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Indirect detection of myelin water by T2-relaxation during the RF pulse

Al-Abasse, Yosef (2015) MSFT01 20151
Medical Physics Programme
Medical Radiation Physics, Lund
Abstract
Introduction: The axonal myelin sheath is the main cause of magnetic resonance imaging (MRI) contrast between gray matter (GM) and white matter (WM) in the brain. It encloses a small pool of myelin water (MW) with a short T2 of about 15 ms. Common signal equations of MRI sequence assume an instantaneous RF excitation pulse followed by free relaxation. As known from ultra-short echo time (UTE) MRI, deviations from this signal behavior may occur when the relaxation times are shorter than or of the same order of magnitude as the duration of radio frequency (RF) excitation pulse. In this MSc thesis project, it was studied whether such transverse in-pulse relaxation effects can be used to detect MW by increasing the RF pulse duration to the... (More)
Introduction: The axonal myelin sheath is the main cause of magnetic resonance imaging (MRI) contrast between gray matter (GM) and white matter (WM) in the brain. It encloses a small pool of myelin water (MW) with a short T2 of about 15 ms. Common signal equations of MRI sequence assume an instantaneous RF excitation pulse followed by free relaxation. As known from ultra-short echo time (UTE) MRI, deviations from this signal behavior may occur when the relaxation times are shorter than or of the same order of magnitude as the duration of radio frequency (RF) excitation pulse. In this MSc thesis project, it was studied whether such transverse in-pulse relaxation effects can be used to detect MW by increasing the RF pulse duration to the range of the MW T2. By such an approach, the in-pulse relaxation effects, which alter the MRI signal, would arise mainly from MW.

Material and methods: By numerical integration of the Bloch equations, in-pulse relaxation effects on the magnetization were studied for rectangular (RECT), Gaussian and sinc-shaped (SINC) pulses of 10 ms pulse duration in order to find the optimal RF pulse for imposing different degrees of saturation onto the longitudinal magnetizations of MW and intra–/extra- axonal water with optimal excitation profile, i.e., minimal degradation in the signal intensities. The effect of longitudinal relaxation during the RF pulse was neglected because T1 of MW and intra– and extra-axonal water (IE-water) is much longer than the duration of the RF pulse. The fast low angle shot (FLASH) pulse sequence of a 3 T MR scanner (Siemens Magnetom Skyra) was modified to provide two pulse durations of 0.5 ms and 10 ms using a Gaussian RF pulse. FLASH MRI at variable flip angles was carried out on three cream phantoms (12 %, 27 % and 40 % fat content), a formalin-fixated pig brain and a healthy volunteer. The measurement with short pulse duration served as a reference to the difference in apparent T1.

Results: The simulations indicated that in-pulse relaxation would result in a reduced partial saturation of MW magnetization which is largely independent on flip angle and amounting to between 6 % (SINC) and 21 % (RECT). A Gaussian shape (11 % reduced partial saturation) was implemented experimentally, as this shape was less sensitive to frequency offsets, due to the shape of the excitation profile, than the RECT. With the long pulse, the apparent T1 was about 25 % shorter in WM and 10 % shorter in GM, for both the fixated brain and in vivo, and the effects were thus much larger than expected from the simulations. The spatial distribution of the T1 reduction showed more pronounced reduction in the WM, where MW is localized. The simulations for the cream phantoms indicated a halving of the in-pulse relaxation effects compared with the MW and IE-water in vivo, and thus no mapping of the apparent T1 was performed.

Conclusion: The reference measurement was most likely affected by magnetization transfer (MT) effects from macromolecules, for which saturation is also influenced by RF pulse duration. Thus, the shorter the pulse duration, the more the MT effects are pronounced. Compared with in-pulse relaxation effects, the MT effects seemed to be dominating, and further
studies are needed to separate these effects. (Less)
Popular Abstract (Swedish)
Magnetkameran är en icke-invasiv metod som används för att avbilda mjukvävnad i kroppen med hög spatial upplösning. Myelinskidan som omger axonerna i det centrala nervsystemet ger upphov till kontrasten mellan vit och grå substans i konventionella magnetkamerabilder. Det är av intresse att detektera nedbrytningen av myelinskidan eftersom förekomst av sådan kan ge en indikation på tidigare stadier av den neurologiska sjukdomen multipel skleros (MS). Idag finns det utmaningar inom magnetresonansfysiken (MR-fysiken) vid direkt detektering av myelinskidan eftersom signalen från den avtar på några få mikrosekunder. En radiofrekvent (RF) puls används för att flippa ned magnetiseringsvektorn till det transversella planet och därmed generera MR... (More)
Magnetkameran är en icke-invasiv metod som används för att avbilda mjukvävnad i kroppen med hög spatial upplösning. Myelinskidan som omger axonerna i det centrala nervsystemet ger upphov till kontrasten mellan vit och grå substans i konventionella magnetkamerabilder. Det är av intresse att detektera nedbrytningen av myelinskidan eftersom förekomst av sådan kan ge en indikation på tidigare stadier av den neurologiska sjukdomen multipel skleros (MS). Idag finns det utmaningar inom magnetresonansfysiken (MR-fysiken) vid direkt detektering av myelinskidan eftersom signalen från den avtar på några få mikrosekunder. En radiofrekvent (RF) puls används för att flippa ned magnetiseringsvektorn till det transversella planet och därmed generera MR signalen. I konventionell signalbeskrivning antas att RF-pulsen appliceras momentant, och är direkt följd av fri relaxation. MR-signalen som erhållits med en konventionell (d.v.s. kort) RF-puls är inte selektivt känslig för myelin-vatten utan består av signal från både myelin-vatten och intra– och extra cellulärt vatten (IE-vatten). Dessutom är valet av RF-pulsen avgörande, eftersom olika RF-pulser innehar olika egenskaper som potentiellt kan användas för att urskilja de två populationerna. I detta arbete förlängdes RF- pulsen till 10 ms för att kunna separera dessa två populationer genom skillnader i den transversella relaxationen, T2. Myelin-vattnet relaxerar snabbare (T2,myelin vatten ≈ 15 ms) än IE- vatten (T2,IE-vatten ≈ 80 ms). Som en följd av detta kan relaxationseffekterna under den långa RF- pulsen inte försummas, vilket leder till att den klassiska signalekvationen måste modifieras. Effekter från den longitudinella relaxationen försummades eftersom T1 antogs vara mycket längre än RF-pulsen. Som referens användes mätning med kort RF-puls (0,5 ms) i denna studie. För att erhålla robusta mätresultat genererades en skenbar T1-karta med den långa RF-pulsen, och denna jämfördes sedan med motsvarande skenbara T1-karta för den korta RF-pulsen. Den resulterande bilden avslöjade var den största skillnaden mellan de skenbara T1-kartorna fanns, och denna skillnad antogs bero på relaxationseffekter hos myelin-vattnet.

