eEdE-159-6797
Magnetic resonance guided laserinduced thermal ablation therapy:
a visual review of key concepts
Authors: Poletto D1, Vale F2, Murtagh R1
University of South Florida Morsani College of Medicine,
Department of Radiology1; Department of Neurosurgery2
Disclosures
o The authors have no conflicts of interest to
disclose.
Purpose
o The use of lasers for thermal ablation of
brain lesions has recently become a viable
option
• largely due to the development of real-time,
intra-operative thermal magnetic resonance
imaging (MRI)
o As MR guided laser thermal ablation therapy
(MR-LiTT) becomes more widely practiced, the
radiologist should become familiar with several
key concepts of this therapy
Approach/Methods
o Review procedure indications and pre-operative
imaging needed for surgical planning
o Discuss the procedure itself, focusing on intraoperative, real time imaging
o Examine post-operative imaging findings and
potential complications
• Teaching points will be illustrated with images obtained
using the Visualase Thermal Therapy System (Medtronic
Neurosurgery, Louisville, CO) at a large tertiary care
facility
Findings/Discussion
Laser Induced Thermal Therapy
o 980 nm diode laser and
coaxial applicator system
• laser supplies an optical fiber with
a 1 cm long light diffusing tip
• light distribution shape is
cylindrical to ellipsoid
• absorption of light photons by
tissue causes coagulation necrosis
• fiber surrounded by catheter
1.65mm in diameter
• sterile saline infused through
catheter for cooling
Figure 1. Disposable coaxial laser applicator.
(Medtronic Neurosurgery, Louisville, CO)
Laser Induced Thermal Therapy
o Laser ablation controlled
through computer workstation
• houses 15 W laser generator
• software integrates with standard MRI
• displays thermal maps for real time,
intra-operative monitoring
• thermal maps can be used to create
irreversible damage estimates
• allows user to set temperature limits
that cause deactivation of laser if
exceeded
Figure 2. Workstation with computer interface
and laser generator. (Medtronic Neurosurgery,
Louisville, CO)
Procedure Indications
o FDA approved to coagulate/necrotize soft tissue under MRI
guidance in neurosurgery
o Current uses include 1-5:
•
•
•
•
•
•
primary or metastatic tumors, usually smaller lesions
recurrence despite prior resection, chemotherapy, radiation
high risk surgical candidates
surgical inaccessibility of tumor
radiation necrosis
epileptogenic foci
Figure 3. MRI images
from two different
candidates for MR-LiTT
(1.5 T Phillips Acheiva)
Left: T1 post contrast of
right frontoparietal
glioblastoma multiforme
Right: T2 of left mesial
temporal lobe sclerosis.
Pre-procedural Imaging
o stereotactic MRI brain with gadolinium
• performed after stereotactic frame placement on patient
• volumetric sequences allow for laser trajectory planning and
determination of frame, arc settings
• alternatively can perform stereotactic non-contrast computed
tomography (CT) and fuse with prior MRI, or use optical navigation
software
Video 1. Click for
video of pre-operative,
non-contrast CT (Philips
Brilliance 16P, 120 kV,
138 mAs, slice
thickness =1.00mm)
showing components of
the stereotactic frame
placed prior to laser
ablation on a patient
with mesial temporal
lobe sclerosis.
Laser Applicator Placement
o Stereotactic arc attached to frame
• enables laser catheter placement at previously determined coordinates
• optimal laser trajectory is along long axis of the lesion, dividing the
lesion into equal parts
o Stab incision, twist drill hole through skull, dura puncture
o Laser applicator system inserted into brain and secured with
custom bone anchor
Figure 4. Graphic
depicting configuration of
stereotactic arc and frame
with guiding device.
Cranial bone anchor is
placed and laser
introduced through the
anchor. Frame/arc then
removed (Medtronic
Neurosurgery, Louisville,
CO).
Final Pre-treatment Planning
o Patient transferred to MRI
suite, placed in scanner
o 3 dimensional T1 GRE
sequence acquired
• check applicator placement,
choose plane(s) for ablation
monitoring
• plane should contain whole
applicator and lesion
o Test pulse applied (3-4W,
<60 seconds) by laser
Figure 5. Applicator placement into mesial
temporal lobe confirmed with gradient echo T1
sequence. Applicator appears as a linear
hypointense artifact caused by air within the outer
catheter.
