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Initial steps toward MRI-based treatment planning for external radiotherapy

Thörnqvist, Sara (2009)
Medical Physics Programme
Abstract
"Introduction; The aim of this study was to examine the possibilities for MRI-based radiotherapy treatment planning. The work included evaluation of geometric image distortions in MR-images, investigations of how segmentation of bone could be performed from images acquired with the ultra short time of echo (UTE) pulse sequence and assessment of dosimetric differences between treatment planning based on these images as compared to the original CT-based treatment plans.

Methods: A phantom with a well-defined rod geometry was used for the studying of geometric distortions in the imaging plane in MR-images. The ability of the UTE pulse sequence to visualize bone and other tissues with short T2-relaxation, was investigated by imaging the... (More)
"Introduction; The aim of this study was to examine the possibilities for MRI-based radiotherapy treatment planning. The work included evaluation of geometric image distortions in MR-images, investigations of how segmentation of bone could be performed from images acquired with the ultra short time of echo (UTE) pulse sequence and assessment of dosimetric differences between treatment planning based on these images as compared to the original CT-based treatment plans.

Methods: A phantom with a well-defined rod geometry was used for the studying of geometric distortions in the imaging plane in MR-images. The ability of the UTE pulse sequence to visualize bone and other tissues with short T2-relaxation, was investigated by imaging the head of four healthy volunteers and bone, air and soft tissues were segmented from these images. The dosimetric accuracy expected from such segmented images, as well as from images where the head was treated as one homogenous volume, was compared to the gold standard of using CT-images. The dosimetric evaluation involved nine patients with existing conformal treatment plans and three patients with existing intensity modulated treatment plans.

Results: The geometric distortions in the imaging plane were in most cases less than the 2 mm demand in radiotherapy within the investigated volume and with the vendor’s distortion correction applied. Segmentation was feasible using the UTE pulse sequence although some areas were misclassified. For conformal radiotherapy, dose calculations based on the segmented images showed a very close resemblance to the original CT-based plans in regard to dose volume histogram (DVH), dose to planning target volume (PTV) and dose to organ at risk (OAR) although approximating the head as one homogenous volume gave clinically acceptable deviations as well. This trend was also seen for the intensity modulated radiotherapy (IMRT) plans with the same optimization as in the original treatment plan however, making a new optimization based on the derived plans resulted in large deviations in DVH and dose to OAR, possibly a result from misclassifications in the segmentation.

Conclusion: Adding the UTE pulse sequence to the T1W- and the T2W-pulse sequences used when imaging patients obviously result in a prolonged scan time (of about 10 min) but has the advantage of making a more accurate treatment planning possible and could lead to MRI being used even in more complicated cases however, more patients are needed for evaluation. To completely fulfill the aim of this study, scanning a patient with both modalities is necessary since it could reveal the performance of the segmentation method and enable a dosimetric evaluation of the MRI-images to be conducted." (Less)
Abstract (Swedish)
En ofta använd behandlingsmetod mot cancer är strålbehandling. Inför en sådan behandling undersöks patienten ofta med en typ av röntgenundersökning som kallas datortomografi (eng. computed tomomgraphy, CT). Röntgenbilderna visar hur mycket av strålningen som absorberas av kroppen, vilket innebär att man tydligt ser skillnad mellan ben, som absorberar mer av strålningen än vävnad och luft, som strålningen passerar lättare. Olika organ, som t ex urinblåsa och tarmar, är dock svårare att särskilja. Att CT-bilder utnyttjas beror framförallt på att samma fysiska fenomen används vid strålbehandling vilket bl.a. utnyttjas för att beräkna stråldosen till tumören i ett dosplaneringssystem. Det finns andra medicinska bilder där mjukvävnad framträder... (More)
En ofta använd behandlingsmetod mot cancer är strålbehandling. Inför en sådan behandling undersöks patienten ofta med en typ av röntgenundersökning som kallas datortomografi (eng. computed tomomgraphy, CT). Röntgenbilderna visar hur mycket av strålningen som absorberas av kroppen, vilket innebär att man tydligt ser skillnad mellan ben, som absorberar mer av strålningen än vävnad och luft, som strålningen passerar lättare. Olika organ, som t ex urinblåsa och tarmar, är dock svårare att särskilja. Att CT-bilder utnyttjas beror framförallt på att samma fysiska fenomen används vid strålbehandling vilket bl.a. utnyttjas för att beräkna stråldosen till tumören i ett dosplaneringssystem. Det finns andra medicinska bilder där mjukvävnad framträder tydligare än i CT:n, så är fallet med magnetresonanskameran (MR). Här genereras bilden genom att protoner i olika vävnader och organ samverkar med magnetfält istället för strålning. Med MR-bilder kan cancertumörers utsträckning och riskorgan ses tydligare. Friska organ, vilka utgör sk riskorgan då de på grund av deras läge, i förhållande till tumören, påverkas av behandlingen genom att de också kan få en betydande stråldos. Läkarna använder ofta både bilder från CT och MR när de bestämmer storleken på tumör och riskorgan. MR-bilderna saknar information om hur strålarna absorberas men det kan dock lösas genom att segmentera dessa bilder dvs att man delar upp bilden i olika mindre områden utifrån dess information.

