Principles of Magnetisation and Relaxation in MRI

3051 words (12 pages) Essay

21st Sep 2017 Health Reference this

Disclaimer: This work has been submitted by a university student. This is not an example of the work produced by our Essay Writing Service. You can view samples of our professional work here.

Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of UKEssays.com.

NAME: NUR AFIFAH BINTI ABDUL RAHIM

LIST OF FIGURES

Figure 1: The T1 relaxation curve.

Figure 2: The T2 decay curve.

Figure 3 : FID and spin echo.

Figure 4: The images of brain shows different image contrast.

Figure 5: T1 post-contrast demonstrating edema.

Figure 6: The sequences of RF pulse in a partial saturation recovery.

 

 

 

 

 

 

INTRODUCTION

MRI images are obtained by measuring how rapidly hydrogen nuclei of different tissues return to their resting energy states after being excited by a strong magnetic field (MacDonald-Jankowski, 2006). The principles of MRI should be understood clearly to enable us produce good quality images for diagnosis.

Get Help With Your Essay

If you need assistance with writing your essay, our professional essay writing service is here to help!

Find out more

As the protons precess in the external magnetic field, they slowly align themselves in either parallel or anti parallel to the external magnetic field according to the energy level they possessed. At this rate, more protons align parallel to the external magnetic field (Bo) hence produce net vector magnetization (NMV) at longitudinal magnetization, Mz which value is maximum while the transverse magnetization, Mxy is zero (Borisjuk et al., 2013). MR signal is only can be detected on Mxy, so we need to tilt Mz to Mxy.

1. Radiofrequency (RF) Pulse

1.190° and 180° RF pulse

RF pulse is commonly used in MRI for excitation, refocusing and inversion (Pang and Shen, 2007). When the RF pulse is given, the protons will gain energy and resonance will occur if the energy given equals to the precessional frequency of the protons. The amplitude and duration of RF pulse determine the magnitude of the flip angle (Hancock, 2014). 90° RF pulse is used to tilt the vector away from longitudinal magnetization, Mz and transferred completely towards transverse plane results in transverse magnetization, Mxy (Hsu and Lowe, 2006). 180° pulse is required to rephrase or refocusing the protons and it rotates Mz directly opposite to external magnetic field, Bo with the number of protons are larger in anti-parallel states (Kobzar et al., 2012).

2. Relaxation Time

As the RF pulse is turned off, the transverse magnetization slowly disappear while longitudinal magnetization reappeared, (Gultekin and Gore, 2005).

2.1T1 Recovery – Longitudinal Relaxation

T1 relaxation can be defined as the time when the magnetization has returned to 63% of its original value (Lakrimi et al., 2011). T1 relaxation, also known as spin-lattice relaxation occur because as the vector from Mxy returns to Mz, they give up energy to the environment or lattice (Piechnik et al., 2013) .

Figure 1: The T1 relaxation curve (Hancock, 2014).

2.2T2 /T2* Decay – Transverse Relaxation

T2 decay also known as spin-spin relaxation time determine the time of protons loss their phase as well as loss MR signal after RF pulse is turned off. It is also can be defined as the time taken to loss 63% of the transverse magnetization (Lakrimi et al., 2011). T2 relaxation result from loss of energy to the adjacent nuclei (Key, 2007). When the RF pulse is send in, all protons are in phase (phase coherence). However, the protons will lose it phase after RF pulse is turned off. Phase incoherent is caused by two factors which are the inhomegeneities if the external and internal magnetic field. The inhomogeneity of external magnetic field will cause additional transverse relaxation known as T2* decay (Chen et al., 2005).

Figure 2: The T2 decay curve (Hancock, 2014).

3. Free Induction Decay (FID)

FID can be defined as loss of MR signal due to T2* effects. FID decays faster with the time constant T2* because of the inhomogeneities of external magnetic field (Yoshioka et al., 2009). However, the signal can be recovered by using 180° RF pulse and the signal produced is known as spin echo signal (Chavhan et al., 2009).

C:UsersUiTMPicturesfid.png

Figure 3 : FID and spin echo (Yoshioka et al., 2009) .

4. Image Contrast

TR is time interval between applications of another 90 degree pulse in a sequence (Hancock, 2014). TE is the time determines how much T2 decay is allowed to occur in the middle of a 90 degree pulse before signal is read (Fonseca, 2013).

