Transcranial sonography of the substantia nigra by tissue harmonic imaging

Introduction: Transcranial ultrasound (TCS) of midbrain structures has recently gained in importance in diagnosing parkinsonian disorders. In previous studies, alterations in the echo pattern of the substantia nigra (SN) were defined by conventional B-mode imaging (CBI). However, tissue harmonic imaging (THI) may promise an even better visualization of midbrain parenchyma. This study gives a direct comparison between CBI and THI by depicting the SN-echogenicity.

Methods: 248 subjects aged from 18–83yrs were investigated with a 2.5MHz phased-array probe (Sonoline Elegra, Siemens, Erlangen, Germany) through the temporal bone window. The SN size was planimetrically measured using both CBI and THI. Two investigators defined independently the echogenicity of the SN in each subject.

Results: In 168 subjects, SN size could be measured on both sides (n=336). SN echogenic size defined by CBI and THI significantly correlated (r: 0.7, p=0.01). Examination with THI demonstrated larger area of echogenic SN than measurements with B- mode (mean area±SD: 23,5±7,8mm² vs. 17,2±10,1mm²; p=0.00)

Conclusion: Both CBI and THI may be applied to assess the SN-echogenicity. The present study is the first to demonstrate that the echogenic size of the SN is substantially influenced by the imaging mode itself. The normal range of SN size should be redefined for THI if this new imaging mode will be applied as a diagnostic tool.