OBJECT: The authors report different MRI patterns in patients with essential 
tremor (ET) or obsessive-compulsive disorder (OCD) after transcranial MR-guided 
focused ultrasound (MRgFUS) and discuss possible causes of occasional MRgFUS 
failure.
METHODS: Between March 2012 and August 2013, MRgFUS was used to perform 
unilateral thalamotomy in 11 ET patients and bilateral anterior limb capsulotomy 
in 6 OCD patients; in all patients symptoms were refractory to drug therapy. 
Sequential MR images were obtained in patients across a 6-month follow-up 
period.
RESULTS: For OCD patients, lesion size slowly increased and peaked 1 week after 
treatment, after which lesion size gradually decreased. For ET patients, lesions 
were visible immediately after treatment and markedly reduced in size as time 
passed. In 3 ET patients and 1 OCD patient, there was no or little temperature 
rise (i.e., < 52°C) during MRgFUS. Successful and failed patient groups showed 
differences in their ratio of cortical-to-bone marrow thickness (i.e., skull 
density).
CONCLUSIONS: The authors found different MRI pattern evolution after MRgFUS for 
white matter and gray matter. Their results suggest that skull characteristics, 
such as low skull density, should be evaluated prior to MRgFUS to successfully 
achieve thermal rise.