Download mri in radiation treatment planning and assessment

MRI IN RADIATION
TREATMENT PLANNING
AND ASSESSMENT
Yue Cao1 and Lili Chen2
Departments of Radiation Oncology and Radiology,
University of Michigan1
Department of Radiation Oncology,
Fox Chase Cancer Center2
Outline
I. OVERVIEW OF MR IMAGING
I.1 Principles of Magnetic Resonance Imaging
I.2 Contrasts of MRI
I.3 Radiographic Response
I.4 Artifacts and Geometric distortions
II. CURRENT MRI IN RADIATION TREATMENT PLANNING
II.1 Target Definition
II.2 Treatment planning and simulation
II.2.1 MRI simulator
II.2.2 Assessment and Correction for Geometric distortion
II.2.3. Creation of MR DRR
II.3 Use of MRI in place of CT for treatment planning
III. PHYSIOLOGICAL AND METABOLIC MR TECHNIQUES AND
APPLICATIONS
III.1 Physiological and metabolic MR techniques
III.1.1 Diffusion MR technique
III.1.2 Cerebral blood volume MR technique
III.1.3 CBF MR technique
III.1.4 Vascular permeability MR technique
III.1.5 Proton MR spectroscopy
III.2 Applications of physiological and metabolic MR in Radiation Oncology
OVERVIEW OF MR IMAGING
I.1 Principles of Magnetic
Resonance Imaging
I.1.1 Signal Source
– Nuclear magnetic moments or
spins of water protons
– randomly oriented
– lining up with a external
magnetic field
– precessing about the direction of
B0
– Larmor relationship:
ϖ0=γB0
– Magnetization:
Longitudinal Mz=Mo
Transverse Mxy=0
a
b
a) randomly oriented spins in a sample
b) alignment of spins in a static magnetic
field.
OVERVIEW OF MR IMAGING
I.1 Principles of Magnetic
Resonance Imaging
I.1.2 RF Excitation
• Applying a small radiofrequency
(RF) field of amplitude B1 to
create the transverse
magnetization Mxy
• B1 oscillating at the Larmor
frequency
• B1 perpendicular to B0
• B1 rotates the magnetization away
from B0
• The flip angle depends upon the
amplitude and duration of B1
• Applying B1 long enough, the
transverse magnetization will be
equal to M0
B0
M0
M
α
Mxy
OVERVIEW OF MR IMAGING
I.1 Principles of Magnetic
Resonance Imaging
I.1.3 Free Induction Decay
– Following a 90o RF pulse, M0
freely precesses about B0
– Inducing oscillating voltage in the
receiver coil
– Magnetization is decay due to the
incoherent precessing frequencies
of spins
• Some of spins precess about B0
faster than others
The envelope of the FID signals can
be described as
S=S0 exp(-t/T*2)
OVERVIEW OF MR IMAGING
I.1 Principles of Magnetic Resonance
Imaging
I.1.4 The Spin Echo
• After a 90o RF pulse, spins start dephasing
• Applying a 180o RF pulse τ seconds after
the 90o pulse, the fast spins will be behind
the low spins
• Then, the spins will come back into phase
with each other after another τ seconds.
• The varying magnetic field induced in the
receiver coil will form an echo, therefore it
is called a spin echo.
• The magnitude of spin echo
– S= S0 exp( - TE/T2), TE=2τ
• The amplitude of SE is decreased with an
increase in TE due to incomplete refocuses
of some of incoherent processes
OVERVIEW OF MR IMAGING
I.1 Principles of Magnetic
Resonance Imaging
I.1.5 T1 Relaxation Time
• After FID or forming a SE, the
magnetization eventually returns to
its initial rest state (along B0) by a
process called T1 or spin-lattice
relaxation.
– Two methods to measure T1
Saturation recovery
S=S0 [1-exp(-t/T1)]
Inversion recovery
S=S0 [1-2 exp(-t/T1)]
OVERVIEW OF MR IMAGING
I.1 Principles of Magnetic Resonance Imaging
I.1.6 2D Fourier Imaging
Frequency
gradient
RF pulse
time
time
Phase
gradient
Frequency encoding
If a magnetic field gradient is applied
along the x axis, the spins along the x
axis will experience a differential
magnetic field according to their
position, rotate with different
frequencies, and induce voltages with
different oscillating frequencies in the
receiver coils. Applying a onedimensional FT to the induced voltages
will yield MR signals resolved along the
x axis.
Phase encoding
If a gradient is applied along y axis for
a short time period before recording
the induced voltages, the spins along
the y axis will have different phases due
to that the spins that experience a
larger field will rotate faster than the
spins that are exposed to a smaller field.
