**3.1. Clinical problem**

combination of these effects will result in a 3D US image resolution that is best near the transducer and the rotational axis, while being poorest away from the transducer and rota‐

3D rotational scanning with an end-firing transducer is most sensitive to the motion of the transducer and patient since the axis of rotation is in the center of the 3D US image. Any motion during the 3D scan will cause a mismatch in the acquired 2D US images, resulting in artifacts in the center of the 3D US image. Artifacts in the center of the 3D US image will also occur if the axis of rotation is not accurately known; however, proper calibrations can remove this source of potential error. Thus, for interventional applications such as 3D US-guided prostate biopsy or brachytherapy, the rotational scanning mechanism is typically supported by a

)

**Figure 2.** The 3D US of the prostate displayed using the multi-planar reformatting approach: (a) An end-firing TRUS prostate cube-view 3D image, allowing the sides to be translated and angles to reveal the desired anatomy. (b) A 3D

Some 3D US-guided interventional procedures are making use of 3D scanning techniques that do not require a mechanical scanning device. In this approach, the user holds and manipulates a conventional US transducer to cover the patient's anatomy being investigated. Since construction of a 3D US image requires that the position and orientation of the conventional transducer be known, free-hand scanning requires a method to track the positions and orientations of the transducer as it is being moved. All methods to accomplish this task require a sensor to be mounted on the transducer to allow measurement of the conventional 2D

Over the past 2 decades, several approaches for free-hand scanning have been developed: tracked 3D US with articulated arms, free-hand 3D US with acoustic sensing, free-hand 3D US with magnetic field sensing, and image-based sensing (speckle decorrelation). The method used most commonly is the magnetic field sensing approach with several companies providing

TRUS image acquired using a side-firing transducer using the mechanical rotation approach.

transducer's position and orientation as it is moved over the body.

tional axis.

stabilization apparatus [16, 34, 40].

6 Advancements and Breakthroughs in Ultrasound Imaging

*2.2.4. Free-hand scanning with position sensing*

(a) (b)

Hepatocellular carcinoma (HCC) is the fifth most common diagnosed malignancy and the third most frequent cause of cancer related deaths worldwide [52]. Incidence is particularly high in Asia and sub-Saharan Africa due to the large incidence of hepatitis B and C, both of which are complicated by hepatic cirrhosis, which is the greatest risk factor for HCC. Recently, increasing trends in HCC have been reported from several Western countries [53]. Further‐ more, the liver is the second most common site of metastatic cancer arising in other organs.

When feasible, surgical resection or liver transplant is the accepted standard therapeutic approach, and currently has the highest success rate of all treatment methods for primary and metastatic liver cancer. Unfortunately, only 15% of patients are candidates for surgery [54, 55]. Patients who do not qualify for surgery usually are offered other therapeutic solutions such as chemotherapy and radiotherapy, but unfortunately have variable limited success rates.

Minimally invasive percutaneous techniques, such as radio-frequency (RF) and microwave (MW) ablation of malignant tissue in the liver is a rapidly expanding research field and treat‐ ment tool for those patients who are not candidates for surgical resection or transplant. In some cases this acts as a bridge to liver transplantation [54, 56]. Due to low complications rates and shorter recovery times, the indications for these minimally invasive procedures are constantly increasing. However, these methods have a higher local recurrence rate than surgical resec‐ tion, mostly due to insufficient or inaccurate local ablation of cancerous cells [56, 57].

Microwave energy-induced tissue heating by near-field probes is emerging as a common thermal treatment of liver tumors [58]. Application of MW for tumor ablation has multiple advantages over other techniques, including higher treatment temperatures and the ability to create larger uniformly shaped ablation zones in shorter time periods. However, the accurate placement of the probe is critical in achieving the predicted treatment goal [59]. The current standard of care uses CT images for planning and 2D US image guidance for intra-operative guidance of the ablation probe(s) into the target lesion. However, this approach suffers from several disadvantages, such as: (1) 2D US imaging requires physicians to mentally integrate many 2D images to form an impression of the anatomy and pathology, leading to more variability in guidance during interventional procedures; (2) 2D US does not permit the viewing of planes parallel to the skin, (3) liver deformation and motion artifact due to breathing reduces targeting accuracy, (4) 2D US-based for measurement of tumor volume needed for the treatment plan is variable and at times inaccurate, and (5) the detection and tracking of the needle delivering the thermal energy in the liver is crucial for accurate placement of the needle relative to the tumor, but can be difficult using 2D US. 3D US imaging of the liver and target may help to overcome these disadvantages resulting in improved accuracy of probe placement and improved ablation of the lesion.

logram linkage, which is mounted on the carriage of the linear slide. A second geared DC motor is used to generate the tilt motion, allowing for independent control of the two systems.

