How Much Tesla Is Too Much?

Johnny U. V. Monu, MB, BS; MSc

Professor of Radiology and Orthopedics

University of Rochester School of Medicine

Rochester, New York

Historical Timeline – Clinical Imaging

• 1970’s MRI clinical imaging at 0.6 T

• 1980’s 1.5 T imaging for clinical use

• 1999 8.0T scanner Ohio state University

• 2002 3.0T was approved for clinical use

• 2007 9.4 T scanners University of Chicago

• 2007 U of Minnesota, MGH and NIH all have 7.0 T MRI for research

Overview

• Currently there are 11T, 9T 7T and 4T MRI in research facilities all over the world

• 7 Tesla scanners operate in up to 35 research clinics world wide

• The more powerful the magnet - - The better the images - The faster the imaging time

- The more patient through put

• 10 -12 T seem to be the upper limit for current technology

Where We Are

• Nomenclature - <1.0 T Low field - > 1.0T < 3.0T High field

- > 3.0T Ultra High field

• 1.5T systems are still the work horse for MR imaging

• 3T MRI scanners are already saturating the developed world

• The vendors have started planning marketing to Africa

High Field Strength MRI Scanners

Outwardly very similar

1.5T 3.0T

7.0T Scanner – smaller foot print

Regulation and Use

• The FDA has limited the power of the MRI for clinical use to 8T

• Europe (IEC) limited MRI power to 4T but tends to follow the lead of the US

• 2007 Theysohn et al evaluated 102 patients on whole body ultra high field (7.0T) MRI systems

Magn Reson Mater Phy 2008 21:63

• 2014 Chang et al. used 11.7T magnet to perform MRI microcopic exam of the meniscal root ligaments

Skeletal Radiol 2014 43:1395

Higher Field MRI Scanner-Advantages

• Has higher signal to noise

• Has higher contrast to noise

- Use of less contrast agent

• Images have higher spatial resolution

- Ability to image smaller structures with improved resolution

- Thinner slices can be obtained

• Images can be acquired at faster times

- Less motion degradation

Advantages – Higher SNR and Improved Resolution

1.5T

3T

Advantages – Higher SNR and Improved Resolution

1.5T

3T

Advantages – Higher SNR

TR3383/TE46/ET 3

TR3500/TE37/ET 8

1.5T

3T

Advantages – Higher SNR

TR 1383/TE 10/ET 3

TR 2000/TE 19/ET 6

1.5T

3T

Advantages – Higher SNR

TR 3300/TE 54/ET 12

TR 3500/TE 36/ET 8

3T

1.5T

Patient with Resolving Lateral Friction Syndrome

1.5 T Knee Image

3.0 T Knee Image

Fat Saturation Better at 3T

SL ligament and LT ligament clearly seen

Smaller Structures Better Visualized – Intrinsic Wrist Ligaments

Improved Resolution

Deltoid

Ligament

Plantar Fascia

Mass Great Toe

Bluish colored

mass with skin

discoloration

Underside of the

great toe

T1W, IntW+Fat and Post Contr T1W +Fat Sat

UltraHigh Field Imaging Advantageous In -

• Sports medicine type patients with injury to tendons, ligaments and

cartilage

• Small parts imaging such as small joints of hands and feet

• Some structures sub-optimally imaged in the 1.5T scanner such as

cartilage better imaged at 3T or higher

• Fine detail studies such as evaluating bony trabecular in

osteoporosis

• Functional MRI and MRI spectroscopy better at higher field

strengths

Reality Check

• Scanning parameters must be optimized to realize the

promises of

• Trade off between high resolution scanning versus faster

scanning

• Higher Field strength scanners not efficient at acquiring

T1W SE images

• Consider using FSE T1W sequences

Complaints at UltraHigh Field Scanners

• Acoustic noise increased

• Nausea

• Vertigo

• Flashing lights

• Headaches

• Metallic taste in mouth

• Induced currents inside the

body

• Heat deposited within

tissue higher (SAR issues)

• Missile effects around the

scanner

Other Inherent Limitations

Stronger magnetic field

• Electronic device advisory more stringent

• Susceptibility effects more pronounced - Field inhomogeneity - Chemical shift

• Motion degradation more severe

• Hardware availability e.g. Coils are limited - Ability to build coils in house - Some vendors maintain coil monopoly