4.1 T1-Weighted Image (T1WI)

If short TR and TE are applied, T1-weighted image (T1WI) will be produced. Anatomy of a part is good to be demonstrated by using T1WI. Tissue with short T1 recovery for example fat will appear bright on the image as it emit high signal while tissue with long T1 recovery (water) is vice versa.

4.2T2-Wieghted Image (T2WI)

If long TR and TE are used, T2-weighted image (T2WI) will be produced. TE long is needed so that differences in T2 can be manifested. Tissue with long T2 (water) will appear bright on the image while tissue with short T2 (fat) will appear dark.

4.3 Proton Density Weighted Image (PDWI)

If long TR and short TE are applied, the image that will be produced is known as proton density weighted image (PDWI). The images produces are influenced by the number of protons they have. The larger the number of protons, the higher the signal will be produced.

File:T1t2PD.jpg

Figure 4: The images of brain shows different image contrast. a) T1-weighted image – CSF (black), grey matter (grey), white matter (light grey) b) T2-weighted image – CSF (white), grey matter (grey), white matter (dark grey) c) PD weighted image – CSF (dark), grey matter (light grey) white matter (dark grey) (Weegenaar, n.d.).

5. Effect of Pathological and Contrast Media (CM) to Relaxation Time

Pathological processes in some tissue will alter the relaxation time hence produces changes in the signal intensity and contrast of the tissues in MR images (Berquist, 2012). Pathologic tissues usually have longer T1 and T2 compared to normal tissue, hence, the tissue will manifest high contrast compared to its surrounding (Yoshioka et al., 2009).

C:UsersAbd RahimPicturesfid.png

Table 1: The table shows different pathologies that affect image contrast in MRI (Hesselink, n.d.).

Both T1 and T2 relaxation time can be shortened by using contrast media to detect pathological tissue (Saeed and Wilson, 2012). Pathological tissue in TI image will appear bright as CM increase the signal intensity while in T2 image, CM will produces broader lines with increased intensity (Yoshioka et al., 2009).

T1 CM3.png

Figure 5: T1 post-contrast demonstrating edema throughout the right fronto-parietal region with an enhancing, necrotic lesion within the anterior right frontal lobe (Chambless et al., 2010).

6. Cell Characteristics Factor That Affect Relaxation Times

Different characteristics of cell will affect both T1 and T2 relaxation time (Shen, 2012). The molecules of tissue which are closely packed together will induce faster energy transfer between the particles. This characteristic is related to T2 relaxation time (Westbrook et al., 2011).

Find out how UKEssays.com can help you!

Our academic experts are ready and waiting to assist with any writing project you may have. From simple essay plans, through to full dissertations, you can guarantee we have a service perfectly matched to your needs.

View our services

Besides, molecules that have low inherent energy also have short T1 relaxation time. It is related to the ability of molecular lattice to easily absorb energy from hydrogen nuclei. It is related in T1 relaxation time (Westbrook et al., 2011).

7. Partial Saturation Pulse Sequences

The pulse sequence parameters should be adjusted so that a high signal can be achieved (Pinus and Mohamed, 2010). Partial saturation is different from saturation recovery pulse sequence based on the time interval given between pulses. (Holma and Sidarosa, 2006). In partial saturation pulse sequence, TR short is applied so that the protons have not fully relaxed (Jordan et al., 2013). T1WI will be produced in this pulse sequence. However, the sequences is said to be saturated if the second pulse is applied immediately after the first pulse, hence produce PDWI (Hashemi et al., 2010).

C:UsersAbd RahimPicturesx1y0.png

Figure 6: The sequences of RF pulse in a partial saturation recovery (Hashemi et al., 2010).

CONCLUSION

The principles of MRI should be understood clearly to obtain images that can use for diagnosis. The importance of knowing the principles also need to be applied not just in theory but also during performing MRI examination. The functions of each parameters used in MRI also should be recognized as they play vital role in producing different type of images.

(1091 words)

REFERENCES

Berquist, T. H. 2012. MRI of the Musculoskeletal System. Sixth ed. Philadelphia, USA: Lippincott Williams & Wilkins.

Borisjuk, L., Rolletschek, H. & Neuberger, T. 2013. Nuclear magnetic resonance imaging of lipid in living plants.