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**Figure 3.** Schematic diagram of the hybrid 3D US scanner for used in the focal liver ablation procedure. The diagram

shows the start and end positions of the hybrid (linear and tilt) scan.

The use of 3D US-guidance for focal liver tumor ablation is based on the fact that the use of 3D US will show the features of liver masses and the hepatic vasculature more clearly, allow guidance of the ablation probes to the target more accurately, and allow more accurate monitoring of the ablation zone during the procedure and at follow up.

#### **3.2. 3D US Scanner for focal liver tumor ablation**

We have developed 3D US guidance systems for improving cancer diagnosis and treatment by introducing hardware and software innovations [21, 60-64]. Our previous efforts have been extended to the development of a 3D US-guidance system for treating HCC. Specialized hardware and software tools are used that allow 3D acquisition of 3D US images, real-time registration of the pre-operative CT to intra-operative 3D US images, and tracking of the ablation probes during insertion into the target. This is accomplished by registering previously acquired contrast CT images that show the location of the target lesion to near real-time 3D US images, plus providing visualization and guidance tools to guide the procedure.

The 3D US scanning system consists of: a hand-held electro-mechanical motor/encoder assem‐ bly to move a conventional 2D US imaging transducer in a fan shaped, linear or hybrid motion to a maximum angular limit of 60 degrees and/or 30 mm linear extent to acquire a series of 2D US images;and,aPCequippedwithadigitalframegrabberandsoftware components tocontrolthe motor assembly, acquire 2D images, reconstruct them in 3D, and visualize them in 3D.

#### *3.2.1. Mechanical design*

The handheld 3D scanning device is motorized and constructed with two mechanical systems for generating a linear and tilt scanning motions of the transducer is shown schematically and photographically in Figs. 3 and 4. The linear scanning system is operated with a geared DC motor and lead screw providing linear translation. The tilt motion is generated via a paralle‐ logram linkage, which is mounted on the carriage of the linear slide. A second geared DC motor is used to generate the tilt motion, allowing for independent control of the two systems.

Microwave energy-induced tissue heating by near-field probes is emerging as a common thermal treatment of liver tumors [58]. Application of MW for tumor ablation has multiple advantages over other techniques, including higher treatment temperatures and the ability to create larger uniformly shaped ablation zones in shorter time periods. However, the accurate placement of the probe is critical in achieving the predicted treatment goal [59]. The current standard of care uses CT images for planning and 2D US image guidance for intra-operative guidance of the ablation probe(s) into the target lesion. However, this approach suffers from several disadvantages, such as: (1) 2D US imaging requires physicians to mentally integrate many 2D images to form an impression of the anatomy and pathology, leading to more variability in guidance during interventional procedures; (2) 2D US does not permit the viewing of planes parallel to the skin, (3) liver deformation and motion artifact due to breathing reduces targeting accuracy, (4) 2D US-based for measurement of tumor volume needed for the treatment plan is variable and at times inaccurate, and (5) the detection and tracking of the needle delivering the thermal energy in the liver is crucial for accurate placement of the needle relative to the tumor, but can be difficult using 2D US. 3D US imaging of the liver and target may help to overcome these disadvantages resulting in improved accuracy of probe placement

The use of 3D US-guidance for focal liver tumor ablation is based on the fact that the use of 3D US will show the features of liver masses and the hepatic vasculature more clearly, allow guidance of the ablation probes to the target more accurately, and allow more accurate

We have developed 3D US guidance systems for improving cancer diagnosis and treatment by introducing hardware and software innovations [21, 60-64]. Our previous efforts have been extended to the development of a 3D US-guidance system for treating HCC. Specialized hardware and software tools are used that allow 3D acquisition of 3D US images, real-time registration of the pre-operative CT to intra-operative 3D US images, and tracking of the ablation probes during insertion into the target. This is accomplished by registering previously acquired contrast CT images that show the location of the target lesion to near real-time 3D

The 3D US scanning system consists of: a hand-held electro-mechanical motor/encoder assem‐ bly to move a conventional 2D US imaging transducer in a fan shaped, linear or hybrid motion to a maximum angular limit of 60 degrees and/or 30 mm linear extent to acquire a series of 2D US images;and,aPCequippedwithadigitalframegrabberandsoftware components tocontrolthe

The handheld 3D scanning device is motorized and constructed with two mechanical systems for generating a linear and tilt scanning motions of the transducer is shown schematically and photographically in Figs. 3 and 4. The linear scanning system is operated with a geared DC motor and lead screw providing linear translation. The tilt motion is generated via a paralle‐

US images, plus providing visualization and guidance tools to guide the procedure.

motor assembly, acquire 2D images, reconstruct them in 3D, and visualize them in 3D.

monitoring of the ablation zone during the procedure and at follow up.

and improved ablation of the lesion.

