Dickson, J.

Fersht, N.

Kayani, I.

Maclean, J.

O'Meara, C.

Short, S.

Stacey, C.

Sullivan, K.

Elsevier - Publisher Connector 

PD-0594: The introduction of simultaneous PET/MRI to radiotherapy planning 

2nd ESTRO Forum 2013  S229 
	
PD-0593   
The impact of a Dixon sequence in creating a pseudo CT scan from 
MR images using a Gaussian mixture regression model 
D. Andreasen1, J.L. Andersen2, R.H. Hansen3, K. Van Leemput1, J.M. 
Edmund2 
1Technical University of Denmark, Department of Informatics and 
Mathematical Modelling, Lyngby, Denmark  
2Copenhagen University Hospital Herlev, Department of Oncology, 
Herlev, Denmark  
3Copenhagen University Hospital Herlev, Department of Radiology, 
Herlev, Denmark  
 
Purpose/Objective: For RT based on MRI only, a promising approach is 
to obtain a substitute CT scan from the MR images (a so-called pseudo 
CT, pCT) using a Gaussian mixture regression (GMR) model. The GMR 
model has previously been investigated on 3T MR images using a dual 
ultra-short echo time (dUTE) sequence and was shown to give 
sufficient information for training the GMR model. The dUTE sequence 
provides contrast between bone and tissue using dual echo times but 
at 1 T, chemical shift artifacts at the second echo time may cause 
voxels containing water and fat to behave like bone. The multiecho-
Dixon (mDixon) MR sequence provides contrast between water and fat 
which could potentially remove this problem and provide valuable 
information for the GMR model. In this study, we investigate the 
robustness of the GMR model on predicting pCT scans from dUTE MR 
images of a 1 T scanner and how adding an mDixon sequence affects 
the generated pCT. 
Materials and Methods: Head scans of 2 patients fixated for whole 
brain RT were acquired on a 1 T open MR scanner with flex coils. dUTE 
sequences were obtained at flip angles 10 and 25 degrees, 
respectively. Echo- and repetition times TE1/TE2/TR were 0.09/3.5 
/7.1 ms with a voxel resolution of 1x1x1 mm and a 256 mm FOV. The 
mDixon was acquired with TE1/dTE/TR equal to 6.9/3.5/16 ms, a 
voxel resolution of 1x1x1.5 mm and a 250.5 mm FOV. CT head scans 
were acquired with a voxel resolution of 0.6x0.6x2 mm and a 220 mm 
FOV. The CT was registered to the high angle TE1 UTE using a mutual 
information algorithm and all MR scans were internally registered. All 
scans were resliced to the dUTE resolution and cropped to the 
smallest FOV. The MR images were low- and high-pass filtered 
creating two new images per filtered image. The MR images, their 
filtered counterparts and the CT image were considered as random 
variables and the voxel intensities a sample from their underlying 
distribution. A GMR model was initialized with 20 centers using k-
means clustering and an EM algorithm was used to train the model on 
the data from one of the patients.The model was then applied on the 
other patient to generate the pCT. A model using only the dUTE 
images and one adding the mDixon images were trained.A comparison 
using the real CT to calculate the mean absolute prediction error 
(MAPD) of the pCT in bins of 20 HU was carried out. 
Results: The pCTs of one patient using the extended model is shown 
in the figure. Qualitatively (upper images) and quantitatively (lower 
graph), the results are similar to those previously reported for 3T 
using dUTE only. A reduction in MAPD can be observed in the bone 
region(>500 HU) by adding mDixon to the model. 
Conclusions: The robustness of a GMR model on 1T MR images was 
demonstrated. The model was further expanded with an mDixon 
sequence which reduced the prediction error of predicted CT values 
>500 HU. Although a study based on larger amounts of data should be 
carried out, there is an indication that the mDixon sequence improves 
CT prediction from dUTE MR images. 
   
