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DOI: 10.1148/rg.254055027
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Fundamental Physics of MR Imaging1

Robert A. Pooley, PhD

1 From the Department of Radiology, Mayo Clinic, 4500 San Pablo Rd, Jacksonville, FL 32224. From the AAPM/RSNA Physics Tutorial at the 2004 RSNA Annual Meeting. Received February 11, 2005; revision requested March 22 and received April 22; accepted April 25. The author has no financial relationships to disclose.


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  		Figure 1.  Electrons flowing along a wire. An electric current in a loop of wire will produce a magnetic field (black arrow) perpendicular to the loop of wire. e– = electron.

 


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  		Figure 2.  Hydrogen proton. The positively charged hydrogen proton (+) spins about its axis and acts like a tiny magnet. N = north, S = south.

 


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  		Figure 3.  Main magnetic field. A large electric current in loops of wire at superconducting temperatures will produce a very large magnetic field. N = north, S = south.

 


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  		Figure 4.  Alignment of protons with the B0 field. With no external magnetic field, hydrogen protons (+) are oriented randomly. When the protons are placed in a strong magnetic field (B0), a net magnetization will be produced parallel to the main magnetic field.

 


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  		Figure 5.  Coordinate system. For a typical 1.5-T cylindrical-bore imaging unit, the z axis (longitudinal direction) is often aligned with the main magnetic field; the plane perpendicular to this is called the transverse plane.

 


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  		Figure 6.  Precession. Precession of a spinning top and nuclear precession are similar in that an external force combined with the spinning motion causes precession.

 


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  		Figure 7.  Larmor equation. The Larmor equation allows us to determine the frequency of precession of a proton in a magnetic field.

 


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  		Figure 8.  Absorption of RF energy. Left: Prior to an RF pulse, the net magnetization (small black arrow) is aligned parallel to the main magnetic field and the z axis. Center and right: An RF pulse at the Larmor frequency will allow energy to be absorbed by the protons, thus causing the net magnetization to rotate away from the z axis.

 


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  		Figure 9.  Longitudinal (T1) relaxation. Application of a 90° RF pulse causes longitudinal magnetization to become zero. Over time, the longitudinal magnetization will grow back in a direction parallel to the main magnetic field.

 


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  		Figure 10.  Definition of T1. T1 is a characteristic of tissue and is defined as the time that it takes the longitudinal magnetization to grow back to 63% of its final value.

 


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  		Figure 11.  T1-weighted contrast. Different tissues have different rates of T1 relaxation. If an image is obtained at a time when the relaxation curves are widely separated, T1-weighted contrast will be maximized. Mag = magnetization.

 


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  		Figure 12.  Transverse (T2*) relaxation. Immediately after application of a 90° RF pulse, transverse magnetization is maximized; it then begins to dephase due to several processes (Table). The signals from these dephasing protons begin to cancel out, and the MR signal decreases.

 


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  		Figure 13.  Measurement of the MR signal. A magnetic field (black arrow) that is near and perpendicular to a loop of wire will produce an electric current in the loop. The current can be digitized and stored for later reconstruction into an MR image.

 


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  		Figure 14.  Definition of T2. T2 is a characteristic of tissue and is defined as the time that it takes the transverse magnetization to decrease to 37% of its starting value.

 


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  		Figure 15.  T2-weighted contrast. Different tissues have different rates of T2 relaxation. If an image is obtained at a time when the relaxation curves are widely separated, T2-weighted contrast will be maximized. Mag = magnetization.

 


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  		Figure 16.  Mechanism of spin echo. After transverse magnetization has begun to dephase in the transverse plane, application of a 180° RF pulse will rotate the proton spins to the opposite axis. This rotation will allow the spins to rephase and form an echo.

 


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  		Figure 17.  Formation of spin echoes. Application of a 90° RF pulse results in an immediate signal (called a free induction decay [FID]), which rapidly dephases due to T2* effects. Application of a 180° RF pulse will allow formation of an echo at a time TE. Multiple 180° pulses will form multiple echoes. Mag = magnetization.

 


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  		Figure 18.  Pulse sequence diagram. A pulse sequence diagram can be used to show the relative timing of certain events during an MR imaging acquisition. The timing of RF pulses, the signal formed from these pulses, and the digitization of the signal is shown. TE is shown as the time to the echo, and the repetition time (TR) is shown as the time it takes to go through the pulse sequence once. This pulse sequence uses a 90° RF pulse with a 180° RF pulse to rephase spins to form an echo. T1- and T2-weighted images may be created with this pulse sequence. ADC = analog-to-digital converter; in all pulse sequence diagrams, G = gradient.

 


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  		Figure 19.  Parameters for T1 weighting. Short TE (producing minimal T2 weighting) and intermediate TR (producing maximal T1 weighting) will result in a T1-weighted image.

 


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  		Figure 20.  Parameters for T2 weighting. Long TE (producing maximal T2 weighting) and long TR (producing minimal T1 weighting) will result in a T2-weighted image.

 


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  		Figure 21.  Parameters for proton density weighting. Short TE (producing minimal T2 weighting) and long TR (producing minimal T1 weighting) will result in a proton density–weighted image.

 


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  		Figure 22.  Multiecho spin-echo pulse sequence. This sequence uses a 90° RF pulse with multiple 180° RF pulses to form multiple echoes. Each echo can be used to create a separate image data set with different contrast weighting. The gray highlighting shows the differences between this pulse sequence and the basic spin-echo sequence.

 


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  		Figure 23.  Turbo spin-echo pulse sequence. This sequence uses a 90° RF pulse with multiple 180° RF pulses. Multiple echoes are formed, and the data are used to create a single data set. Multiple rows of raw data are filled during one TR period; this feature allows the pulse sequence to be run fewer times, thus saving imaging time.

 


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  		Figure 24.  Inversion-recovery pulse sequence. This sequence is similar to the basic spin-echo sequence with the addition of an initial 180° inversion pulse. This sequence can be used to suppress the appearance of unwanted signals (eg, those due to fat or fluid). TI = inversion time.

 


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  		Figure 25.  Inversion of the signal in the inversion-recovery sequence. After initial inversion of the longitudinal magnetization, T1 relaxation occurs and the signals from different tissues cross the zero axis at different times. When the signal to be suppressed crosses the zero axis, a 90° RF pulse will rotate all other signals into the transverse plane for image formation. TI = inversion time.

 


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  		Figure 26.  Gradient-recalled-echo pulse sequence. This sequence is similar to the spin-echo sequence except that the initial RF pulse is less than 90° and there is no 180° RF pulse. Signal dephasing and rephasing by means of gradient pulses results in formation of a gradient echo, which is used to produce T1- or T2*-weighted images.

 




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