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The effects of computed tomography image characteristics and knot spacing on the spatial accuracy of B-spline deformable image registration in the head and neck geometry.

Brouwer CL, Kierkels RG, van 't Veld AA, Sijtsema NM, Meertens H - Radiat Oncol (2014)

Bottom Line: Using BKS = 15 mm for the three clinical cases resulted in an average MRD < 1.0 mm.The accuracy decreased with decreasing image feature content, decreasing image contrast, and higher noise levels.Our results indicate that DIR accuracy in clinical CT images (typical noise levels < ± 100 HU) will not be effected by the amount of image noise.

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Affiliation: University of Groningen, University Medical Center Groningen, Department of Radiation Oncology, Groningen, the Netherlands. c.l.brouwer@umcg.nl.

ABSTRACT

Objectives: To explore the effects of computed tomography (CT) image characteristics and B-spline knot spacing (BKS) on the spatial accuracy of a B-spline deformable image registration (DIR) in the head-and-neck geometry.

Methods: The effect of image feature content, image contrast, noise, and BKS on the spatial accuracy of a B-spline DIR was studied. Phantom images were created with varying feature content and varying contrast-to-noise ratio (CNR), and deformed using a known smooth B-spline deformation. Subsequently, the deformed images were repeatedly registered with the original images using different BKSs. The quality of the DIR was expressed as the mean residual displacement (MRD) between the known imposed deformation and the result of the B-spline DIR.Finally, for three patients, head-and-neck planning CT scans were deformed with a realistic deformation field derived from a rescan CT of the same patient, resulting in a simulated deformed image and an a-priori known deformation field. Hence, a B-spline DIR was performed between the simulated image and the planning CT at different BKSs. Similar to the phantom cases, the DIR accuracy was evaluated by means of MRD.

Results: In total, 162 phantom registrations were performed with varying CNR and BKSs. MRD-values < 1.0 mm were observed with a BKS between 10-20 mm for image contrast ≥ ± 250 HU and noise < ± 200 HU. Decreasing the image feature content resulted in increased MRD-values at all BKSs. Using BKS = 15 mm for the three clinical cases resulted in an average MRD < 1.0 mm.

Conclusions: For synthetically generated phantoms and three real CT cases the highest DIR accuracy was obtained for a BKS between 10-20 mm. The accuracy decreased with decreasing image feature content, decreasing image contrast, and higher noise levels. Our results indicate that DIR accuracy in clinical CT images (typical noise levels < ± 100 HU) will not be effected by the amount of image noise.

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Registration accuracy for varying image feature content. The MRD as function of BKS for phantoms with varying disc spacings (phantom XV-XVII) (see phantom properties in Table 1). Abbreviations: MRD = mean residual displacement, BKS = B-spline knot spacing.
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Figure 4: Registration accuracy for varying image feature content. The MRD as function of BKS for phantoms with varying disc spacings (phantom XV-XVII) (see phantom properties in Table 1). Abbreviations: MRD = mean residual displacement, BKS = B-spline knot spacing.

Mentions: To study the effect of varying feature content on the registration accuracy, phantoms with different disc spacing were created (see phantom XV-XVII in Table 1 and Figure 1). The registration of the phantom images with the smallest disc spacing (i.e. 1 mm in between discs) showed similar MRDs for all BKSs (Figure 4). Increased disc spacing (i.e. 7 and 19 mm in between discs; phantom XVI and XVII), resulted in MRDs > 2.0 mm for BKS < 8.0 mm (Figure 4).For three head-and-neck cancer patients a simulated moving image was created using a rescan CT of each individual patient. For each patient, the residual displacement is shown for three representative axial cross-sections throughout the body volume of the planning CT (Figure 5AI-CI). The largest residual displacement was observed near tissue-air transitions and within homogenous regions, such as the brain. Figure 5AII-CII shows the corresponding distribution of the deformation vector length of the known imposed and the resulting registration. The deformations between the planning CT and the rescan CT were of similar magnitude as the artificially imposed Gaussian shaped B-spline deformation to the phantom images. The distribution of the residual displacements after a registration with BKS = 15 mm is shown in Figure 5AIII-CIII. For all patients the MRD was < 1.0 mm.Figure 6 plots the MRD as function of BKS for patient A-C. The registrations resulted in MRDs of < 1.0 mm for all cases, using a BKS < 30 mm. The MRD was lowest using a BKS between 10–20 mm. The neck flexion of patient B was different from patient A and C, and therefore more brain tissue was included in the image of patient B. The brain tissue included higher residual displacements than those in the neck region. The highest MRD for patient C was found in homogeneous tumor tissue.


