The Study Of Effects Of Ribs In High Intensity Focused Ultrasound Beam Path On Acoustic Field And Biological Focal Region

BACKGROUNDHigh intensity focused ultrasound (HIFU) is emerging as a non-invasive tumor treatment technology in recent years. The physical principles of HIFU treatment are that the ultrasound is focused in the target tissue by transducer, then the energy is concentrated in the target tissue, target tissue is heated rapidly to 65℃, resulting in protein denaturation, so that target tissue are ablated in non-invasive manner. HIFU, as a noninvasive, effective and feasible modality, have been used in sorts of tumor and non-tumor disease. However, there are some problems in the clinical application of HIFU. Many target tissues needed HIFU therapy lie in the ribcage's acoustic shadow. The ribs in HIFU beam path result in acoustic field distortion. Because of the acoustic absorption of ribs, the ultrasonic energy attenuates exponentially. In order to ensure safe and effective treatment, it is important to study the effects of ribs in HIFU beam path on acoustic field and BFR. In this paper, directing the beam axis through the intercostals space, lied at the rib edge and on the centre of middle rib. For each of these axial arrangements, the plane of the front surface of ribs was placed 3cm, 6cm or 9cm in front of the focus. Hydrophone plots were recorded to note the acoustic field. HIFU performed in the manner of a single dot in vitro, the position, profile and volume of BFR were measured, and the change of ribs surface were observed. At the same time, the paper showed that the effect of different f-number of the transducer through the ribs on BFR, advice for selection transducer in clinical use was provided.OBJECTIVE1. To investigate the effects of ribs in HIFU beam path on acoustic field.2. To investigate effects of ribs in HIFU beam path on the position, profile and volume of BFR in the same acoustic power.To study the effect of ribs in HIFU beam path on HIFU treatment when increasing acoustic power in safety to overcome the energy attenuation of the ribs in beam path.3. To investigate the effects of the f-number of the transducer through the ribs on BFR.METHODS1. The acoustic measurement system was used to note the acoustic field. This system was consisted of hydrophone, sound-absorption tank, sound-absorption target, sound absorption material, 3-d motion device, ultrasonic generator, oscilloscope and computer. The parameters of therapeutic transducer were: frequency-1.0MHz, diameter-200mm, focal length-130mm. The ribs were made by polystyrene (ρ=1.05g/cm2, c=2340m/s,α=0.2dB/cm). The width of rib was 10mm, the intercostals space was 20mm, the thickness of rib was 5mm. Directing the beam axis through the intercostals space, lied at the rib edge and on the centre of middle rib. For each of these axial arrangements, the plane of the front surface of ribs was placed 3cm, 6cm or 9cm in front of the focus. Hydrophone plots were recorded to note the acoustic field.2. A clinical HIFU device, Model-JC HIFU Tumor Therapeutic System (Chongqing Haifu(HIFU)Technology, Chongqing, China) was used. This system consists of a therapeutic transducer and a diagnostic transducer under the control of a centre console. The therapeutic transducer emits high intensity ultrasound which can ablate the targeted tissues, and the diagnostic transducer emits imaging ultrasound which is used to guide and monitor the treatment in real-time. The acoustic power was measured by radiation pressure method. Liver in vitro of bovine was prepared for experiment. The ribs were the same as part 1. The experiment holder was put in therapy bed, and the liver was placed in holder, the hold was filled with degassed water. The ribs were placed between the focal plane and transducer, directing the beam axis through the intercostals space, lied at the rib edge and on the centre of middle rib. For each of these axial arrangements, the plane of the front surface of ribs was placed 3cm, 6cm or 9cm in front of the focus. The depth of exposure in liver was 20mm. The parameters of HIFU exposure were: acoustic power 250W, exposure time 10s, and exposure mode was dot. After HIFU exposure, The position, the maximum length (L, mm), width (W, mm) and depth (D, mm) of the coagulated necrosis in the liver were measured, and the volume (V, mm3) was calculated according to the following formula: V =π6×L×W×DSPSS and excel statistical software were performed for the statistical analysis.3. Model-JC HIFU Tumor Therapeutic System was used.Increase the acoustic power;compensate the energy attenuation of the ribs in beam path.The transducer type was 1.0MHz-200mm-130mm;the HIFU exposure time and mode were same as before.Directing the beam axis through the intercostals space,lied at the rib edge and on the centre of middle rib.For each of these axial arrangements,the plane of the front surface of ribs was placed 6cm,9cm in front of the focus.The depth of exposure in liver was 20mm.After HIFU exposure,The position,the maximum length (L,mm), width (W,mm) and depth (D,mm) of the coagulated necrosis in the liver were measured,and the volume (V, mm3) was calculated according to the following formula: V =π6×L×W×DSPSS and excel statistical software were performed for the statistical analysis. 