Improved visualization of high-intensity focused ultrasound lesions

Abstract

Spectral parameter imaging in both the fundamental and harmonic of backscattered radiofrequency (RF) data was used for immediate visualization of high-intensity focused ultrasound (HIFU) lesion sites. A focused 5-MHz HIFU transducer with a coaxial 9-MHz focused single-element diagnostic transducer was used to create and scan lesions in chicken breast and freshly excised rabbit liver. Bmode images derived from the backscattered RF signal envelope were compared with midband fit (MBF) spectral parameter images in the fundamental (9-MHz) and harmonic (18-MHz) bands of the diagnostic probe. Images of HIFU-induced lesions derived from the MBF to the calibrated spectrum showed improved contrast (~ 3 dB) of tumor margins versus surround compared to images produced from the conventional signal envelope. MBF parameter images produced from the harmonic band showed higher contrast in attenuated structures (core, shadow) compared to either the conventional envelope (3.3 dB core; 11.6 dB shadow) or MBF images of the fundamental band (4.4 dB core; 7.4 dB shadow). The gradient between the lesion and surround was 3.4 dB/mm, 6.9 dB/mm and 17.2 dB/mm for B-mode, MBF-fundamental mode and MBF-harmonic mode respectively. Images of threshold and 'popcorn' lesions produced in freshly excised rabbit liver were most easily visualized and boundaries best defined using MBF-harmonic mode.

High-Frequency Ultrasound M-Mode Imaging for Identifying Lesion and Bubble Activity During High-Intensity Focused Ultrasound Ablation

“…Early studies on the use of ultrasound-induced hyperthermia (1–2 W/cm 2 , 30 min) for treatment of melanoma tumor xenografts showed that treated tissue had a significant decrease in spectral slope by 0.480 dB/MHz and a significant increase in intercept by more than 10 dB as compared to untreated tissue (Silverman et al 1986). Later work by the same group reported changes in spectral parameters on lesions induced by HIFU in ex-vivo liver (700 W/cm 2 , 5 s) (Lizzi et al 1997) and more recently in ex-vivo chicken breast and rabbit liver (5.6–10 kW /cm 2 , 15–30 s) (Silverman et al 2006). The ablated tissue in these studies exhibited midband fit of 6 to 8 dB higher than surrounding tissue, with further enhanced contrast of ~12 dB when the MBF was computed from the second harmonic signal (Silverman et al 2006).…”

“…Later work by the same group reported changes in spectral parameters on lesions induced by HIFU in ex-vivo liver (700 W/cm 2 , 5 s) (Lizzi et al 1997) and more recently in ex-vivo chicken breast and rabbit liver (5.6–10 kW /cm 2 , 15–30 s) (Silverman et al 2006). The ablated tissue in these studies exhibited midband fit of 6 to 8 dB higher than surrounding tissue, with further enhanced contrast of ~12 dB when the MBF was computed from the second harmonic signal (Silverman et al 2006). The aforementioned values of change in midband fit, intercept, and slope are in line with the trends reported for the corresponding threshold values in this study (+3.3 dB, +7.8 dB, and −0.17 dB/MHz), and quantitatively close to the optimal thresholds determined in this study for temporal maximum midband fit Δ M max of +11 dB and temporal minimum slope of −0.50 dB/MHz (Table 1).…”

“…For example, calibrated spectral parameters of fundamental and harmonic frequencies (Lizzi et al 1997; Silverman et al 2006), attenuation and/or ultrasound backscatter (Anand and Kaczkowski 2004; Ribault et al 1998; Zhang et al 2009; Zhong et al 2007), temperature (Amini et al 2005; Arthur et al 2010; Liu and Ebbini 2010; Miller et al 2002; Seip and Ebbini 1995; Straube and Arthur 1994), thermal diffusivity (Anand and Kaczkowski 2008), strain or stiffness (Eyerly et al 2010; Fahey et al 2005; Kallel et al 1999; Lizzi et al 2003; Maleke and Konofagou 2008; Shi et al 1999; Souchon et al 2005; Zhang et al 2008), and echo-decorrelation (Mast et al 2008), have been exploited. However, these methods can be subject to artifacts from tissue movement, problematic over the large range of temperature variations in HIFU ablation, and disrupted by cavity formation (Miller et al 2002; Zheng and Vaezy 2010).…”

The origins of nonlinear enhancement in ex vivo tissue during high intensity focused ultrasound ablation

“…Treatment planning requires a good understanding of the acoustic field, and at the high intensities needed to thermally ablate tissue, the effect of nonlinearity cannot be ignored. Likewise, potential ultrasound-based treatment monitoring methods such as tissue harmonic imaging rely on detecting nonlinear acoustic phenomena in tissue [3].…”

Characterization of the pancreas in vivo using EUS spectrum analysis with electronic array echoendoscopes

“…Spectrum analysis of backscattered radio-frequency (RF) ultrasound signals in US imaging 19-23 has also been exploited for objective, quantitative tissue characterization. The method has proven to be effective for identifying tissue changes due to prostate cancer, 24-26 breast cancer, 27 ocular cancer, 28 lymph node metastases from cancers of the breast 29 and colon, 30-33 liver disease, 34 intravascular plaque, 35 and hyperthermic lesions 36-38 and has also been implemented to perform real-time tissue-type imaging. 39 In brief, the method of spectrum analysis extracts parameters from the ultrasound backscattered from local inhomogeneities in tissue.…”