How Much Tesla Is Too Much?

Transcription

1 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

2 Historical Timeline Clinical Imaging 1970 s MRI clinical imaging at 0.6 T 1980 s 1.5 T imaging for clinical use T scanner Ohio state University T was approved for clinical use T scanners University of Chicago 2007 U of Minnesota, MGH and NIH all have 7.0 T MRI for research

3 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 T seem to be the upper limit for current technology

4 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

5 High Field Strength MRI Scanners 3.0T 1.5T Outwardly very similar

6 7.0T Scanner smaller foot print

7 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 : Chang et al. used 11.7T magnet to perform MRI microcopic exam of the meniscal root ligaments Skeletal Radiol :1395

8 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

9 Advantages Higher SNR and Improved Resolution 1.5T 3T

10 Advantages Higher SNR and Improved Resolution 1.5T 3T

11 Advantages Higher SNR 1.5T 3T TR3383/TE46/ET 3 TR3500/TE37/ET 8

12 Advantages Higher SNR 1.5T 3T TR 1383/TE 10/ET 3 TR 2000/TE 19/ET 6

13 Advantages Higher SNR 1.5T 3T TR 3300/TE 54/ET 12 TR 3500/TE 36/ET 8

14 Patient with Resolving Lateral Friction Syndrome 3.0 T Knee Image 1.5 T Knee Image

15 Fat Saturation Better at 3T

16 Smaller Structures Better Visualized Intrinsic Wrist Ligaments SL ligament and LT ligament clearly seen

17 Improved Resolution Deltoid Ligament Plantar Fascia

18 Mass Great Toe Bluish colored mass with skin discoloration Underside of the great toe

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

20 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

21 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

22 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

23 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

24 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 ($ m) twice as much as a 1.5T scanner ($ m) 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

25 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

26 Thank you

27 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 (8): e 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:

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29 7.0T Scanner smaller foot print

30 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

31 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

32 Advantages Higher SNR and Improved Resolution 1.5T 3T

33 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

34 Fat Saturation Better at 3T T2W T1W

35 Improved Resolution Improved visualization of ligaments and cartilage

36 Deltoid Lesion

37 Vascular Map

38 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 T seem to be the upper limit for current technology

39 Enchondroma Index Finger Observe articular cartilage

40 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

41 High Field Strength MRI Scanner - 3Tesla (3T) Unit 3T MRI scanner has - Double the strength of a 1.5T scanner x Earth s magnetic field Research tool for over 20 years Clinical use for nearly 15 years

42 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

43 Smaller Structures Better Visualized - Collateral Ligaments

44 Great Toe - osteoarthritis Bony spurs [arrow] more easily seen

45 Ulna Collateral Ligament Injury

46 Cartillage fragment and defect

47 Posterior Talo-fibular Ligament

48 Advantages Higher SNR 1.5T 3T