• confirm adequate coverage
of lesion by ablation zone
Thermal Ablation
o Outer catheter primed with
saline
• cools fiber and surrounding
tissue to prevent tissue
carbonization
o Temperature limits set by user
• placed near adjacent critical
structures to prevent damage
• 1-3 points, < 50° C
o Treatment dose: 10-15W, 30180 seconds per ablation
Figure 6. Light diffusing tip of
laser applicator system (Medtronic
Neurosurgery, Louisville, CO).
o Number of ablations dependent
on size of lesion
• can retract optical fiber inside
cooling catheter (mm)
Intra-operative Monitoring
o Magnetic resonance thermal
imaging (MRTI)
o Dynamic thermal maps
• color-coded fast spoiled
gradient echo sequence (SPGR)
• 5 seconds per acquisition, run
repeatedly for real-time
monitoring
o Thermal imaging achieved by
shifts in proton resonance
frequency 6
Figure 7. Color coded thermal map showing
ablation of right amygdala and hippocampus in
a patient with medial temporal lobe epilepsy
(MTLE). Treatment included 3 ablations at
12W, for approximately 180 seconds each.
• linear relationship to
temperature due to
temperature dependent
alterations in hydrogen bonds
of H20
Intra-operative Monitoring
Video 2. Click for video of intra-operative thermal imaging (SPGR, FOV = 220 mm,
matrix 256 x 256, TE = 20 ms, TR = 20 ms, flip angle 20°) from the same patient.
Video is without color overlay to show hypointense signal in ablation zone, caused by
temperature dependent prolongation of T1 relaxation 7.
Intra-operative Monitoring
o Irreversible damage
estimates
• Color-coded images created
using an Arrhenius model of
cell death
• mathematical formula based
on the time and temperature
dependence of protein
denaturation 2
• calculated per voxel
o Thermal maps and damage
estimates superimposed on
original T1 images for anatomic
reference
Figure 8. Irreversible damage estimate in
the same patient, generated from dynamic
thermal maps, showing total ablation zone
in orange.
o Procedure concluded when
irreversible damage estimate
covers target area
Immediate Post-procedure Imaging
o T1 post gadolinium (Gd)
sequence
• Performed prior to removal of
laser applicator
• confirm adequate size of total
ablation zone
• zone contained within an
enhancing rim
• represents irreversibly
damaged tissue with a rim of
deoxyhemoglobin 7-8
o Followed by return to
operating room for applicator
removal or additional laser
ablations
Figure 9. Immediate post-procedure T1+ Gd (1.5
T Philips Achieva) sequence showing enhancing
rim of ablation zone surrounding the laser
applicator.
Post-operative Care
o Patients admitted for overnight observation
o Discharge often possible the following day
o Post-operative steroid taper and seizure
prophylaxis
o Potential complications:
•
•
•
•
•
damage to adjacent structures
hemorrhage
worsening edema
short term memory loss
seizure
Post-operative Imaging
o Day 1 post-operative
• T1 shows central
hyperintensity with
hypointense rim; represents
methemoglobin produced by
coagulation necrosis
surrounded by peripheral
edema 7-8
• corresponding reversal of this
pattern on T2, FLAIR
Figure 10. Click to view a day 1 post-operative MRI
after ablation for MTLE showing the changes listed
above (1.5T Philips Achieva; T1, T2, FLAIR, DWI, ADC).
• axonal swelling at periphery
of ablation may account for rim
of restricted diffusion 7
Post-operative Imaging
o Day 90 post-operative
• T1 shows decreased central
hyperintensity; T1 + Gd shows
decreasing peripheral
enhancement
• decreasing edema on T2,
FLAIR sequences
• resolution of restricted
diffusion
Figure 11. Click to view a 3 month post-operative
MRI in a different patient, also after ablation for
MTLE, showing the changes described above (3T GE
HDX; T1, T1+ Gd, T2, FLAIR, DWI, ADC).
• changes begin within one
month of ablation, continue
over 4-6 months 8-9
Post-operative Imaging
o Volume estimates
• track changes in ablation zone
size by measuring volume of
peripherally enhancing lesion on
T1+ Gd
• rapid increase in ablation zone
volume over 24 hour period, often
reaching >200% 4, 10
• slower growth can then occur for
a period of 1-2 weeks
Figure 12. Click to view coronal T1+ Gd MRI
obtained at 24 hrs post, and 6 m post ablation
for MTLE, showing decreasing size of
enhancing rim (3T GE HDX).
