334
Biomedical Signal and Image Processing
Since the ultrasound image is built up by assuming that sound travels in straight
lines, regions of tissue that are affected by refraction will be displayed incorrectly.
A different class of registration problems is concerned with registration of image
sequences that follow a process that changes with time. Acquisition of images over
time and subsequent registration can be used to study dynamic processes such as tissue perfusion, blood flow, and metabolic or physiological processes. One example of
such a process is imaging of the heart where images are acquired in synchrony with
the heartbeat, synchronized to the ECG or blood pressure waveform. Synchronized
or “gated” acquisitions allow averaging of images over multiple cardiac cycles to
reduce image noise in nuclear medicine and MR imaging. In a similar way, temporal
registration of x-ray images of the heart before and after injection of contrast material allows synchronous subtraction of mask images. These types of image registration methods assume that the imaging cycle does not change between periods, or for
the heart, from beat to beat. A similar principle applies to images acquired at different stages of the breathing cycle, although the breathing cycle is less reproducible,
and therefore registration errors will be greater.
One serious consideration in image registration is the use of nonguided freehand
imaging. The interpretation of the acquired image relies solely on the anatomical
knowledge of the operator, and the mental interpretation of the angle and position
of the transducer by the clinician. Most other ultrasound imaging modalities have a
built-in feedback mechanism that relates each scan with respect to the previous scan
by mechanical and/or electronic feedback.
16.13 COMPARISON OF CT, MRI, AND ULTRASONIC IMAGES
Ultrasonic imaging has many overlaps with x-ray CT imaging as far as the one-toone image formation is concerned; both mechanisms assume rectilinear propagation
of the image carrier. However, ultrasound does not have the same penetration capabilities as the x-ray used in CT scanning, and ultrasound is less harmful than x-ray.
On the other hand, MRI, due to its very high resolution and image quality, provides detailed physiological information, while ultrasound only gives flow feedback.
Compared to MRI, the main advantage of ultrasound is the low cost and portability
of ultrasound machines. In addition, as will be discussed later, the presence of ironbased or other ferromagnetic materials is prohibited in the MRI room, which calls
for the use of often costly tools such as titanium-based surgical tools in the MRI
room. No such restriction is applied to ultrasound. Moreover, to ease of conducting
ultrasound imaging allows using the machine for imaging of almost all types of
patients without the use of sedatives. However, in order to perform MRI on some
children and even adults with claustrophobia, the patients need to be sedated or even
put to sleep to allow image acquisition.
16.14 BIOEFFECTS OF ULTRASOUND
The following bioeffects of ultrasonic imaging deserve attention: thermal effects,
mechanical and cavitational effects, cellular and subcellular effects, biochemical
effects, and finally the effects of ultrasound on organs and systems.
Biomedical Signal and Image Processing
Since the ultrasound image is built up by assuming that sound travels in straight
lines, regions of tissue that are affected by refraction will be displayed incorrectly.
A different class of registration problems is concerned with registration of image
sequences that follow a process that changes with time. Acquisition of images over
time and subsequent registration can be used to study dynamic processes such as tissue perfusion, blood flow, and metabolic or physiological processes. One example of
such a process is imaging of the heart where images are acquired in synchrony with
the heartbeat, synchronized to the ECG or blood pressure waveform. Synchronized
or “gated” acquisitions allow averaging of images over multiple cardiac cycles to
reduce image noise in nuclear medicine and MR imaging. In a similar way, temporal
registration of x-ray images of the heart before and after injection of contrast material allows synchronous subtraction of mask images. These types of image registration methods assume that the imaging cycle does not change between periods, or for
the heart, from beat to beat. A similar principle applies to images acquired at different stages of the breathing cycle, although the breathing cycle is less reproducible,
and therefore registration errors will be greater.
One serious consideration in image registration is the use of nonguided freehand
imaging. The interpretation of the acquired image relies solely on the anatomical
knowledge of the operator, and the mental interpretation of the angle and position
of the transducer by the clinician. Most other ultrasound imaging modalities have a
built-in feedback mechanism that relates each scan with respect to the previous scan
by mechanical and/or electronic feedback.
16.13 COMPARISON OF CT, MRI, AND ULTRASONIC IMAGES
Ultrasonic imaging has many overlaps with x-ray CT imaging as far as the one-toone image formation is concerned; both mechanisms assume rectilinear propagation
of the image carrier. However, ultrasound does not have the same penetration capabilities as the x-ray used in CT scanning, and ultrasound is less harmful than x-ray.
On the other hand, MRI, due to its very high resolution and image quality, provides detailed physiological information, while ultrasound only gives flow feedback.
Compared to MRI, the main advantage of ultrasound is the low cost and portability
of ultrasound machines. In addition, as will be discussed later, the presence of ironbased or other ferromagnetic materials is prohibited in the MRI room, which calls
for the use of often costly tools such as titanium-based surgical tools in the MRI
room. No such restriction is applied to ultrasound. Moreover, to ease of conducting
ultrasound imaging allows using the machine for imaging of almost all types of
patients without the use of sedatives. However, in order to perform MRI on some
children and even adults with claustrophobia, the patients need to be sedated or even
put to sleep to allow image acquisition.
16.14 BIOEFFECTS OF ULTRASOUND
The following bioeffects of ultrasonic imaging deserve attention: thermal effects,
mechanical and cavitational effects, cellular and subcellular effects, biochemical
effects, and finally the effects of ultrasound on organs and systems.