Chambless, L. B., Angel, F. B., Abel, Y. W., Xia, F. & Weaver, K. D. 2010. Delayed cerebral radiation necrosis following treatment for a plasmacytoma of the skull.

Chavhan, G. B., Babyn, P. S., Thomas, B., Shroff, M. M. & Haacke, E. M. 2009. Principles, Techniques, and Applications of T2*- based MR Imaging and Its Special Applications.

Chen, Q., Marble, A. E., Colpitts, B. G. & Balcom, B. J. 2005. The internal magnetic field distribution, and single exponential magnetic resonance free induction decay, in rocks. Journal of Magnetic Resonance, 

Fonseca, M. G. 2013. Spin Echo Magnetic Resonance Imaging.

Gultekin, D. H. & Gore, J. C. 2005. Temperature dependence of nuclear magnetization and relaxation.

Hancock, J. 2014. MRI Study Guide, Australian Instistute of Radiography.

Hashemi, R. H., Bradley, W. G. & Lisanti, C. J. 2010. MRI: The Basics. Third edition ed. Philadelphia, USA: Lippincott, Williams & Wilkins.

Holma, D. A. & Sidarosa, K. 2006. Slice profile optimization in arterial spin labeling using presaturation and optimized RF pulses. Magnetic Resonance Imaging.

Hsu, J.-J. & Lowe, I. J. 2006. Encoding to the longitudinal magnetization for MR imaging and flow velocity mapping.

Jordan, C. D., Saranathan, M., Bangerter, N. K., Hargreaves, B. A. & Gold, G. E. 2013. Musculoskeletal MRI at 3.0 T and 7.0 T: A comparison of relaxation times and image contrast. European Journal of Radiology, 82, 734-739.

Key, A. W. 2007. Magnetic Resonance Imaging.

Kobzar, K., Ehni, S., Skinner, T. E., Glaser, S. J. & Luy, B. 2012. Exploring the limits of broadband 90 and 180 universal rotation pulses.

Lakrimi, M., Thomas, A. M., Hutton, G., Kruip, M., Slade, R., Davis, P., Johnstone, A. J., Longfield, M. J., Blakes, H., Calvert, S., Smith, M. & Marshall, C. A. 2011. The principles and evolution of magnetic resonance imaging.

Macdonald-Jankowski, D. S. 2006. Magnetic Resonance Imaging. Part 1: The Basic Principles.

Pang, Y. & Shen, G. X. 2007. Improving excitation and inversion accuracy by optimized RF pulse using genetic. 30 January 2007 ed.

Piechnik, S. K., Ferreira, V. M., Lewandowsk, A. J., Ntusi, N. A., Banerjee, R., Holloway, C., Hofman, M. B., Sado, D. M., Maestrini, V., White, S. K., Lazdam, M., Karamitsos, T., Moon, J. C., Neubauer, S., Leeson, P. & Robson, M. D. 2013. Normal variation of magnetic resonance T1 relaxation times in the human population at 1.5 T using ShMOLLI. Journal of Cardiovascular Magnetic Resonance.

Pinus, A. B. & Mohamed, F. B. 2010. fMRI Scanning Methodologies.

Saeed, M. & Wilson, M. 2012. Value of MR contrast media in image-guided body interventions. World Journal of Radiology.

Shen, J. 2012. Preserving the excitation profile of small flip angle RF pulses in the presence of rapid transverse relaxation. Journal of Magnetic Resonance.

Westbrook, C., Roth, C. K. & Talbot, J. 2011. MRI in practice, West Sussex, USA, Blackwell Publishing Ltd.

Yoshioka, H., Schlechtweg, P. M. & Kose, K. 2009. Magnetic Resonance Imaging.

1

NAME: NUR AFIFAH BINTI ABDUL RAHIM

LIST OF FIGURES

Figure 1: The T1 relaxation curve.

Figure 2: The T2 decay curve.

Figure 3 : FID and spin echo.

Figure 4: The images of brain shows different image contrast.

Figure 5: T1 post-contrast demonstrating edema.

Figure 6: The sequences of RF pulse in a partial saturation recovery.

