Basics of MRI Part I - PDF Free Download

Basics of MRI Part I Mathew J. Dixon, D.O. Chairman Department of Radiology Memorial Health University Medical Center Savannah, GA

Objectives Brief History Concept of MRI Creation of a Magnetic Field Concepts of T1 and T2 Contrast Clinically Useful Information Image identification Safety Common Artifacts

History First scan performed July 3, 1977 on first scanner that was named Indomitable There were appx.. 20,000 clinical MR units in the U.S. in 2000

How does MRI work? About 2/3 of the body is composed of water There are differences in the composition of water in various tissues and organs Pathological processes frequently have a water composition that is different from normal tissues

How does MRI work? Water is composed of hydrogen and oxygen Hydrogen nuclei are capable of acting like a microscopic compass When placed in a magnetic field, the hydrogen nuclei will align with the field like a compass When submitted to radiowaves,, the nuclei will change their alignment/energy state This allows differences in tissues to be measured since the nuclei will re-align at different rates when in different tissues

HUH!

Let s s Review

Place water containing tissues in scanner http://www.magnet.fsu.edu/education/tutorials/magnetacademy/mri/ images/mri-scanner.jpg

Figure 1. Electrons flowing along a wire Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 3. Main magnetic field Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 2. Hydrogen proton Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 4. Alignment of protons with the B0 field Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 6. Precession Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 7. Larmor equation Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 8. Absorption of RF energy Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 9. Longitudinal (T1) relaxation Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 10. Definition of T1 Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 11. T1-weighted contrast Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 12. Transverse (T2*) relaxation Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

T2* and T2 Dephasing

Figure 14. Definition of T2 Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 15. T2-weighted contrast Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

T1 and T2 Relaxation Processes Occur Simultaneously

Figure 13. Measurement of the MR signal Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Still Confused?

Good News!

You only need to understand about 10% of everything we just covered to interpret MRI

Hey, this MRI stuff is just like making decisions at The White House!

Image Identification

Figure 11. T1-weighted contrast Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Figure 15. T2-weighted contrast Pooley, R. A. Radiographics 2005;25:1087-1099 Copyright Radiological Society of North America, 2005

Image Identification STIR

Image Identification FLAIR

Image Identification

Image Identification Qayyum, A. et al. Radiology 2005;237:507-511 Copyright Radiological Society of North America, 2005

Common Artifacts Motion-Respiration, Flow, Cardiac, Patient Susceptibility Chemical Shift Aliasing (Wraparound)

Figure 7a. Motion-related artifacts (Respiratory) Zhuo, J. et al. Radiographics 2006;26:275-297 Copyright Radiological Society of North America, 2006

Figure 7b. Motion-related artifacts (Pulsation) Zhuo, J. et al. Radiographics 2006;26:275-297 Copyright Radiological Society of North America, 2006

Figure 14. Sagittal MR image shows a magnetic susceptibility artifact that resulted from the presence of metallic dental fillings Copyright Radiological Society of North America, 2006 Zhuo, J. et al. Radiographics 2006;26:275-297

Safety General Safety During Pregnancy Orbital Foreign Bodies Foreign Bodies Elsewhere Aneurysm Clips Orthopedic Implants Pacers Stents Heart Valves Spine Stimulators

Question #1 Electric current flowing through a wire will produce: A. Hydrogen protons B. The gyromagnetic ratio C. A magnetic field D. T1 contrast

Question #1 Electric current flowing through a wire will produce: A. Hydrogen protons B. The Exist in the body with or without current The gyromagnetic ratio Is unaltered by current It is fixed for different nuclei 42.6 MHz/T for hydrogen protons C. A magnetic field D. T1 contrast Correct Answer Further proton manipulation will be necessary to create T1 contrast

Explanation #1 Electrons flowing in a wire will create a magnetic field perpendicular to the current. Pooley, R. A. Radiographics 2005;25:1087-1099

Question #2 Under normal circumstances and with respect to their magnetization, how are protons oriented in the body? A. Randomly B. Along the Z-axis Z (longitudinal) C. Along the X-axis X (transverse) D. Along the Y-axis Y (transverse)

Question #2 Under normal circumstances and with respect to their magnetization, how are protons oriented in the body? A. Randomly Correct answer B. Along the Z-axis Z (longitudinal) Protons may align along this axis, but this will not be universal and should be temporary until an external magnetic field is applied C. Along the X-axis X (transverse) Protons may align along this axis, but this typically occurs after an external magnetic field and a 90 degree RF pulse are applied D. Along the Y-axis Y (transverse) Protons may align along this axis, but this typically occurs after an external magnetic field and a 90 degree RF pulse are applied

Explanation #2 Protons are distributed randomly in the body until their nuclei are manipulated with external forces. Pooley, R. A. Radiographics 2005;25:1087-1099

Question #3 Which of the following image(s) is/are T1-weighted? A. B. C. D.

Explanation #3 Which of the following image(s) is/are T1-weighted? A. B. C. D. On T1 white matter and fat are bright and CSF and fluid are much darker, which is seen on image A. and D. The CSF in B. is dark because this is a FLAIR sequence, which is heavily T2 weighted B. and C. are incorrect because the white matter is very dark on both Kaplan et al. Musculoskeletal MRI 2001; 1:4.

Question #4 Which imaging artifact is frequently utilized in adrenal imaging? A. Aliasing B. Pulsation C. Gibbs D. Chemical Shift

Question #4 Which imaging artifact is frequently utilized in adrenal imaging? A. Aliasing is incorrect because it refers to imaging wrap and has no place in adrenal imaging B. Pulsation is incorrect since this is an artifact created by repetitive movement such as arterial flow and is also not useful in adrenal imaging unless attempting to detect vascularity within a lesion C. Gibbs is incorrect since Gibbs artifact is a group of bright or dark lines seen adjacent and parallel to regions of abrupt signal change D. Chemical Shift is correct

Explanation #4 D. Chemical shift is correct because it is caused by the difference of chemical shift between fat and water This artifact manifests itself as a misregistration between fat and water pixels in an image This artifact allows identification of microscopic fat in lesions, which is very useful in identifying lipid rich adrenal adenomas http://www.mr-tip.com/serv1.php?type=art

Figure 18a. (a) In-phase MR image acquired with an echo time of 2.2 msec Zhuo, J. et al. Radiographics 2006;26:275-297 Copyright Radiological Society of North America, 2006

Question #5 Which of the following image(s) is/are T2-weighted? A. B. C. D.

Explanation #5 Which of the following image(s) is/are T2-weighted? A. B. C. D. On T1 white matter and fat are bright and CSF and fluid are much darker, which fits images A. and D. The white matter in B. and C. is very dark, indicating that they are T2 weighted. CSF in B is dark because this is a FLAIR sequence, which suppresses CSF signal, but this is still T2 weighted. Kaplan et al. Musculoskeletal MRI 2001; 1:4.