I mitt examensarbete undersöktes en viss typ av MR-bilder som inte tidigare använts vid universitetssjukhuset i Lund, sk ultra short time of echo (UTE). Denna metod gör att områden med ben och luft kan särskiljas, i motsats till vanliga MR-bilder där sådana områden inte har någon eller låg signal. Detta gör det omöjligt, att utifrån bilderna, särskilja dem åt. Skillnaden i hur mycket strålning som absorberas är dock stor mellan luft och ben vilket troligtvis påverkar stråldosberäkningen. I UTE-bilder från bildtagning av fyra friska frivilligaförsökspersoners huvud, har ben, luft och mjukvävnad segmenterats. Segmenteringen fungerar bra men vissa problem uppstår vid tunna ben och vid området utanför skallen. För att jämföra hur stor skillnad i dos som kan fås från segmenterade bilder jämfört CT-bilder gjordes segmenteringen utav ben, luft och mjukvävnad även i CT-bilder. De segmenterade områdena i CT-bilderna tilldelades därefter ett värde som relaterar till hur mycket av strålningen som absorberas vilket möjliggör en dosberäkning. Resultaten visar att segmentering i huvudet gav dosvärden nära dem från de icke-segmenterade CT-bilderna för enklare behandlingar men för mer komplicerade behandlingar avvek de beräknade stråldoserna mer. I det sistnämnda fallet bör fler patienter inkluderas för att dra fullständiga slutsatser. För att undersöka hur väl segmenteringen fungerar och hur noggrant dosen kan beräknas med UTE-bilderna krävs att en patient genomgår både en MR- och en CT-undersökning samt att dosplaneringssystemet accepterar enbart MR-bilder. UTE-bilderna verkar dock mycket lovande för att möjliggöra noggranna dosberäkningar. (Less)
Please use this url to cite or link to this publication:
author
Thörnqvist, Sara
supervisor
organization
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Strålterapi
language
English
id
2157097
date added to LUP
2011-09-14 12:02:35
date last changed
2013-09-05 10:24:10
@misc{2157097,
  abstract     = {"Introduction; The aim of this study was to examine the possibilities for MRI-based radiotherapy treatment planning. The work included evaluation of geometric image distortions in MR-images, investigations of how segmentation of bone could be performed from images acquired with the ultra short time of echo (UTE) pulse sequence and assessment of dosimetric differences between treatment planning based on these images as compared to the original CT-based treatment plans.

Methods: A phantom with a well-defined rod geometry was used for the studying of geometric distortions in the imaging plane in MR-images. The ability of the UTE pulse sequence to visualize bone and other tissues with short T2-relaxation, was investigated by imaging the head of four healthy volunteers and bone, air and soft tissues were segmented from these images. The dosimetric accuracy expected from such segmented images, as well as from images where the head was treated as one homogenous volume, was compared to the gold standard of using CT-images. The dosimetric evaluation involved nine patients with existing conformal treatment plans and three patients with existing intensity modulated treatment plans.

Results: The geometric distortions in the imaging plane were in most cases less than the 2 mm demand in radiotherapy within the investigated volume and with the vendor’s distortion correction applied. Segmentation was feasible using the UTE pulse sequence although some areas were misclassified. For conformal radiotherapy, dose calculations based on the segmented images showed a very close resemblance to the original CT-based plans in regard to dose volume histogram (DVH), dose to planning target volume (PTV) and dose to organ at risk (OAR) although approximating the head as one homogenous volume gave clinically acceptable deviations as well. This trend was also seen for the intensity modulated radiotherapy (IMRT) plans with the same optimization as in the original treatment plan however, making a new optimization based on the derived plans resulted in large deviations in DVH and dose to OAR, possibly a result from misclassifications in the segmentation.

Conclusion: Adding the UTE pulse sequence to the T1W- and the T2W-pulse sequences used when imaging patients obviously result in a prolonged scan time (of about 10 min) but has the advantage of making a more accurate treatment planning possible and could lead to MRI being used even in more complicated cases however, more patients are needed for evaluation. To completely fulfill the aim of this study, scanning a patient with both modalities is necessary since it could reveal the performance of the segmentation method and enable a dosimetric evaluation of the MRI-images to be conducted."},
  author       = {Thörnqvist, Sara},
  keyword      = {Strålterapi},
  language     = {eng},
  note         = {Student Paper},
  title        = {Initial steps toward MRI-based treatment planning for external radiotherapy},
  year         = {2009},
}