8 Advancements and Breakthroughs in Ultrasound Imaging

*3.2.1. Mechanical design*

**3.2. 3D US Scanner for focal liver tumor ablation**

**Figure 3.** Schematic diagram of the hybrid 3D US scanner for used in the focal liver ablation procedure. The diagram shows the start and end positions of the hybrid (linear and tilt) scan.

The 3D scanning system parameters can be set by the user: *Scanning mode:* Three different modes of linear, tilt and combined (or hybrid, a combination of both linear and tilt imaging modes to maximize the field-of-view) are available depending on the anatomy of body parts being scanned and the image requirements. *Scan Extent:* Maximum extent of linear translation (typically 2.5 cm) or tilt angle (typically 60 deg) can be set individually to the extremes values. *Scan Spacing:* Elevational linear and angular spacing can be set to optimize the trade-off between the scanning time and the scan spacing. *Frame-Rate:* The rate at which images are digitized by the frame grabber is set (typically 15 frames/s). *Scanning Depth:* Maximum scanning depth can be set prior to each scan for accurate reconstruction of the volumes.

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Since the hybrid scanning mode involves coordination between two acquisition methods, it was tested in terms of accuracy of 3D image generation. We used two custom made phantoms with known geometry. The validation experiments where performed using the handheld 3D US scanning device in hybrid scanning mode using a two-dimensional conventional curved

*Geometrical Error in 3D reconstruction*: This test was designed to measure the accuracy of the 3D reconstruction of the 3D hybrid scanner in three directions. The test phantom was made of a grid of known dimensions made with 0.1 mm thick nylon monofilament threads wrapped around an accurately machined frame to form a 4-layer grid. Each layer was slightly shifted from the layer above to avoid acoustic shadowing. The distance between any two layers was 1cm. The phantom was submerged in a 15% glycerol solution [61] and imaged at different depth settings. The acquired 3D US images were then viewed and analyzed by measuring the distances between the images of the monofilaments and comparing them to the expected

)

**Figure 6.** (a) Photograph of the 3D monofilament thread grid, which was used to validate the 3D reconstruction of

*Error in 3D volume measurements:* In the second test, we assessed the accuracy in measuring volumes using our system. For this experiment, several spherical phantoms with different sizes were made of tissue mimicking agar [65]. The volume of each of these spherical phantoms was measured prior to embedding them in a cube of tissue mimicking agar phantom. The spherical

the ultrasound image. (b) The 3D ultrasound image of the phantom, showing the grid of threads.

array ultrasound transducer used for abdominal applications (Toshiba, PVT-375BT).

(a) (b)

*3.2.2. Validation methods*

values.

**Figure 4.** Photograph of hybrid scanner with abdominal ultrasound transducer mounted and ready for scanning.

The 3D scanning device has three modes of operation: a linear translation, in which the transducer (oriented perpendicular to the surface or at an angle for Doppler imaging) is translated along a straight line parallel to the patient's surface. This motion generates a rectangular volume shown in Fig. 5a. The second mode generates a tilt motion (or wobbling), in which the transducer is rotated about its face resting on the patient's skin surface (Fig. 5b). The third mode is a combination of the first two modes that creates a combined (or hybrid) motion. The transducer is rotated as it is moved along a surface covering a larger volume than either of the first two modes (Fig. 5c). For example, if transducer with linear array is used at 15cm depth setting on the ultrasound machine (typical depth for abdominal imaging), hybrid scanning gives a volume that is three times larger than the linear mode and 47% larger than the tilt mode only.

**Figure 5.** Schematic diagrams showing the three modes of operation of the mechanical compound 3D US scanning device. On the left is the schematic of the linkage and the right are the linear, tilt and hybrid motions.

The 3D scanning system parameters can be set by the user: *Scanning mode:* Three different modes of linear, tilt and combined (or hybrid, a combination of both linear and tilt imaging modes to maximize the field-of-view) are available depending on the anatomy of body parts being scanned and the image requirements. *Scan Extent:* Maximum extent of linear translation (typically 2.5 cm) or tilt angle (typically 60 deg) can be set individually to the extremes values. *Scan Spacing:* Elevational linear and angular spacing can be set to optimize the trade-off between the scanning time and the scan spacing. *Frame-Rate:* The rate at which images are digitized by the frame grabber is set (typically 15 frames/s). *Scanning Depth:* Maximum scanning depth can be set prior to each scan for accurate reconstruction of the volumes.
