, total exposure dose, and pixel size of detector in MAR-CT gives better results than FBP tomosynthesis. Artifact spread function (ASF) chart demonstrates that FBP tomosynthesis results in maximum removal of

Metal artifacts influence image quality by reducing contrast and obscuring detail, thus impairing the ability to detect structures of interest and making diagnosis impossible. The objective of this report is to evaluate the clinical application of digital linear tomosynthesis in imaging a phantom and hip prosthesis using a relatively new tomosynthesis instrument and applying a selection of reconstruction algorithms. Tomosynthesis images were

Artifacts caused by high-attenuation features in hip prostheses were observed in digital linear tomosynthesis reconstruction as a result of the small number of projections and

compared with the results from artifact reduction processing and a FBP algorithm.

**2.4 Evaluation of artifact reduction for prosthesis imaging** 

metal artifacts.

narrow angular range typically employed in tomosynthesis imaging. Gomi et al. (Gomi et al 2008 , 2009) developed artifact reduction methods based on a modified Shepp–Logan reconstruction filter kernel by taking into account the additional weight of direct current components in the frequency domain space. Processing leads to an increase in the ratio of low frequency components in an image (see appendix). Artifact reduction processing was performed with a basic and FBP algorithm. Artifact reduction processing provides a method of filtering that can be used in combination with the backprojection algorithm to yield sliced images with desired properties by means of tomosynthesis.

Fig. 6. Comparison of images obtained from artifact reduction tomosynthesis (W= 0.06), FBP tomosynthesis, SAA tomosynthesis, and maximum likelihood tomosynthesis (ML, four subsets & 15 iterations) of the center plane. Artifact reduction tomosynthesis provided better visualization of the hip prosthesis by eliminating blurring and reducing artifacts above and below visualized planes.

The quality of CT images is governed by the strength of artifacts, which depends on numerous factors such as size, shape, density, atomic number and position of metal objects, patient size, and patient's cross-section shape. For small implants manufactured from relatively light metals (e.g., titanium), the effects of beam-hardening and scattering are low. Therefore, the corrupted CT values as well as noise-induced streaking artifacts that pose a major problem to image quality can be neglected. In such cases, the digital linear tomosynthesis approach to artifact reduction processing appears to be promising for the reduction of artifacts stemming from metals with a relatively high atomic number (Fig.6). Improvement of an artificial image (part of undershooting) by an artifact reduction

processing is accepted (Fig.7). The effectiveness of the artifact reduction processing method in enhancing the visibility of a prosthetic case was quantified in terms of removal of artifacts. The potential of artifact reduction processing for digital linear tomosynthesis in the evaluation of hip prostheseswas demonstrated. Artifact reduction processing tomosynthesis realize that an especially normal bone and adhesion of an artificial bone become observable (Fig.8-9).

### **2.5 Potential artifacts**

Ideally, structures in a given plane of interest should be clearly displayed in the corresponding tomosynthesis reconstruction plane, whereas structures located outside of that plane should not be visible. Essentially, the limited angular range of the tomosynthesis image acquisition geometry dictates that the spatial resolution is limited in the dimension perpendicular to the detector plane. As a result, out-of-plane structures cannot be

X-Ray Digital Linear Tomosynthesis Imaging of Arthoroplasty 103

**(+15mm)**

Fig. 8. Case 1 patient (81-year-old woman; Rheumatism, post THA). AP radiographs of the hip joint prostheses are demonstrated. AP radiograph is difficult to visualize 3D information

in an AP radiograph as shown. The use of artifact reduction tomosynthesis (w=0.06) allowed better visualization of the right hip joint prosthesis caused by the blurring of

anatomic structures above and below the visualized planes.

**X-ray radiography**

**Off-focus plane**

**FBP**

**Artifact reduction**

**(-15mm) In-focus-plane Off-focus plane**

completely removed from the reconstruction plane. Out-of-plane structures are present in every reconstruction plane, but most are not visible because the various low-amplitude structures from projections overlap each other in the reconstruction plane, and therefore are blurred. Out-of-plane structures from high-attenuation features cannot be blurred. They appear as multiple replicates of the particular feature in every reconstruction plane except for the one in which the actual high-attenuation feature is located. At one projection angle, these ghosting features are distributed along the line made by the X-ray source and actual feature (Fig.10).