If this process is done N times and
magnitude of y gradient-is different for
each time, we will obtain N signals with
N different phases. Again, applying a
one-dimensional Fourier transform to
the N signals with N different phases
will yield the MR signals resolved along
the y axis.
time
OVERVIEW OF MR IMAGING
I.2 Basic MRI Contrasts
– T1 weighted image
• TE short, TR ~<T1
• Brighter for tissue w shorter T1
– T2 weighted image
• TE ~<T2, TR long
• Brighter for tissue w longer T2
– FLAIR T2 weighted image
• Fluid-attenuation inversion recovery
– Post Gd-DTPA T1 weighted image
• Brighter for tissue that is uptake contrast
– Contrast agent – Gd-DTPA
• Shortening T1 Æ increase SI in T1WI
• Shortening T2* or T2 Æ decrease SI in
T2*WI or T2WI
OVERVIEW OF MR IMAGING
I.3 Treatment (radiographic)
Response
• Surrogate measure
– Longest dimension, two longest
dimensions, and volume of the
tumor
– 2000 NCI guideline
• Longest dimension
CR: disappearing tumor
PR: decrease > 30%
SD: increase <20% or decrease <30%
PD: increase >20% or new lesions
Pre Tx
Post Tx
OVERVIEW OF MR IMAGING
I.4 Artifact and Geometric distortion
• Artifact and geometric distortion
–
–
–
–
–
Inhomogeneity in the external magnetic field
Inhomogeneity in the RF field
any non-linearity and asymmetry in gradients
Eddy currents
a large change in magnetic susceptibility
around tissue boundaries
– Chemical shifts of water and fat
– surgical clips and dental work
– High field worse, fast sequences worse
(EPI, GE, …)
•
Motion artifacts
– Respiration motion
– Cardiac motion
– Involuntary motion
CURRENT MRI IN RADIATION
TREAMENT PLANNING
II.1 Target Definition
– Advantages of MRI
•
•
•
•
Superior soft tissue contrasts
Richness in various contrasts
Widely available
One stop to obtain anatomic, metabolic and functional images
– Radiation treatment planning
• Routinely used in brain tumor including primary and metastasis
• Increasing its use in prostate cancer Å better delineation of prostate
and rectal volume
• a leaser extent in H/N, liver, pancreas, …
CURRENT MRI IN RADIATION
TREAMENT PLANNING
?
CT
Post-Gd T1W
FLAIR
CURRENT MRI IN RADIATION
TREAMENT PLANNING
II.2 Treatment planning
and simulation
II.2.1 MRI simulator
– A system at FCCC
• 0.23 T
• Flat patient table top
• 47 cm separation between
two poles
• Patient weight up to 400
lb
A Philips 0.23 permanent magnet
CURRENT MRI IN RADIATION
TREAMENT PLANNING
II.2 Treatment planning and simulation
II.2.2 Assessment and Correction for Geometric
distortion
• Sources of geometric distortion
– Hardware and software of the scanner
– Patient induced
• System dependent factors
– Field strength: the lower field the less distortion
– Filed of view: the smaller FOV the less distortion
» The farther distance from the isocenter results in the large
distortion
CURRENT MRI IN RADIATION
TREAMENT PLANNING
II.2 Treatment planning and simulation
II.2.2 Assessment and Correction for Geometric
distortion
• FCCC 0.23 system
– 139 Hz/pixel in FE Æ insignificant distortion caused by
chemical shifting and susceptibility
– Distortion caused by imperfect gradient systems
» Distortion of 2-3 mm if 12-18 cm is offset from the
isocenter
» Gradient distortion correction program Æ
negligible distortion for the patient lateral size < 38 cm
7 mm distortion for the patient later size > 38 cm
CURRENT MRI IN RADIATION
TREAMENT PLANNING
CT
Uncorrected MRI
GDC MRI
CURRENT MRI IN RADIATION
TREAMENT PLANNING
II.3 Use of MRI in place of CT for treatment
planning
– II.2.3. Creation of MR DRR
• Bony structures appear as signal voids on MRI
• Bony structures can be contoured and assigned a bulk
density
– e.g., 2.0g/cm3 to pubic symphysis, femoral heads, acetabulum
and sacrum for prostate patient
– Accuracy of this method Æ 2-4 mm compared to CT DRR
CURRENT MRI IN RADIATION
TREAMENT PLANNING
MRI-DRR
CT-DRR
CURRENT MRI IN RADIATION
TREAMENT PLANNING
II.3 Use of MRI in place of CT for treatment
planning
– gold standard
• MR and CT image fusion with CT-based dose
calculation
– MRI-based treatment planning for prostate
• Additional errors induced by the fusion process
• substantial differences in bladder and rectal filling
• redundant CT imaging session
CURRENT MRI IN RADIATION
TREAMENT PLANNING
II.3 Use of MRI in place of CT for treatment
planning
– MRI-based treatment planning for prostate
• Major concern: lack of electron density information for
accurate dose calculation and heterogeneous anatomy
• Several Monte Carlo studies have shown that there is no
clinically significant difference in dose calculation
between homogeneous and heterogeneous geometry for
the pelvic region.
