Biomedical Engineering Reference
In-Depth Information
CT equipment is large and stationary so a person can fit inside, and as a result, it is rela-
tively expensive to operate. Consecutive pictures of a moving heart are now achievable
through synchronization to ECG signals. The resolution of CT images is typically 1 mm.
CT scanning creates superb images of the brain, bone, lungs, and soft tissue, making it com-
plementary to ultrasound.
Although the taking of CT images requires training, it is not difficult. Interpretation
of CT cross-sectional
images demands considerable experience for a definitive
diagnosis.
16.6.3 Magnetic Resonance Imaging
For magnetic resonance imaging, the patient is placed in a strong static magnetic field
created by a large enclosing electromagnet. The resolution is mainly determined by
the gradient or shape of the magnetic field, and it is typically 1 mm. Images are calcu-
lated by reconstruction algorithms based on the sensed voltages proportional to the
relaxation times. Tomographic images of cross-sectional slices of the body are computed.
The imaging process is fast and reasonably safe, since no ionizing radiation is used. Care
must be taken to keep ferromagnetic materials away from the powerful magnets used,
and there are limits to the strength of the applied magnetic fields and how quickly
they are switched. Because the equipment needed to make the images is expensive,
exams are costly.
MRI equipment has several degrees of freedom such as the timing, orientation, and fre-
quency of magnetic fields; therefore, a high level of skill is necessary to acquire diagnosti-
cally useful images. Diagnostic interpretation of images involves both a thorough
knowledge of the settings of the system and experience.
16.7 IMAGE FUSION
As implied by the comparison of images and the discussion of PET imaging in
Section 16.5.2, the physics of each imaging modality reveals different characteristics of tis-
sue properties and functions. In order to obtain a more complete picture, two (or more)
imaging modalities can be combined. Image fusion is the simultaneous display of two dif-
ferent types of images either side by side or superimposed. To demonstrate these different
points of view, Figure 16.56 compares four types of imaging views of a metastases and is
explained next.
One of the leading types of image fusion is PET/CT, where specialized scanners, spa-
tially coregistered, take two sets of images of the same subject in one convenient instrument
[2]. As an example, images of a patient with cancer given FDG are shown in Figure 16.56.
The top left PET images in frontal and transverse views (A and B) display the uptake of
FDG as a small, dark region (see arrow), signifying hypermetabolic activity and the likeli-
hood of a metastases; however, pinpointing the anatomical location of the cancer nodule
is ambiguous in these views. The CT view (D), displays good resolution as well as tissue
structures but not a clear indication of the nodule. The fusion image (E), a combination of