PD-0594   
The introduction of simultaneous PET/MRI to radiotherapy planning 
J. Maclean1, K. Sullivan1, I. Kayani2, J. Dickson3, C. Stacey4, C. 
O'Meara2, S. Short1, N. Fersht1 
1UCLH NHS Foundation Trust, Radiotherapy, London, United Kingdom  
2UCLH NHS Foundation Trust, Nuclear Medicine and Radiology, 
London, United Kingdom  
3UCLH NHS Foundation Trust, Nuclear Medicine, London, United 
Kingdom  
4UCLH NHS Foundation Trust, Radiotherapy Physics, London, United 
Kingdom  
Purpose/Objective: PET/CT aids target volume definition for many 
tumour sites including meningiomas. Recently simultaneous PET/MR 
(mMR) imaging has become available, but applications in radiotherapy 
planning are undefined. We performed a feasibility study using a 
Siemens Biograph 3T mMR for radiotherapy planning in meningiomas 
using 68Ga-DOTATATE as PET tracer (binds to somatostatin receptors) 
and gadolinium (Gd) enhanced MRI. 
Materials and Methods: Two phantoms (bespoke and Lucy) were 
scanned to assess image distortion and accuracy of mMR co-
registration to planning CT. A customised flat acrylic baseboard 
compatible for mMR and CT was designed and manufactured in-house. 
Body surface coils were used in conjunction with the patient’s 
thermoplastic (TP) radiotherapy head shell. A TP bridge was fitted 
over the shell to support the body surface coil. A healthy volunteer 
underwent mMR scan in the TP shell to establish tolerability (60cm 
bore scanner), shell durability(3T MRI) and anatomic clarity.  
4 patients with meningiomas in different regions underwent mMR 
followed by PET/CT for radiotherapy planning.  
mMR imaging protocol: Part 1 without TP shell ( non-contrast T2 & 
diffusion = 35 mins), attenuation correction, immediately followed by 
Part 2 with TP shell (T1 -/+ Gd), DCEMRI, simultaneous PET images = 
15 minutes). 68Ga-DOTA (100MBq median) was injected prior to Part 1 
to allow 40 minutes uptake before PET imaging in Part 2 (approx 
50mins total in scanner). Patients had PET/CT scan approx 20 mins 
after mMR.  
We assessed patient tolerability, software compatibility, biological 
target volume(BTV) on mMR and PET/CT, co-registration and PET SUV 
with and without the shell. 
Results: Phantom work showed that co-registration and image 
distortion resulted in <1mm uncertainties in all regions assessed. The 
shell and baseboard were compatible with mMR, but attenuated PET 
signal 2.5-19% (increasing attenuation from anterior to posterior 
location). However, the shell increased co-registration accuracy (and 
would permit co-registration in the neck) and did not affect BTV 
delineation. Patients tolerated the protocol.  
mMR images were incompatible with current planning software used 
for PET/CT BTV delineation. A complex solution involving a number of 
different software was developed. BTV was largely the same on mMR 
and PET/CT, but distinguishing tumour from the pituitary was clearer 
on mMR. In PET negative regions where standard MRI/ CT was 
suspicious for tumour, more complex MR sequences (e.g. diffusion) 
can be evaluated on the same slice on mMR, which may increase 
certainty about volume definition. 
Conclusions: It is feasible to use mMR with 68Ga-DOTA and Gd contrast 
for radiotherapy planning, although our current software pathway is 
complex and a simpler solution should be developed. Further study is 
required to establish if mMR permits more accurate BTV delineation. 
Use of mMR may reduce the overall numbers of scans as separate MRI 
is not required and there is potential for post-RT PET research. 
Scanning in shell improves co-registration but attenuates PET SUV. 
    
PD-0595   
Impact of PET reconstruction algorithm on dose painting strategies 
I. Skjei Knudtsen1, W. van Elmpt2, M. Öllers2, D. de Ruysscher3, E. 
Malinen4 
1Department of Physics, University of Oslo, Oslo, Norway  
2Department of Radiation Oncology (MAASTRO), Maastricht University 
Medical Centre, Maastricht, The Netherlands  
3Department of Radiation Oncology, University Hospital Leuven, 
Leuven, Belgium  
4Department of Medical Physics, Oslo University Hospital, Oslo, 
Norway  
 
Purpose/Objective: High FDG-uptake within a lung tumor is 
hypothesized to be an indicator of treatment resistance. Current 
treatment planning techniques are able to design dose distributions 
according to the FDG intensity within tumors, i.e. dose-painting 
strategies. Since the apparent FDG distribution in the PET image is 
dependent on PET reconstruction parameters, these parameters also 


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PD-0594: The introduction of simultaneous PET/MRI to radiotherapy planning

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