**2. 3D ultrasound imaging systems**

#### **2.1. Benefits of 3D ultrasound imaging**

Conventional 2D US imaging systems making use of 1D transducer arrays allow users to manipulate the hand-held US transducer freely over the body in order to generate images of organs and pathology. While this capability is sufficient for many interventional procedures such as breast biopsy, some interventional procedures require 3D image visualization, which 3D US imaging attempts to provide. More specifically:


The following sections review approaches used in generation of 3D US images based on 1D. An emphasis is placed on the geometric accuracy of the generated 3D images as well as the use of this technology in interventional and quantitative monitoring applications.

#### **2.2. Mechanical 3D US scanning systems**

Mechanical 3D US systems make use of mechanisms using motors to translate, tilt, or rotate a conventional 2D US transducer. A sequential digitized series of 2D US images and their relative positions and orientation are acquired rapidly by a computer as the 2D US transducer is moved, while the 3D US image is reconstructed. Since the scanning geometry in mechanical 3D US systems is predefined and precisely controlled by a mechanical motorized system, the relative position and orientation of the acquired 2D US images are known accurately and precisely. These mechanical 3D scanning systems allow the user to optimize the image resolution by adjusting the angular or spatial interval between the acquired 2D image [10].

Two approaches have been used in the development of mechanical 3D US scanning systems: integrated 3D US transducers with the scanning mechanism within the transducer housing; and external mechanical fixtures that hold the housing of a conventional 2D US transducers. Both approaches have been successfully used for a variety of clinical applications including interventional applications.

#### *2.2.1. Wobbling or tilting mechanical 3D US scanners*

In this chapter we focus on the recent development of 3D US imaging as it applies to imageguided interventions. The chapter will briefly review how 3D US images are obtained and then will provide two examples of recent development of 3D US- guided interventional procedures.

Conventional 2D US imaging systems making use of 1D transducer arrays allow users to manipulate the hand-held US transducer freely over the body in order to generate images of organs and pathology. While this capability is sufficient for many interventional procedures such as breast biopsy, some interventional procedures require 3D image visualization, which

**•** Freely manipulating the conventional US transducer during the interventional procedure over the anatomy to generate 2D US images requires that users mentally integrate many 2D images to form an impression of the anatomy and pathology in 3D. In cases of interventions of complex anatomy or pathology, this approach leads to longer procedures and may result

**•** Since the conventional 2D US imaging transducer is held and manipulated manually, it is difficult to relocate the 2D US image at the exact location and orientation in the body at a later time. Since monitoring the progression of the interventional procedure often requires imaging of the same location (plane) of the anatomy, manual manipulation of a 2D US image

**•** Conventional 2D US imaging does not permit viewing of planes parallel to the skin – often called C-mode. This approach is, at times, suboptimal since interventional procedures sometimes require an arbitrary selection of the image plane for optimal viewing of the

**•** Planning the interventional procedure and therapy monitoring often require accurate lesion volume measurements. Since conventional 2D US imaging only provides a cross-section of the lesion, measurements of organ or lesion volume is variable and at times inaccurate. The following sections review approaches used in generation of 3D US images based on 1D. An emphasis is placed on the geometric accuracy of the generated 3D images as well as the

Mechanical 3D US systems make use of mechanisms using motors to translate, tilt, or rotate a conventional 2D US transducer. A sequential digitized series of 2D US images and their relative positions and orientation are acquired rapidly by a computer as the 2D US transducer is moved, while the 3D US image is reconstructed. Since the scanning geometry in mechanical 3D US systems is predefined and precisely controlled by a mechanical motorized system, the relative position and orientation of the acquired 2D US images are known accurately and precisely.

use of this technology in interventional and quantitative monitoring applications.

**2. 3D ultrasound imaging systems**

2 Advancements and Breakthroughs in Ultrasound Imaging

3D US imaging attempts to provide. More specifically:

in variability in guidance of the interventional procedures.

pathology and guiding the interventional procedure.

**2.2. Mechanical 3D US scanning systems**

**2.1. Benefits of 3D ultrasound imaging**

is suboptimal.

Most US system manufacturers offer integrated 3D US transducers that are based on a mechanically-swept transducer or "wobbler". In these systems a 1D US array is wobbled or swept back and forth inside the 3D transducer housing. Digital 2D US images that are generated while the 1D US array is wobbled, which are used in the 3D US image reconstruction. These 3D transducers are larger than conventional 2D US transducers. These types of 3D US transducers are convenient to use but require a special US machine that can control the 3D scanning and reconstruct the acquired 2D images into a 3D image.

Many interventional 3D US-guided interventional systems are currently using external fixtures for mechanical 3D scanning since researchers typically do not get access to the control of the US system for development of novel interventional systems. In this approach, a motorized custom made fixture is used to house the conventional 2D US transducer. A computer is used to control the motor to cause the US transducer to tilt or "wobble". The video stream from the US machine is digitized using an analogue or digital frame grabber. Since the relative angle between the acquired 2D images is known, a 3D image can be reconstructed as the 2D images are acquired.

Although the external mechanical 3D scanning fixtures are bulkier than integrated 3D transducers, they can be used with any US manufacturer's transducer, obviating the need to purchase a special 3D US machine. In addition, the external fixture approach can take advant‐ age of improvements in the US machine (e.g., image compounding, contrast agent imaging) and flow information (e.g., Doppler imaging) without any changes in the scanning mechanism.

