Week 6 and 7 study guide

Increase TR

Scan Time: Increases | SNR: Increases | Spatial Resolution: No Change | Note: Drops T1 tissue contrast.

Decrease TR

Scan Time: Decreases | SNR: Decreases | Spatial Resolution: No Change | Note: Maximizes T1 tissue contrast.

Increase TE

Scan Time: No Change | SNR: Decreases | Spatial Resolution: No Change | Note: Maximizes T2/pathology contrast.

Decrease TE

Scan Time: No Change | SNR: Increases | Spatial Resolution: No Change | Note: Minimizes T2 tissue contrast.

Increase NEX/NSA

Scan Time: Increases | SNR: Increases | Spatial Resolution: No Change | Note: Accumulates signal; reduces random noise.

Decrease Bandwidth

Scan Time: No Change | SNR: Increases | Spatial Resolution: No Change | Note: Increases Chemical Shift; higher min TE.

Increase Bandwidth

Scan Time: No Change | SNR: Decreases | Spatial Resolution: No Change | Note: Decreases Chemical Shift; lower min TE.

Increase Matrix (Fine)

Scan Time: Increases (Phase) | SNR: Decreases | Spatial Resolution: Increases | Note: Reduces partial volume averaging.

Decrease Matrix (Coarse)

Scan Time: Decreases (Phase) | SNR: Increases | Spatial Resolution: Decreases | Note: Blurs fine detail; faster data collection.

Increase FOV

Scan Time: No Change | SNR: Increases | Spatial Resolution: Decreases | Note: Spreads pixels wider; lower spatial detail.

Decrease FOV

Scan Time: No Change | SNR: Decreases | Spatial Resolution: Increases | Note: Packs pixels tighter; severe signal drop.

Increase Slice Thickness

Scan Time: No Change | SNR: Increases | Spatial Resolution: Decreases | Note: Increases partial volume averaging.

Decrease Slice Thickness

Scan Time: No Change | SNR: Decreases | Spatial Resolution: Increases | Note: Minimizes partial volume averaging.

Decrease Phase FOV

Scan Time: Decreases | SNR: Decreases | Spatial Resolution: No Change | Note: High risk of Phase Wrap (Aliasing).

Combination: Decrease NEX + Increase Phase FOV

Scan Time: Balances Out | SNR: Competes | Spatial Resolution: Decreases | Note: Eliminates Phase Wrap (Aliasing).

Which one is the best for resolution

PM	ST	For	Nex
320 x 320	2	10	3
320 x 320	5	10	4
256 x 128	3	14	4
320 x 256	3	20	1

The first one. First find smallet FOV, then highest matrix, then smallest slice thickness

Matrix (Res)	FOV	Slice Thickness
256 × 224	40	4
256 x 256	40	2
512 x 512	40	2
256 x 256	36	3

Which one produces the highest SNR?

The first one. First highest FOV, then highest slice thickness, then course or lower matrix and highest NEX

How to find pixel area?

FOV= 14 cm

M= 256×224

First Fov times 10

14×10=140

2nd: M divided by 140

140/256 × 140/224

0.55 × 0.63= 0.34mm2

How to find voxel volume?

FOV: 24cm

M: 320 × 256

ST: 5

First: FOV x 10

24cm x 10 = 240mm

2nd: FOV divided by Matrix

240/320 × 240/256

0.75 × 0.93 x slice thickness (5) = 3.52mm3

How to find new SNR?

• Original SNR: 100

• Change in NEX: From 2 to 4

• Calculate the factor of increase: Divide new NEX by old NEX:

• 4/2= 2

• find the square rood of factor increase: the square root of 2 is 1.41

• multiply square root by original SNR.

• 1.41 × 100 = 141 ( new SNR)

Rectangular FOV increase?

increase SNR, lower spatial resolution, and reduce the chance of anatomy wrapping into the image. The scan time usually remains the same.

RFOV decreases?

FOV gets smaller. This improves spatial resolution because the pixels are smaller, but SNR decreases because less signal is collected. The risk of wraparound artifacts also increases.