The effects of computed tomography image characteristics and knot spacing on the spatial accuracy of B-spline deformable image registration in the head and neck geometry.

Brouwer CL, Kierkels RG, van 't Veld AA, Sijtsema NM, Meertens H - Radiat Oncol (2014)

Registration accuracy for varying image feature content. The MRD as function of BKS for phantoms with varying disc spacings (phantom XV-XVII) (see phantom properties in Table 1). Abbreviations: MRD = mean residual displacement, BKS = B-spline knot spacing.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4128373&req=5

Figure 4: Registration accuracy for varying image feature content. The MRD as function of BKS for phantoms with varying disc spacings (phantom XV-XVII) (see phantom properties in Table 1). Abbreviations: MRD = mean residual displacement, BKS = B-spline knot spacing.
Mentions: To study the effect of varying feature content on the registration accuracy, phantoms with different disc spacing were created (see phantom XV-XVII in Table 1 and Figure 1). The registration of the phantom images with the smallest disc spacing (i.e. 1 mm in between discs) showed similar MRDs for all BKSs (Figure 4). Increased disc spacing (i.e. 7 and 19 mm in between discs; phantom XVI and XVII), resulted in MRDs > 2.0 mm for BKS < 8.0 mm (Figure 4).For three head-and-neck cancer patients a simulated moving image was created using a rescan CT of each individual patient. For each patient, the residual displacement is shown for three representative axial cross-sections throughout the body volume of the planning CT (Figure 5AI-CI). The largest residual displacement was observed near tissue-air transitions and within homogenous regions, such as the brain. Figure 5AII-CII shows the corresponding distribution of the deformation vector length of the known imposed and the resulting registration. The deformations between the planning CT and the rescan CT were of similar magnitude as the artificially imposed Gaussian shaped B-spline deformation to the phantom images. The distribution of the residual displacements after a registration with BKS = 15 mm is shown in Figure 5AIII-CIII. For all patients the MRD was < 1.0 mm.Figure 6 plots the MRD as function of BKS for patient A-C. The registrations resulted in MRDs of < 1.0 mm for all cases, using a BKS < 30 mm. The MRD was lowest using a BKS between 10–20 mm. The neck flexion of patient B was different from patient A and C, and therefore more brain tissue was included in the image of patient B. The brain tissue included higher residual displacements than those in the neck region. The highest MRD for patient C was found in homogeneous tumor tissue.

Bottom Line: Using BKS = 15 mm for the three clinical cases resulted in an average MRD < 1.0 mm.The accuracy decreased with decreasing image feature content, decreasing image contrast, and higher noise levels.Our results indicate that DIR accuracy in clinical CT images (typical noise levels < ± 100 HU) will not be effected by the amount of image noise.

View Article: PubMed Central - HTML - PubMed

Affiliation: University of Groningen, University Medical Center Groningen, Department of Radiation Oncology, Groningen, the Netherlands. c.l.brouwer@umcg.nl.

ABSTRACT

Objectives: To explore the effects of computed tomography (CT) image characteristics and B-spline knot spacing (BKS) on the spatial accuracy of a B-spline deformable image registration (DIR) in the head-and-neck geometry.

Methods: The effect of image feature content, image contrast, noise, and BKS on the spatial accuracy of a B-spline DIR was studied. Phantom images were created with varying feature content and varying contrast-to-noise ratio (CNR), and deformed using a known smooth B-spline deformation. Subsequently, the deformed images were repeatedly registered with the original images using different BKSs. The quality of the DIR was expressed as the mean residual displacement (MRD) between the known imposed deformation and the result of the B-spline DIR.Finally, for three patients, head-and-neck planning CT scans were deformed with a realistic deformation field derived from a rescan CT of the same patient, resulting in a simulated deformed image and an a-priori known deformation field. Hence, a B-spline DIR was performed between the simulated image and the planning CT at different BKSs. Similar to the phantom cases, the DIR accuracy was evaluated by means of MRD.

Results: In total, 162 phantom registrations were performed with varying CNR and BKSs. MRD-values < 1.0 mm were observed with a BKS between 10-20 mm for image contrast ≥ ± 250 HU and noise < ± 200 HU. Decreasing the image feature content resulted in increased MRD-values at all BKSs. Using BKS = 15 mm for the three clinical cases resulted in an average MRD < 1.0 mm.

Conclusions: For synthetically generated phantoms and three real CT cases the highest DIR accuracy was obtained for a BKS between 10-20 mm. The accuracy decreased with decreasing image feature content, decreasing image contrast, and higher noise levels. Our results indicate that DIR accuracy in clinical CT images (typical noise levels < ± 100 HU) will not be effected by the amount of image noise.

Show MeSH
Related in: MedlinePlus