Rationale and Objectives
Magnetic resonance imaging with an endorectal coil allows high-resolution imaging of prostate cancer and the surrounding normal organs. These anatomic details can be used to direct radiotherapy. However, organ deformation introduced by the endorectal coil makes it difficult to register magnetic resonance images for treatment planning. In this study, plug-ins for the volume visualization software VolView were implemented on the basis of algorithms from the National Library of Medicine’s Insight Segmentation and Registration Toolkit (ITK).
Materials and Methods
Magnetic resonance images of a phantom simulating human pelvic structures were obtained with and without the endorectal coil balloon inflated. The prostate not deformed by the endorectal balloon was registered to the deformed prostate using an ITK thin plate spline (TPS). This plug-in allows the use of crop planes to limit the deformable registration in the region of interest around the prostate. These crop planes restricted the support of the TPS to the area around the prostate, where most of the deformation occurred. The region outside the crop planes was anchored by grid points.
Results
The TPS was more accurate in registering the local deformation of the prostate compared with a TPS variant, the elastic body spline. The TPS was also applied to register an in vivo T 2 -weighted endorectal magnetic resonance image. The intraprostatic tumor was accurately registered. This could potentially guide the boosting of intraprostatic targets. The source and target landmarks were placed graphically. This TPS plug-in allows the registration to be undone. The landmarks could be added, removed, and adjusted in real time and in three dimensions between repeated registrations.
Conclusion
This interactive TPS plug-in allows a user to obtain a high level of accuracy satisfactory to a specific application efficiently. Because it is open-source software, the imaging community will be able to validate and improve the algorithm.
Magnetic resonance imaging (MRI) enhanced by an endorectal coil provides anatomic details of both prostate tumor and the surrounding normal tissues . Incorporating information from MRI studies into radiotherapy treatment may enable the direction of radiation to boost resistant areas, without increasing the dose to surrounding tissues. Various image registration methods have recently been studied to incorporate MRI information for radiation treatment planning and monitoring . Because the information needed for radiation treatment planning is concentrated in and around the prostate, the thin plate spline (TPS) approach appears to be an efficient and accurate way to perform deformable registration . Here, we report our implementation and assessment of plug-ins based on algorithms available in the open source Insight Segmentation and Registration Toolkit (ITK; National Library of Medicine, Bethesda, MD). In particular, we adapted a physics-based ITK TPS plug-in to run interactively in the volume visualization software VolView version 2.0 (Kitware, Inc, Clifton Park, NY) . The interactive nature of the plug-in allows landmarks to be added, removed, and adjusted in real time between repeated registrations. We evaluated the accuracy of the TPS in registering the local deformation of the prostate compared with a TPS variant, the elastic body spline (EBS). To illustrate potential clinical utility, we also applied the TPS to register an in vivo T 2 -weighted endorectal magnetic resonance image. The intraprostatic tumor was accurately registered. This could potentially guide the boosting of intraprostatic targets.
Materials and methods
Magnetic Resonance Images
A deformable pelvic phantom was custom made for our image registration study. Our earlier studies have been reported . Pertinent to this study, the magnetic resonance numbers for the prostate, seminal vesicles, bladder, and rectal wall were specified to simulate those of humans. The phantom was made of tissue-substitute materials by blending epoxy resins, urethanes, water-based polymers, and other proprietary materials (CIRS, Inc, Norfolk, VA). The University of Texas M.D. Anderson Cancer Center’s institutional review board approved this retrospective study.
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Image Registration Work Flow
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Adaptation of the ITK TPS as a VolView Plug-In
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μ∇2u→(x→)+(μ+λ)∇[∇∙u→(x→)]=f→(x→), μ
∇
2
u
→
(
x
→
)
+
(
μ
+
λ
)
∇
[
∇
•
u
→
(
x
→
)
]
=
f
→
(
x
→
)
,
where u→(x→) u
→
(
x
→
) is the displacement from the original position x→ x
→ ; ∇2 ∇
2 and ∇ ∇ are the Lapacian and gradient operators, respectively; the force field f→(x→)=c→r(x→) f
→
(
x
→
)
=
c
→
r
(
x
→
) ; position vector r→(x→)=∣∣x→∣∣=[x21+x22+x23]1/2 r
→
(
x
→
)
=
|
x
→
|
=
[
x
1
2
+
x
2
2
+
x
3
2
]
1
/
2 ; μ and λ are the coefficients that describe the physical properties of the materials; and ∇∙u→(x→) ∇
•
u
→
(
x
→
) is the divergence of u→(x→) u
→
(
x
→
) . The solution to Equation 1 given the force field is
u→(x→)=G(x→)c→, u
→
(
x
→
)
=
G
(
x
→
)
c
→
,
where
G(x→)=[αr(x→)I−3x→x→T]r(x→). G
(
x
→
)
=
[
α
r
(
x
→
)
I
−
3
x
→
x
→
T
]
r
(
x
→
)
.
The Poisson ratio υ = λ/[2(λ + μ)], α = 12(1 − υ) − 1, and I is the identity matrix. The form of G(x→) G
(
x
→
) for the ITK TPS implementation used in this study is as follows:
G(x→)=Ir(x→). G
(
x
→
)
=
I
r
(
x
→
)
.
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Variants of the TPS
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G(x)=r(x)2log[r(x)]×I; G
(
x
)
=
r
(
x
)
2
log
[
r
(
x
)
]
×
I
;
the EBS,
G(x)={[12×(1−v)−1]r[x]2×I−3xxT}×r(x), G
(
x
)
=
{
[
12
×
(
1
−
v
)
−
1
]
r
[
x
]
2
×
I
−
3
x
x
T
}
×
r
(
x
)
,
where ν is the Poisson ratio; the elastic body reciprocal spline,
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G(x)={[8×(1−v)−1]r[x]×I−3xxT/r(x)}; G
(
x
)
=
{
[
8
×
(
1
−
v
)
−
1
]
r
[
x
]
×
I
−
3
x
x
T
/
r
(
x
)
}
;
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G(x)=r(x)3×I. G
(
x
)
=
r
(
x
)
3
×
I
.
These variants are based on variants plugged into the solution of G ( x ) . In this study, we investigated the accuracy of the TPS and the ITK EBS for the deformable registration of endorectal magnetic resonance images of the prostate.
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Landmark Placement
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Results
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TPS Deformable Manual Alignment
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Comparison of TPS Variants
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Clinical Example
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Future Work
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Conclusion
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