• Adequate to use homogeneous density in prostate
CURRENT MRI IN RADIATION
TREAMENT PLANNING
II.3 Use of MRI in place of CT for treatment
planning
– MRI-based treatment planning for prostate
• Compared to the plan based upon CT and heterogeneous geometry
for 15 plans
– The mean differences:
» <2.6% for isocenter dose
» 1.25 ± 0.83 % for the maximum doses
» 0.54 ± 0.4% for D95 of the PTV
» 1.0± 0.49% for D17 of the rectum
» 1.09 ± 0.31% D35 of the rectum
» 0.96 ± 0.53% for D25 of the bladder
» 0.66 ± 0.71% and D50 of the bladder
CURRENT MRI IN RADIATION
TREAMENT PLANNING
90%
CT - based plan
100%
100%
90%
MRI - based plan
CT-based and MRI-based IMRT plans. The isodose lines are
100%, 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20% and 10%.
CURRENT MRI IN RADIATION
TREAMENT PLANNING
100
CT-based PTV
MR-based PTV
CT-based rectum
MR-base rectum
CT-based bladder
MR-based bladder
Volume (%)
80
60
40
20
0
0
10
20
30
40
50
Dose (Gy)
60
70
80
90
100
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.1 Physiological and metabolic MR
techniques
– Diffusion imaging
– Diffusion tensor imaging
– Vascular imaging including blood volume, blood flow,
mean transit time, and vascular permeability
– Metabolic imaging (1H, 31P, 13C, 19F,…)
– Do not blindly use commercial software without evaluation
(e.g., XXX)
– Understand limitations of technologies
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.1 Physiological and metabolic MR techniques
III.1.1 Diffusion MR
• The random motion of the water protons and the inhomogeneity of a magnetic
field cause de-phasing of spins
• Applying dephasing and refocused field gradients to sensitize an image to water
proton motion
• The diffusional signal loss by the gradient application is given
– S=S0 exp(-bD)
– large diffusion results in great in signal loss
Low diffusion
in tumor
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.1 Physiological and metabolic MR techniques
III.1.2 Cerebral blood volume and Cerebral blood
flow MR techniques
– Using DCE imaging, several vascular properties
such as CBV, CBF, MTT(=CBV/CBF) and
vascular permeability can be estimated.
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
1100
1000
900
S(t)
III.1 Physiological and metabolic MR
techniques
III.1.2 Cerebral blood volume MR
techniques
– DCE T2* weighted images with
an i.v. injected contrast bolus
– An image volume can be
acquired within 1-2 seconds
– The acquisition continues for 1-2
minutes
– The equation on the right hand
side provides a good estimate of
CBV
– An overestimate or underestimate
can occur in abnormal conditions
800
700
600
500
400
0
20
40
60
80
time (s)
CBV ∝
∫
⎡ S0 ⎤
dt
ln ⎢
⎥
⎣ S (t ) ⎦
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.1 Physiological and metabolic MR techniques
III.1.2 Cerebral blood flow MR techniques
– CBF can be estimated from DCE T2* weighted MRI and a artery input
function
AIF (t ) =
1 ⎡ S MCA0 ⎤
ln ⎢
⎥
TE ⎣ S MCA (t ) ⎦
Tiss (t ) =
⎡ S
⎤
1
ln ⎢ tiss 0 ⎥
TE ⎣ S tiss (t ) ⎦
t
Tiss (t ) = ∫ AIF (τ ) R(t − τ ) dτ
0
– The residual function R is determined by deconvolusion computation
(SVD)
– The amplitude of the residual function R is proportional to blood flow.
