T1/T2 Imaging Improves Lesion Visualization After MRgFUS
Technique enhances follow-up assessment of chronic lesions often unseen on T2 imaging
T1/T2-weighted ratio imaging offers substantial improvements over T2-weighted imaging for visualizing lesions created by MR-guided focused US (MRgFUS) in the brains of patients with tremors, according to a study in Radiology.
The technique could improve how clinicians assess treatment in patients with this common neurological disorder.
“The T1/T2-weighted ratio provides a way to better visualize chronic MRgFUS-induced lesions by amplifying the contrast that the standard T2 sequence does not sufficiently capture,” said study lead author Carlos A. Sanchez-Catasus, PhD, from Clinica Universidad de Navarra in Pamplona, Spain.
For patients with tremor disorders, that visibility gap can be clinically significant.
Medical literature indicates that essential tremors (ET) happen during actions such as using utensils to eat and are significantly more prevalent than those related to tremor-dependent Parkinson’s Disease (TDPD).
Patients with more serious cases that are resistant to standard medications may undergo thalamotomy, a treatment that uses MRgFUS to create a precise thermal lesion in the thalamus, the brain area linked with tremor-causing circuits. The lesion disrupts these circuits, offering immediate symptom relief with high success rates.
But understanding treatment durability depends on follow-up imaging.
“Imaging lesions after treatment is important for knowing areas to target if the patient has a recurrence of symptoms,” Dr. Sanchez-Catasus said. “It’s also useful for monitoring the expected physiological tissue response and tracking the natural evolution of the targeted area over time.”
Limitations of T2 Imaging in Long-Term Follow Up
Despite its importance in follow up, T2-weighted MRI, the standard for visualizing MRgFUS thalamotomy lesions, has limitations in long-term assessment, becoming particularly critical to evaluate when there is a recurrence of symptoms.
Once the edema from the procedure resolves at around six months after treatment, the lesion tends to be difficult to see on the T2-weighted image. This makes it difficult, if not impossible, to correlate symptoms with the lesion location.
To address this loss of visibility, T1/T2-weighted ratio enhances contrast in the image by dividing the T1-weighted scan by the T2-weighted one.
In the study, Dr. Sanchez-Catasus and colleagues compared this method with T2-weighted imaging in 54 patients, including 27 with ET and 27 with TDPD.
Lesion normalized contrast was higher for T1/T2-weighted ratio images than for T2-weighted images. “We found similar results in patients with tremor recurrence,” Dr. Sanchez-Catasus said. “In addition, the T1/T2-weighted ratio imaging had higher between-evaluator agreement than T2-weighted imaging.”
These advantages may be especially valuable in patients with recurrent tremors, where lesion visibility is critical for reassessment.
Looking ahead, Dr. Sanchez-Catasus and colleagues plan to investigate whether the T1/T2-weighted ratio allows visualization of chronic lesions that are not visible in the T2 sequence. It will provide insights into tremor recurrence.
“Some studies show that at one year, approximately half of patients show no discernible signal abnormalities on the T2 sequence, which is the current standard for follow-up after MRgFUS,” Dr. Sanchez-Catasus said. “It is of great interest to reveal the lesion trace that standard T2 scans may miss, to obtain a quantitatively superior view of the ablation site.”
In patients with recurrent tremors, invisible T2 findings create uncertainty about whether recurrence reflects inadequate initial treatment, Dr. Sanchez-Catasus said. If the T1/T2-weighted ratio makes the invisible lesion “reappear,” it could finally confirm whether the targeting was correct.
“It would allow us to determine whether the disappeared lesion was simply too small or biologically soft, lacking the permanent architectural change necessary for long-term relief,” he said.
Tackling Variability in T1/T2 Imaging
While T1/T2 ratio imaging is not a new technology, it has been plagued with a lack of homogeneity stemming from the use of different scanners and the appearance of different artifacts on individual T1 and T2 scans, according to Nahyun Celina Jo, MD, a neuroradiology fellow at Johns Hopkins University in Baltimore.
In a commentary accompanying the study, Dr. Jo praised the authors for using lesion normalized contrast, defined as the signal intensity of the lesion on six-month follow-up scans normalized to a reference region on pretreatment imaging, to evaluate post-MRgFUS thalamotomy lesions.
“Using this T1-over-T2 weighted image makes it so much easier to visualize the lesion that was treated before,” she said.
“One of the advantages of the T1/T2-weighted image set is that you can extract the information from the T1 and the T2 maps that we routinely obtain,” added Max Wintermark, MD, MBA, professor and the chair of neuroradiology at the University of Texas MD Anderson Center in Houston. “It’s a simple approach that seems to be beneficial.”
With only 54 patients in the retrospective study, larger cohort prospective studies are needed.
Dr. Sanchez-Catasus also would like to see a comparative analysis with susceptibility-weighted imaging, noting that some studies suggest this sequence can detect more persistent lesions.
“Susceptibility-weighted imaging could provide additional complementary information,” Dr. Sanchez-Catasus said. “This issue should be addressed in future studies.”
For More Information
Access the Radiology study, “Advancing Chronic Lesion Assessment After MRI-guided Focused Ultrasound Using T1/T2-weighted Ratio Maps,” and the related editorial, “T1/T2-weighted Ratio to Improve MRI-guided Focused Ultrasound-induced Chronic Lesion Contrast in Patients with Tremor Syndrome.”
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