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

• Camouflage

• Irregular borders

• Low contrast

• Motion (blurring)

In-Plane ResolutionIn-Plane Resolution

This is a photo that has been taken at 165x256 resolution

In-Plane ResolutionIn-Plane Resolution

This is a photo that has been taken with 329x512 resolution

In-Plane ResolutionIn-Plane Resolution

Original Resolution 720x1150

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

How detailed must the image be?

• The goal is to select the optimal voxel size for the anatomy and pathology you want to see

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 averagingThe 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

Signal-to-NoiseSignal-to-Noise

Signal-to-NoiseSignal-to-Noise

Signal-to-NoiseSignal-to-Noise

Signal-to-NoiseSignal-to-Noise

AVERAGING

BANDWIDTH

LARGE (WIDE) SMALL (NARROW)

BANDWIDTH = 1/(DWELL TIME)

DWELL TIME

SHORT sample of echo LONG sample of echo

PELVIS

• FOV 340• RFOV 65• Slice 7mm• Matrix 256• Scan Percent 60• NSA 4• Time 5:54

KNEE

• FOV 160• RFOV 80• Matrix 256• Scan Percent 90• Turbo Spin Echo• Slice 4 mm• NSA 2• Knee Coil• Time 2:28

WRIST

• Gradient Echo• 3D Volume Acq.• FOV 100• RFOV 80• Matrix 256• Scan Percent 80• Slice 2.4 mm• Time 3:52

PROTOCOL FACTORS AFFECTING

IMAGE DETAIL IMAGE NOISE

• Slice Thickness• Matrix Size• FOV

• Slice Thickness• Matrix Size• FOV• Coil Selection• Averaging• Bandwidth• TR and TE• Field Strength