Motion Correction in Emission Tomography - Correction Techniques in Emission Tomography - page 158

as the activity is spread over a larger image volume. It has been shown that the
respiratory motion may lead to incorrect staging of tumors [25]. Small tumors
may even succeed in evading detection [50]. In addition to image blur, another
disadvantage of motion has been observed and quantified in recent years on
PET images: attenuation artifacts in the case of hybrid scanners where CT- or
MRI-based attenuation maps are used for attenuation correction of the PET
data.
As the motion correction techniques are similar for SPECT and PET to-
mographic images, or else can be easily modified to be applied to the other
modality, the correction techniques in this chapter will mainly be described
with reference to PET imaging only.
The degrading motion in PET/SPECT studies can be divided into cate-
gories in accordance with the source of the motion. Three major sources of
motion can be differentiated in emission tomography:

• Body motion, caused by willing or unwilling motion of the patient

• Respiratory motion, caused by the motion of the diaphragm and lungs

• Cardiac motion, caused by the beating motion of the heart

To study the effects and the correction techniques for these types of motion,
it would be helpful to first see how large the magnitude of each type of motion
is. In most cases, this also gives an idea about the impact of the motion on
the images.

8.1.1 Magnitude of motion

8.1.1.1 Patient motion

The motion of the patient may be caused involuntarily by tremor or spasms
due to a disease or by motion due to coughing or sneezing by the patient. It
may also be caused by the motion of body parts by patients if they become
tired or feel pain caused by lying in the same position for long periods of
time. The magnitude of this motion is variable. Techniques to minimize the
body motion of the patient include usage of a special mechanism such as the
BodyFix ® system. Other methods include observing the patient during data
acquisition and inducing a trigger signal in the data stream whenever large
patient movements are observed. The data can then be corrected for motion
in a post processing step.

8.1.1.2 Respiratory motion

Respiratory motion of the patients is caused by the movement of the di-
aphragm which leads to expansion or contraction of the lungs. Along with
the lungs other organs in the thorax, such as the heart, which is located be-
tween the lungs and is adjacent to the diaphragm, the liver, the ribs and the