Den ovan beskrivna metoden användes i detta arbete med en Gaussisk puls som testades på tre olika gräddfantom med olika fetthalter (12 %, 27 % och 40 %), på formalinfixerad grishjärna samt i en volontär. Den resulterade differensen i T1,skenbart mellan kort och lång puls visade en observerad effekt på 25 % i vit substans och 10 % i grå substans. Denna effekt var mycket högre än vad simuleringar av Bloch-ekvationerna antydde (d.v.s. 11 %). Magnetiseringsöverföringseffekter (magnetization transfer, MT) från makromolekyler influerade troligen mätningen med den korta RF-pulsen. MT-effekterna blir större ju kortare RF-pulsen blir. Således beror MT-effekterna på pulslängden och detta studerades inte i denna studie. Detta medförde att den stora observerade effekten på differensen i T1,skenbart troligen dominerades av MT. Det krävs därför ytterligare studier, med varierad pulslängd, för att kunna
separera dessa två effekter. (Less)
Please use this url to cite or link to this publication:
author
Al-Abasse, Yosef
supervisor
organization
course
MSFT01 20151
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
7865561
date added to LUP
2015-09-13 12:18:31
date last changed
2017-01-09 16:32:25
@misc{7865561,
  abstract     = {Introduction: The axonal myelin sheath is the main cause of magnetic resonance imaging (MRI) contrast between gray matter (GM) and white matter (WM) in the brain. It encloses a small pool of myelin water (MW) with a short T2 of about 15 ms. Common signal equations of MRI sequence assume an instantaneous RF excitation pulse followed by free relaxation. As known from ultra-short echo time (UTE) MRI, deviations from this signal behavior may occur when the relaxation times are shorter than or of the same order of magnitude as the duration of radio frequency (RF) excitation pulse. In this MSc thesis project, it was studied whether such transverse in-pulse relaxation effects can be used to detect MW by increasing the RF pulse duration to the range of the MW T2. By such an approach, the in-pulse relaxation effects, which alter the MRI signal, would arise mainly from MW.

Material and methods: By numerical integration of the Bloch equations, in-pulse relaxation effects on the magnetization were studied for rectangular (RECT), Gaussian and sinc-shaped (SINC) pulses of 10 ms pulse duration in order to find the optimal RF pulse for imposing different degrees of saturation onto the longitudinal magnetizations of MW and intra–/extra- axonal water with optimal excitation profile, i.e., minimal degradation in the signal intensities. The effect of longitudinal relaxation during the RF pulse was neglected because T1 of MW and intra– and extra-axonal water (IE-water) is much longer than the duration of the RF pulse. The fast low angle shot (FLASH) pulse sequence of a 3 T MR scanner (Siemens Magnetom Skyra) was modified to provide two pulse durations of 0.5 ms and 10 ms using a Gaussian RF pulse. FLASH MRI at variable flip angles was carried out on three cream phantoms (12 %, 27 % and 40 % fat content), a formalin-fixated pig brain and a healthy volunteer. The measurement with short pulse duration served as a reference to the difference in apparent T1.

Results: The simulations indicated that in-pulse relaxation would result in a reduced partial saturation of MW magnetization which is largely independent on flip angle and amounting to between 6 % (SINC) and 21 % (RECT). A Gaussian shape (11 % reduced partial saturation) was implemented experimentally, as this shape was less sensitive to frequency offsets, due to the shape of the excitation profile, than the RECT. With the long pulse, the apparent T1 was about 25 % shorter in WM and 10 % shorter in GM, for both the fixated brain and in vivo, and the effects were thus much larger than expected from the simulations. The spatial distribution of the T1 reduction showed more pronounced reduction in the WM, where MW is localized. The simulations for the cream phantoms indicated a halving of the in-pulse relaxation effects compared with the MW and IE-water in vivo, and thus no mapping of the apparent T1 was performed.

Conclusion: The reference measurement was most likely affected by magnetization transfer (MT) effects from macromolecules, for which saturation is also influenced by RF pulse duration. Thus, the shorter the pulse duration, the more the MT effects are pronounced. Compared with in-pulse relaxation effects, the MT effects seemed to be dominating, and further
studies are needed to separate these effects.},
  author       = {Al-Abasse, Yosef},
  language     = {eng},
  note         = {Student Paper},
  title        = {Indirect detection of myelin water by T2-relaxation during the RF pulse},
  year         = {2015},
}