• followed by decrease in volume
to near pre-treatment size by 1-6
months 2, 4
Post-operative Imaging
o Assessing for recurrence
• local recurrence of tumor within
the ablation zone
• volumetric changes on T1+Gd
• greatest lesion growth usually
within 24 hrs; suggests 24 hr post
scan, rather than immediate post, is
best baseline 10
• expect slower growth for additional
2 weeks 4
• Signal intensity differences
Figure 13. Click to view a 1 year post operative
MRI following mesial temporal lobe ablation,
showing encephalomalacia, and no appreciable
contrast enhancement (3T GE HDX; T1, T1+Gd,
T2, FLAIR, DWI, ADC)
9
• changes in signal intensity on T1,
T2, GRE, and FLAIR from pre- to
post- scans, for multiple time points
• signal intensity changes were
different between success versus
recurrence at all time points on T1
and T2 GRE sequences
Summary/Conclusion
Time
Pre-operative
Important
MR Sequences
Imaging
Findings
1. Stereotactic MR
including T1+Gd
with volumetric
sequences
1. Adequate lesion
localization to plan
laser trajectory
Intra-operative
1. 3D T1 GRE,
volumetric
2. Fast, color-coded,
spoiled GRE
3. Irreversible damage
map
1. Determine imaging
plane for ablation
2. Dynamic thermal
monitoring
3. Estimate of total
ablation zone
Post-operative
1. T1+Gd at multiple
time points
2. Other sequences
(T2, FLAIR, T2 GRE,
DWI, ADC) at
multiple time points
1. Track changes in
volume of total
ablation zone
2. Track changes in
signal intensity
Summary/Conclusion
o Initial studies of MR guided laser thermal
ablation have generated promising results, and the
therapy is likely to become more widely practiced
o Being familiar with the surgical technique, intraoperative MR sequences, and post-operative
imaging appearance will enable the radiologist to
play a more active role in the application of this
therapy and have a greater impact on patient care.
References
1. Jethwa PR, Barrese JC, Gowda A, Shetty A, Danish SF. Magnetic resonance thermometry-guided laserinduced thermal therapy for intracranial neoplasms: initial experience. Neurosurgery. 2012; 71(1 Suppl
Operative): 133-144.
2. Carpentier A, McNichols RJ, Stafford RJ et al. Real-time magnetic resonance guided laser thermal therapy
for focal metastatic brain tumors. Neurosurgery. 2008; 63(1 Suppl 1): ONS21-28.
3. Curry DJ, Gowda A, McNichols RJ, Wilfong AA. MR-guided stereotactic laser ablation of epileptogenic foci in
children. Epilepsy Behav. 2012; 24(4): 408-414.
4. Rao MS, Hargreaves EL, Khan AJ, Haffty BG, Danish SF. Magnetic resonance guided laser ablation improves
local control for postradiosurgery recurrence and/or radiation necrosis. Neurosurgery. 2014; 74(6): 658-667.
5. Carpentier A, Chauvet D, Reina V et al. MR-guided laser-induced thermal therapy (LITT) for recurrent
glioblastomas. Lasers in Surgery and Medicine. 2012; 44: 361-368.
6. Ishihara Y, Calderon A, Watanabe H et al. A precise and fast temperature mapping using water proton
chemical shift. Magn Reson Med. 1995; 34(6): 814-823.
7. Schulze PC, Vitzthum HE, Goldammer A et al. Laser-induced thermotherapy of neoplastic lesions in the brain
– underlying tissue alterations, MRI-monitoring and clinical applicability. Acta Neurochir. 2004; 146: 803-812.
8. Schwabe B, Kahn T, Harth T, Ulrich F, Schwarzmaier HJ. Laser-induced thermal lesions in the human brain:
short- and long-term appearance on MRI. J Comput Assist Tomogr. 1997; 21(5): 818-825.
9. Tiwari P, Danish S, Madabhushi A. Identifying MRI markers associated with early response following laser
ablation for neurological disorders: preliminary findings. PLOS One. 2014; 9(12): e114293.
10. Patel NV, Jethwa PR, Barrese JC et al. Volumetric trends associated with MRI-guided laser-induced thermal
therapy (LITT)for intracranial tumors. Lasers in Surgery and Medicine. 2013; 45: 362-369.
Acknowledgements
o The authors would like to thank Natalie St. Denis, Lisa Distenfield,
and Anil Shetty of Medtronic Neurosurgery, as well as Brad Fernald of
Synaptive Medical, for providing information and images regarding the
Visualase Thermal Therapy System. We also thank Haydy Rojas for
facilitating the IRB approval process.
Author Information
o Dana Poletto, MD
• Resident, Diagnostic Radiology
University of South Florida Morsani College of Medicine
2 Tampa General Circle, STC 7028
Tampa, FL 33606
• dcruite@health.usf.edu
o Fernando Vale, MD
• Division Chief and Vice Chairman, Department of Neurosurgery
and Brain Repair
University of South Florida Morsani College of Medicine
2 Tampa General Circle
Tampa, FL 33606
• fvale@health.usf.edu
o Ryan Murtagh, MD, MBA
• Associate Professor, Diagnostic Radiology
University of South Florida Morsani College of Medicine
2 Tampa General Circle, STC 7028
Tampa, FL 33606
• rmurtagh@health.usf.edu