Cost of Imaging at at Higher Fields

• Quality of images are not the same

- patient sacrifices quality when scanned in 1.5T

• 3T scanner costs about ($2.5-3.5m) twice as much as a 1.5T scanner

($1.0 -1.5m) to purchase

• Special shielding required higher field scanners - increased installation

cost

• Maintenance cost is increased

• Most 3T scanners owned by medical schools and bigger corporations and

research institutions

• Replenishment of cryogens more frequent at higher fields

In Summary - Higher MRI Scanner

• Images have higher resolution - Ability to image smaller structures with improved resolution - Thinner slices can be obtained

• Images can be acquired at faster times - Less motion degradation - More body area can be covered

• For numerous studies diagnostic ability of the radiologist not significantly altered irrespective of scanner type for now

• Costs for a higher field scanners significantly more than that for a 1.5T scanner

Thank you

Suggested Reading

• 9-, 10- or even 11-Tesla - does higher mean better? Professor Siegfried

Trattnig MD, Clinic for Radiodiagnostics, Medical University of Vienna

• Theysohnet al. Subjective acceptance of 7 Tesla MRI for human imaging,

Magn Reson Mater Phy (2008) 21:63–72

• In Vivo High-Resolution 7 Tesla MRI Shows Early and Diffuse Cortical

Alterations in CADASIL. www.plosone.org. 2014 9(8): e106311

• Stafford RJ.High Field MRI: Technology, Applications, Safety, and Limitations

• Chang et al. Morphologic characterization of meniscal root ligaments in the

human knee with magnetic resonance microscopy at 11.7 and 3 T. Skeletal

Radiol (2014) 43:1395–1402

7.0T Scanner – smaller foot print

Advantages 3T Scanner Higher Tissue Contrast

• Has higher contrast to noise. Tissue contrast is determined by a number of variables including – - the TR and TE chosen by the scan operator - the T1 and T2 relaxation times of the tissues being studied - the use of fat saturation

• Since the T1 relaxation times have increased at 3.0T - the TR must be longer to achieve the same type of contrast seen at

1.5T. - Similarly, the TE should be slightly shorter to account for decreases in

T2 relaxation times

• In gradient echo examinations,

- the flip angle should be lower to account for the increased T1 relaxation times

• Since T2* effects are doubled at 3.0T versus 1.5T, TE needs to be shorter at 3.0T to produce similar contrast for those sequences

Disadvantages - Safety Issues

• Specific Absorption Rate (SARs) issues

- Increased heating of tissues

- Becomes an issue when imaging large structures

• not a real problem in MSK except for spine imaging

- Most units will adjust to approved parameters before scanning

• Same disadvantages as other MRI scanners otherwise

Advantages – Higher SNR and Improved Resolution

3T

1.5T

3T MRI Scanner-Advantages

• Has higher signal to noise

• Has higher contrast to noise

- Use of less contrast agent

• Images have higher spatial resolution

- Ability to image smaller structures with improved resolution

- Thinner slices can be obtained

• Images can be acquired at faster times

- Less motion degradation

Fat Saturation Better at 3T

T2W T1W

Improved Resolution

Improved visualization of ligaments and cartilage

Deltoid Lesion

Vascular Map

Overview

• 1999 Ohio state University 8.0T scanner

• 9.4 T scanners University of Chicago

• U of Minnesota, MGH and NIH all have 7.0 T MRI for research

• 10 -12 T seem to be the upper limit for current technology

Enchondroma – Index Finger

Observe articular cartilage

Advantages 3T Scanner Higher Signal to Noise Ratio

• In theory 3T signal to noise ratio [SNR] 2x that of 1.5T - Generally the intrinsic SNR available in a MRI

experiment is a function of • the strength of the main magnetic field • the volume of tissue being imaged • the radiofrequency coil being used

- However image quality at 3.0T influenced by changes to • tissue relaxation times • sensitivity to magnetic susceptibility • the chemical shift difference between fat and water

High Field Strength MRI Scanner - 3Tesla (3T) Unit

• 3T MRI scanner has

- Double the strength of a 1.5T scanner

- 603x Earth’s magnetic field

• Research tool for over 20 years

• Clinical use for nearly 15 years

High Field MRI Scanner-Advantages

• Has higher signal to noise

• Has higher contrast to noise

• Images have higher spatial resolution

• Images can be acquired at faster times

Smaller Structures Better Visualized - Collateral Ligaments

Great Toe - osteoarthritis

Bony spurs [arrow] more easily seen

Ulna Collateral Ligament Injury

Cartillage fragment and defect

Posterior Talo-fibular Ligament

Advantages – Higher SNR

1.5T

3T