# Presentation: 575133

## Peter van Gelderen

**Advanced MRI section, LFMI, NINDS, National Institutes of Health**

**Notes:**
No longer accelerated. Accelerated to indicate that data were undersampled, but not all examples here are faster imaging

## Examples: fMRI and anatomy at 3.0T

**Background:**** High-resolution grey-matter only images from 3D EPI sequence with double inversion preparation 1.15×1.15×1.15 mm****3****, rate-2 SENSE, 10 min 21 sec acquisition time**
***[Ikonomidou et al., ISMRM 2004, 2238]***

**Notes:**
As I mentioned before voxels smaller than grey matter important. These are data acquired using a 3D SENSE EPI sequence with double inversion preparation to be able to reconstruct grey/white/csf.

## Examples: bolus tracking at 3T

- EPI Bolus tracking: 144x112, 1.52mm2x(2+0.5) mm slices
- Sense rate 2, window 45ms, TE 40ms, TR 1s, 12 slices, baseline SNR 50-150

- Amplitude

- Time to peak

## Examples:  perfusion MRI at 7T

## Examples: 7T Anatomy

- Interleaved EPI
- 512x384
- 12 echoes, 32 int.
- TE 23ms
- TR 100ms
- flip 10'
- single slice
- 20s scantime, 6 avg

## Outline

- MR dependence on field strength in general
- SNR
- Other factors
- Optimizing Diffusion Imaging
- Field strength
- RF coils

## MR field strength dependence: M0

## MR field strength dependence: frequency

- Induced energy in coil

## MR field strength dependence: Noise

- Noise sources:
-  Coil, preamp
-  Patient

## MR field strength dependence: SNR

- Signal increases with field strength and volume (voxel size)
- Patient noise increases with field strength if coil is large
- Noise increases with bandwidth and volume of coil

- Combined effect: SNR approximately linear with       for larger coils

## MR field strength dependence: SNR

- From: F. Fera et.al., J MRI **19**:19-26 (2004)

## MR field strength dependence: Other factors

## MR field strength dependence: other factors

- R2* versus field strength

## MR field strength dependence: Other factors

- Limits on acquisition: SNR

## MR field strength dependence: Other factors

- Limits on acquisition: Bluring

## MR field strength dependence: Other factors

- Limits on acquisition: Distortion

- Image

- Phase

## MR field strength dependence: Other factors

- Non MR related

- 7T systems are:
- hard to site (350t of shielding, size)
- expensive
- can induce motion sickness symptoms
- claustrofobic
- more force on gradients: louder, more prone to failure

## MR field strength dependence: Other factors

- B1 homogeneity at 7T

## Optimizing Diffusion Measurements

- Relation to field strength:
- SNR of Diffusion image depends on b-factor
- b-factor determines minimum TE
- longer TE reduces SNR by T2* decay
- T2* depends on field strength

## Optimizing Diffusion Measurements

## Optimizing Diffusion Measurements

## Optimizing Diffusion Measurements

- b*D

- fraction

- SNR of D as function of b and fraction of points at b

## Optimizing Diffusion Measurements

- Optimal fraction at b > 0: 78%

- Fraction at b>0

- SNR

## Optimizing Diffusion Measurements

- Optimal b*D: 1.27.  With D = 0.6*10-9 m2/s, b = 2.1*109 s/m2 = 2100 s/mm2

- b*D

- SNR

## SNR versus TE for G= 4G/cm

- SNR

- TE [ms]

- T2*:

- 

- 70

- 50

- 30

## SNR versus TE for G= 8G/cm

- SNR

- TE [ms]

- T2*:

- 70

- 50

- 30

- 

## SNR versus TE 

- SNR

- TE [ms]

- G= 16 12   8               4     G/cm

## Diffusion SNR and field strength

- Relative SNR of D due to T2* relaxation

## Diffusion SNR and field strength

- SNR of D with field dependent baseline,
- T1 and T2* relaxation, relative to 1.5T and G 4G/cm

- Conclusion (part I):
- higher field only makes sense with stronger gradients

## Optinizing SNR: RF Coils

- Noise sources:
-  Coil, preamp
-  Patient

- SNR depends on coil dimensions

## Signal-to-noise ratio in MRI

- Receive coils play an important role in image SNR

**the signal originates from a single voxel**

## MR field strength dependence: M0

## Surface coil sensitivity

## Arrays of surface coils

- Using an array of surface coils, an SNR-gain can be obtained throughout the brain

## Noise resistance @ 63.8 MHz

## Signal-to-Noise Ratio – Number of Coils

## MR field strength dependence: frequency

- Induced energy in coil

## Prototype 8-Element Coil, 63.8 MHz

**- 8 elements of similar area**
**- Interfaced to Siemens 1.5 T**
**- 8-channel high impedance preamplifier**

## Birdcage vs. 4-element dome coil @ 1.5 T

## NIH 16-Element 3.0 T Brain Array

## Signal-to-Noise Ratio – vs. GE Head Coil

## Parallel Imaging

- Parallel imaging (such as SENSE) allows for
- -reduction of the acquisition window
-                   or
- -increased resolution at the same acquisition window.
- Important at high field because:
- - potential for higher resolution with increased SNR
- - shorter T2* reduces usefull acquisition window
- - bandwidth is limited by gradient switching

## Accelerated imaging

## Geometrical Distortion Reduction

**rate-2 SENSE EPI:**
**12.4 ms readout**

**Acquisition with purposely misadjusted vertical shim**
**1.5 T, 64×48 matrix**

## Conclusion 

- High field by itself not very helpful
- 3T with stronger gradients does improve SNR
- SNR can be improved with better RF coils at all fields,
-      but more so at higer fields
- 3T with stronger gradients and better RF coils may be the best choice

## Acknowledgement

- Sjef Duyn
- Jacco de Zwart
- Viki Ikonomedou

## Diffusion SNR and field strength

- SNR of D with field dependent baseline,
- T1 and T2* relaxation, relative to 1.5T and G 4G/cm

- Conclusion (part I):
- higher field only makes sense with stronger gradients