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
CBV
CBF
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.1.4 Vascular
permeability MR
technique
Kin
– Vascular permeability is
estimated from T1weighted or T2* weighted
DCE images and a
compartmental model
– Two compartmental model
• Vascular space and
extravascular extracellular
space
– Assumption
ΔR1 =constant*C
or
ΔSI =constant*C
blood
inflow
artery
Kout
capillary
vein
blood
outflow
extra-vascular space
Two compartmental model
t − k ( t −τ )
b
Ct (t ) = K in ∫ e
0
C p (τ )dτ + v pC p (t )
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.1.4 Vascular permeability MR technique
Vascular permeability images
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.1.5 Proton MR spectroscopy
– Proton MR spectroscopy (MRS) is a technique that is able
to detect proton metabolites and chemical compounds in
tissue
– In an organic compound, electron bindings of each of
protons with neighbor atoms are different, and generate a
slight different but characteristic magnetic field offset.
– Each of the protons in a compound has a slightly different
resonance frequency even though the compound is
immersed in the same external magnetic field.
– Using this characteristic resonance frequency each proton
can be uniquely identified on a spectrum, and the
concentration of a compound can be estimated relatively to
another compound.
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.1.5 Proton MR
spectroscopy
– Chemical compounds
and metabolites
commonly detected in
brain tissue
• choline-containing
compounds, creatine,
lactate, lipid, and Nacetylaspartate (NAA)
• Proton spectra can be
obtained by 2D or 3D
choline NAA
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.2 Applications of physiological and
metabolic MR in Radiation Oncology
– Applications of physiological and metabolic MR
in radiation therapy are emerging
•
•
•
•
•
Target definition
Early prediction for treatment response
Early prediction for treatment toxicity
optimizing treatment strategies
Endpoints – clinical, pathological, …
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.2 Applications of physiological and
metabolic MR in Radiation Oncology
– Assessment for treatment response
• A study of 28 patients with newly diagnosed GBM
treated by fractionated radiotherapy and
chemotherapy, the pre-radiotherapy volume of the
metabolic abnormality (the region with choline/NAA
> 2.5) within the T2 defined tumor volume predicted
survival.
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.2 Applications of
physiological and
metabolic MR in
Radiation Oncology
–Early assessment for
treatment response
• Another recent study of 23
high-grade gliomas found that
the fractional tumor volume
with high CBV prior to
radiotherapy (RT) predicted
survival
fTV w High-CBV <0.07
P=0.002
fTV w High-CBV >0.07
(month)
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
Pre RT
1 week
3 weeks
Subsequent decreases in the fractional tumor volumes with high-CBV
at week 3 of RT were associated with better survival
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.2 Applications of physiological and
metabolic MR in Radiation Oncology
– Early prediction for treatment toxicity
• assessment of blood-brain or blood-tumor barrier
opening in high-grade gliomas during radiation therapy
using high-resolution post contrast MRI
– A significant increase in the contrast uptake in the initially
non-contrast enhanced portion of the tumor after ~30 Gy and
up to 1 month post RT
– A graduate decrease in the contrast uptake in the initially
contrast enhanced portion of the tumor during RT
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
12.00
Pre RT
After ~30Gy
Gd-DTPA Uptake Index
10.00
8.00
6.00
4.00
2.00
0.00
-2.00
pre-RT
1wk
3wk
1 mon pst
3 mon pst
6 mon pst
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.2 Applications of physiological and
metabolic MR in Radiation Oncology
– Early prediction for hepatic venous perfusion
dysfunction
• Decreases in the regional portal vein perfusion one
month post RT were predicted by the local doses and
the decreases observed after received ~45 Gy in
patients who underwent focal liver radiation
• CT only allows us to acquire DCE images from a
small slab (~2cm) of liver, but MRI allows to us to
image the whole liver
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
250 ml/100g/min
45 ml/100g/min
250 ml/100g/min
0 ml/100g/min
0 ml/100g/min
0 ml/100g/min
Total perfusion
Arterial perfusion
Portal vein perfusion
ΔFpv% 1 month after RT
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
ΔFpv% a
fter 3 wee
ks RT
Gy)
of RT in c
d
n
(e
e
s
o
d
Regional
PHYSIOLOGICAL AND METABOLIC MR
TECHNIQUES AND APPLICATIONS
III.2 Applications of physiological and
metabolic MR in Radiation Oncology
– Early prediction for hepatic venous perfusion
dysfunction
• CT only allows us to acquire DCE images from a
small slab (~2cm) of liver, but MRI allows to us to
image the whole liver
• Prediction of hepatic venous perfusion dysfunction
post RT might have a significant impact on
optimizing or re-optimizing radiation dose in
individual patients
SUMMARY
• Roles of MRI in radiation oncology will continue
increasing, including target definition for treatment
planning, boost volume definition, early assessment
for response, early prediction for radiation toxicity,
and re-optimization for radiation treatment based
upon normal tissue toxicity and tumor response.
• Applications of physiological and metabolic MRI to
RO will increase substantially.