Fig. 7. Comparison of line profiles using artifact reduction processing (*W* = 0.06) and FBP algorithm in the in-focus plane. Artifacts (part of the undershoot) are reduced by artifact reduction processing.

completely removed from the reconstruction plane. Out-of-plane structures are present in every reconstruction plane, but most are not visible because the various low-amplitude structures from projections overlap each other in the reconstruction plane, and therefore are blurred. Out-of-plane structures from high-attenuation features cannot be blurred. They appear as multiple replicates of the particular feature in every reconstruction plane except for the one in which the actual high-attenuation feature is located. At one projection angle, these ghosting features are distributed along the line made by the X-ray source and actual

**0 20 40 60 80 100 120**

Fig. 7. Comparison of line profiles using artifact reduction processing (*W* = 0.06) and FBP algorithm in the in-focus plane. Artifacts (part of the undershoot) are reduced by artifact

**Number of Pixels**

**FBP tomosynthesis Artifact reduction tomosynthesis**

**modified FBP**

**FBP tomosynthesis Artifact reduction tomosynthesis**

**Artifact Reduction Processing W = 0.08**

feature (Fig.10).

**0**

**0.2**

**0.4**

**0.6**

**Pixel Value (Rel.Val.)**

reduction processing.

**0.8**

**1**

**1.2**

Fig. 8. Case 1 patient (81-year-old woman; Rheumatism, post THA). AP radiographs of the hip joint prostheses are demonstrated. AP radiograph is difficult to visualize 3D information in an AP radiograph as shown. The use of artifact reduction tomosynthesis (w=0.06) allowed better visualization of the right hip joint prosthesis caused by the blurring of anatomic structures above and below the visualized planes.

X-Ray Digital Linear Tomosynthesis Imaging of Arthoroplasty 105

The digital linear tomosynthesis images in this review were acquired using linear motion of the X-ray tube and detector. The type of motion used during data acquisition dictates the type of blurring of off-focal-plane objects in the image. Linear motion blurs objects in one dimension only, which leads to linear streak artifacts caused by high-contrast off-focal-plane objects. On the other hand, 3D reconstruction schemes, such as tomosynthesis and CT, require complete knowledge of the X-ray source projection geometry prior to exposure. This limitation precludes much of the potential task-dependent flexibility. This limitation also precludes accurate reconstruction from projections acquired from a patient who moves unpredictably between exposures, as this is geometrically equivalent to not knowing the

Use of digital linear tomosynthesis in imaging of prostheses appears promising. The results of the prosthesis study suggest that digital linear tomosynthesis can improve image quality compared with conventional radiography by removing overlying structures and providing limited 3D information. In addition, the digital linear tomosynthesis method appears to allow for significant improvement of images corrupted by metal artifacts. Digital linear tomosynthesis provided higher quality images than CT. Tomosynthesis is the best solution for cases in which the high-attenuation feature causing the artifacts can be segmented

Artifact reduction processing showed an adequate overall performance, but its effectiveness strongly depended on the image region. Digital linear tomosynthesis images gave good results independent of the type of metal present in the patient and showed good results for the removal of noise artifacts, particularly at greater distances from metal objects. The potential for application of digital linear tomosynthesis to the imaging of prostheses appears promising. Flexibility in the choice of imaging parameters in artifact reduction processing based on the desired final images and realistic imaging conditions

We wish to thank for Shimadzu Corporation for her helpful research assistance in this work.

The 3D Fourier transform of the 3D volume data generated by the backprojection is based

*xyz f x y z j*

where *fxyz* (,,) is the simple backprojection intermediate image, and *x*, *y*, and *z* are real numbers. The meaning of the filtering process performed in 3D Fourier space is described

(,,) (,,) (,,) *FM*

 

*xyz <sup>x</sup> <sup>y</sup> z dx d* ) *<sup>y</sup> dz* (1)

 *<sup>y</sup> z x F <sup>y</sup> z x M <sup>y</sup> <sup>z</sup>* (2)

*F*( , , ) ( , , ) exp (

*x* 

below, and it is mathematically expressed by the following equation (2):

**3. Conclusion** 

projection geometry.

may be beneficial.

**5. Appendix** 

**5.1 FBP algorithm** 

on the following equation (1):

**4. Acknowledgment** 

accurately from the projection.

Fig. 9. Case 2 patient (71-year-old woman; Gonarthrosis, post total knee arthroplasty; TKA). AP radiographs of the knee joint prostheses are demonstrated. AP radiograph is difficult to visualize 3D information in an AP radiograph as shown. The use of artifact reduction tomosynthesis (w=0.06) allowed better visualization of the prosthesis caused by the blurring of anatomic structures above and below the visualized planes.

Fig. 10. Blurring occurs along the sweep direction and results from imaging studies show that a high contrast structure exists out of the slice plane that is continuously perpendicular to the sweep direction.
