Transcript Chapter 10 Lecture - Independent

Chapter 10 Lecture
Mark D. Herbst, M.D., Ph.D.
Image Detail and Noise
• Two image characteristics that reduce the
visibility of anatomy/pathology
– Blurring – level of detail – related to pixel size
– Visual Noise – random bright and dark dots
• These two are related, because if you
improve one, you worsen the other.
Other image characteristics also
impact visibility of
anatomy/pathology
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Camouflage
Irregular borders
Low contrast
Motion (blurring)
How detailed must the image be?
• The goal is to select the optimal voxel size
for the anatomy and pathology you want to
see
What is the MR image?
• Really just a display of the signal
intensities from the voxels in the patient as
pixels. No detail is seen within a voxel
(pixel).
Voxels, FOV, and Matrix size
• Pixel size = FOV / matrix size
• Voxel size = pixel size x slice thickness
• FOV = field of view – can be square or
rectangular (i.e., 25 cm x 25 cm, or 25 cm
x 20 cm)
• Matrix = how many pixels in the image,
square or rectangular (i.e., 256 x 256, or
256 x 192)
Exercise
• What is the voxel size?
– FOV = 25 cm, Matrix = 256 x 256, ST = 1 mm
– FOV = 13 cm, Matrix = 256 x 128, ST = 1 mm
– FOV = 50 cm x 25 cm, Matrix = 512 x 256, ST
= 5 mm
Sources of Noise
• The patient’s body
• Electrical devices – radio, TV, lights,
motors, etc.
Signal-to-Noise Ratio (S/N)
• Two ways to increase S/N
– Increase signal
– Decrease noise
How to increase S/N
• Increase voxel size—this increases S
– Done by increasing ST or increasing pixel size
• Increase field strength
– increases S because S increases with
increasing B0
– Also increases N, but not as much as S, so
S/N increases
How to increase S/N
• Adjust TR and TE – longer TR and shorter
TE lead to increase S, and therefore,
increased S/N
• Use better RF receive coils
– Body coils are worst, small surface coils are
best
– New multicoil designs increase S
How to increase S/N
• Decrease receive Bandwidth (BW)
• BW = 1/(dwell time)
• Problem of increased chem shift artifact
with decreased BW
• If you increase BW to decrease chem
shift, you get more noise
How to increase S/N
• Signal averaging
The number of signals averaged (NSA), or the
number of excitations (NEX) or the number of
acquisitions (NACQ) all mean the same thing.
It is like taking multiple pictures and adding
them together. The noise does not increase
as much as the signal.
S/N ~ sqrt (NEX), so 4 NEX doubles S/N