4. Selected 1.0MHz-200mm-130mm and 1.0MHz-200mm-170mm type transducer for the experiment of BFR,compared the impact of different f-number of transducer on BFR through the ribs.The exposure mode-dot,exposure time-10s,one of the acoustic power was 250W. Directing the beam axis through the intercostals space,lied at the rib edge and on the centre of middle rib.For each of these axial arrangements,the plane of the front surface of ribs was placed 3cm,6cm,9cm in front of the focus.Then the HIFU exposure was carried out,the formation rate,size, shape and location of BFR observed.Another acoustic power was that to increase the power,compensate the energy attenuation of the ribs in beam path,the others were same as before.RESULTS1. The acoustic pressure distribution in axial: Because the ribs in beam path, the acoustic pressure amplitude decrease of 60%-80% compared with free-field.The result showed that the ribs in beam path lead to the acoustic energy attenuation.When the ribs were close to the focal plane (3cm), acoustic energy through intercostals space was large compared with the axial lied at the rib edge and on the centre of middle rib, but compared with the free-field, and its peak value decreased of 60%. When the ribs were far away from the focal plane (6cm, 9cm),the acoustic pressure amplitude were similar.When the ribs were placed in the beam path,-6dB focal region increased of 0.5mm-1.5mm compared with the free-field.The position of BFR relative to free field was close to the transducer about 0.1-2.3mm.2. The acoustic pressure distribution in radial: When the ribs were placed 3cm,6cm in front of the focal plane,the phenomenon of acoustic pressure distribution of multiple peaks emerged.Compared to the free-field, acoustic pressure amplitude decreased. When the ribs were placed 9cm in front of the focal plane,the beam maintained the original shape,acoustic pressure distribution near the free-field,but the acoustic pressure amplitude decreased compared with the free-field.3. The tissue necrosis rate (TNR): When the ribs were placed 3cm,in front of the focal plane, directing the beam axis through the intercostals space,the TNR was 100%,the highest.When directing the beam axis lied at the rib edge and on the centre of middle rib,the TNR decreased sharply.When the ribs were placed 6cm and 9cm in front of the focal plane,the TNR was more than 50%.4. When the ribs were far away from the focal plane (6cm, 9cm), no matter directing the beam axis through the intercostals space, lied at the rib edge or on the centre of middle rib, the ribs were safe.However, when the ribs were close to the focal plane, and this could cause unacceptable damage on the surface of ribs.5. The volume of coagulated necrosis:the relationship between the ribs and beam axis played impact on the volume of coagulated necrosis.When directing the beam axis through the intercostals space, regardless of the location of ribs placed away from the focal plane or close to the focus plane,the volume of coagulated necrosis was large.When the ribs were far away from the focal plane 3cm,6cm, or 9cm, the volume,the length,width and shape of coagulated necrosis was different.6. Increase the acoustic power,compensate the energy attenuation of the ribs in beam path.Then the experiment of BFR was carried out.The results showed the volume of BFR increased,and the surface of ribs was safe.7. The effects of the f-number of the transducer through the ribs on BFR were different.When the acoustic power was 250W,the volume and TNR of BFR were larger in 1.0-200-130(f-number was 0.65) than these in 1.0-200-170(f-number was 0.85).When the acoustic power increased,the results were similar as the front.CONCLUSION1. The ribs in beam path lead to the acoustic energy attenuation. The attenuation of the energy was relative with the relative location between HIFU beam axis and the ribs,and the attenuation of the energy was relative with the ribs far away from the focal plane.2. The ribs in HIFU beam path played great impact on the HIFU AFR, the FWHM increased 0.5mm-1.5mm. The ribs near the focal plane, the impact more obvious. 3. The relative location of ribs and HIFU beam axis was relative with the volume of the BFR.When directing the beam axis through the intercostals space, the energy of HIFU was the largest through the ribs, and the shape of BFR was regulated.So in HIFU treatment, we should make full use of this beam path.4. When the ribs were far away from the focal plane, ribs were safe. When the ribs were closed with the focal plane, ribs were damaged probably.Therefore, for the deep HIFU treatment, the main consideration of impact of ribs was the energy attenuation, just increased the HIFU power to achieve the goals of HIFU treatment.5. When HIFU treatment was carried out, the transducer type of 1.0-200-130(f-number was 0.65) was more suitable for liver cancer lied in the ribcage's acoustic shadow compared with the 1.0-200-170 of the transducer type.