Both approaches used in mechanical 3D US scanning allow short imaging times, ranging from about 3 to 0.2 3D images/s. The 3D images are of high quality and also include B-mode and Doppler information.

Figure 1a is a diagram of the mechanical tilt approach of a conventional 1D array US transducer about an axis parallel to the face of the transducer, and 1b shows the tilting axis away from the face of the transducer. The latter approach is typically used in integrated 3D scanning mech‐ anisms. In both approaches, the acquired 2D US images are arranged as a fan with an adjustable angular spacing, e.g., 1.0°. To generate a 3D image, the housing of the 3D probe or external fixture remains fixed on the skin of the patient while the US transducer is wobbled. The time required to generate a 3D US image depends on the 2D US image update rate and the number of 2D images needed to generate the 3D image. The 2D US image update rate depends on the US machine settings (i.e., depth setting and number of focal zones) and number of acquired 2D US images is determined by the chosen angular separation between the acquired 2D images, and the total scan angle needed to cover the desired anatomy. Typically, these parameters can be adjusted to optimize scanning time, image quality and the size of the volume imaged [11-16]. The most common integrated 3D transducers using the wobbling technique are used for abdominal and obstetrical imaging [17-19].

*2.2.2. Linear mechanical 3D scanners*

generating high quality 3D images [10].

*2.2.3. Endo-cavity rotational 3D scanners*

Linear scanners mechanisms use an external motorized fixture to move the conventional 2D transducer across the skin of the patient. The 2D transducer can be fixed to be perpendicular to the surface of the skin or at an angle for acquiring Doppler images. The spacing between the acquired 2D images is adjustable but constant during the scan so that the acquired 2D images are parallel and uniformly spaced (see Fig. 1c). The velocity of the transducer as it is being scanned is adjusted to obtain 2D images with an appropriate spatial interval for

3D Ultrasound Imaging in Image-Guided Intervention

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The predefined spacing between the acquired 2D US images allows 3D images to be recon‐ structed while the 2D US images are being acquired. In the direction parallel to the acquired 2D US images the resolution of the reconstructed 3D US image will be the same as the original 2D US images. However, in the direction of the 3D scanning, the resolution of the reconstructed 3D image will be equal (if spatial sampling is appropriate) to the elevational resolution of the acquired 2D US images. Thus, the resolution of the 3D US image will be poorest in the 3D scanning direction due to greater spread of the US beam in the elevational direction [21].

This scanning approach is not typically used in interventional applications; however, it has been successfully implemented in many vascular B-mode and Doppler imaging applications,

The endo-cavity rotational 3D scanning approach has been used extensively in 3D US-guided prostate interventional procedures. In this approach an external fixture or internal mechanism is used to rotate an endo-cavity transducer (*e.g*., a transrectal ultrasound (TRUS) probe, see Fig. 1b) about its long axis. Endo-cavity transducers using an end-firing approach are typically used for prostate biopsy. When these types of conventional transducers are rotated by the motorized fixture, the set of acquired 2D images will be arranged as a fan (Fig. 1b), intersecting in the center of the 3D US image, resulting in an image as shown in Fig. 2. To obtain a 3D image

Endo-cavity transducers using a side-firing 1D array are typically used in prostate brachy‐ therapy, cryotherapy and focal therapy. When using these types of conventional transducers, the acquired images will also be arranged as a fan, but intersect at the axis of rotation of the transducer (see Fig. 1a). The side-firing transducer is typically rotated from 80° to 110° to obtain a 3D TRUS image of the prostate [16, 34, 35]. Figure 2 shows that endo-cavity scanning transducer used to image the prostate for 3D US-guided therapy [6, 9, 11, 25, 34, 36-39]

For scanning systems used for 3D US-guided prostate biopsy, the end-firing transducer is rotated by at least 180° about a fixed axis that perpendicularly bisects the transducer array. In this approach, the resolution of the 3D image will not be isotropic. Since the spatial sampling is highest near the rotation axis of the transducer and the poorest away from the axis of rotation of the transducer, thus the resolution of the 3D US image will degrade as the distance from the rotational axis of the transducer is increased. In addition, the axial and elevational resolution will decrease as the distance from the transducer is increased, as discussed above. The

particularly of for carotid arteries [11, 22-30] and tumor vascularization [25, 31-33].

of the prostate as in Fig. 2, an end-firing transducer is typically rotated by 180° [16].

The 3D image resolution will not be isotropic. The resolution in the 3D US image will degrade in the axial direction away from the transducer due to the increasing US beam spread in the lateral and elevational directions of the acquired 2D US images. Since the acquired 2D images used to generate a 3D image are arranged as a fan, the distance between the acquired US images increases with increasing axial distance. Increasing axial distances result in decreasing spatial sampling resulting in further loss of spatial resolution in the elevational direction of the acquired 2D US images of the reconstructed 3D image [20]. Figure 1 

**Figure 1.** Schematic diagrams of 3D US mechanical scanning methods. (a) A side-firing TRUS transducer is mechanical‐ ly rotated and the acquired images have equal angular spacing. The same approach is used in a mechanically-wob‐ bled transducer. (b) A rotational scanning mechanism using an end-firing transducer, typically used in 3D TRUS guided prostate biopsy. The acquired images have equal angular spacing. (c) A linear mechanical scanning mechanism, in which the acquired images have equal spacing. (d) The mechanically tilting mechanism, but integrated into a 3D US transducer. The US transducer is "wobbled" inside the housing of the transducer.