 

 

 

 

 

 

INTRODUCTION

MRI images are obtained by measuring how rapidly hydrogen nuclei of different tissues return to their resting energy states after being excited by a strong magnetic field (MacDonald-Jankowski, 2006). The principles of MRI should be understood clearly to enable us produce good quality images for diagnosis.

As the protons precess in the external magnetic field, they slowly align themselves in either parallel or anti parallel to the external magnetic field according to the energy level they possessed. At this rate, more protons align parallel to the external magnetic field (Bo) hence produce net vector magnetization (NMV) at longitudinal magnetization, Mz which value is maximum while the transverse magnetization, Mxy is zero (Borisjuk et al., 2013). MR signal is only can be detected on Mxy, so we need to tilt Mz to Mxy.

1. Radiofrequency (RF) Pulse

1.190° and 180° RF pulse

RF pulse is commonly used in MRI for excitation, refocusing and inversion (Pang and Shen, 2007). When the RF pulse is given, the protons will gain energy and resonance will occur if the energy given equals to the precessional frequency of the protons. The amplitude and duration of RF pulse determine the magnitude of the flip angle (Hancock, 2014). 90° RF pulse is used to tilt the vector away from longitudinal magnetization, Mz and transferred completely towards transverse plane results in transverse magnetization, Mxy (Hsu and Lowe, 2006). 180° pulse is required to rephrase or refocusing the protons and it rotates Mz directly opposite to external magnetic field, Bo with the number of protons are larger in anti-parallel states (Kobzar et al., 2012).

2. Relaxation Time

As the RF pulse is turned off, the transverse magnetization slowly disappear while longitudinal magnetization reappeared, (Gultekin and Gore, 2005).

2.1T1 Recovery – Longitudinal Relaxation

T1 relaxation can be defined as the time when the magnetization has returned to 63% of its original value (Lakrimi et al., 2011). T1 relaxation, also known as spin-lattice relaxation occur because as the vector from Mxy returns to Mz, they give up energy to the environment or lattice (Piechnik et al., 2013) .

Figure 1: The T1 relaxation curve (Hancock, 2014).

2.2T2 /T2* Decay – Transverse Relaxation

T2 decay also known as spin-spin relaxation time determine the time of protons loss their phase as well as loss MR signal after RF pulse is turned off. It is also can be defined as the time taken to loss 63% of the transverse magnetization (Lakrimi et al., 2011). T2 relaxation result from loss of energy to the adjacent nuclei (Key, 2007). When the RF pulse is send in, all protons are in phase (phase coherence). However, the protons will lose it phase after RF pulse is turned off. Phase incoherent is caused by two factors which are the inhomegeneities if the external and internal magnetic field. The inhomogeneity of external magnetic field will cause additional transverse relaxation known as T2* decay (Chen et al., 2005).

Figure 2: The T2 decay curve (Hancock, 2014).

3. Free Induction Decay (FID)

FID can be defined as loss of MR signal due to T2* effects. FID decays faster with the time constant T2* because of the inhomogeneities of external magnetic field (Yoshioka et al., 2009). However, the signal can be recovered by using 180° RF pulse and the signal produced is known as spin echo signal (Chavhan et al., 2009).

C:UsersUiTMPicturesfid.png

Figure 3 : FID and spin echo (Yoshioka et al., 2009) .

4. Image Contrast

TR is time interval between applications of another 90 degree pulse in a sequence (Hancock, 2014). TE is the time determines how much T2 decay is allowed to occur in the middle of a 90 degree pulse before signal is read (Fonseca, 2013).

4.1 T1-Weighted Image (T1WI)

If short TR and TE are applied, T1-weighted image (T1WI) will be produced. Anatomy of a part is good to be demonstrated by using T1WI. Tissue with short T1 recovery for example fat will appear bright on the image as it emit high signal while tissue with long T1 recovery (water) is vice versa.

4.2T2-Wieghted Image (T2WI)

If long TR and TE are used, T2-weighted image (T2WI) will be produced. TE long is needed so that differences in T2 can be manifested. Tissue with long T2 (water) will appear bright on the image while tissue with short T2 (fat) will appear dark.

4.3 Proton Density Weighted Image (PDWI)

If long TR and short TE are applied, the image that will be produced is known as proton density weighted image (PDWI). The images produces are influenced by the number of protons they have. The larger the number of protons, the higher the signal will be produced.

File:T1t2PD.jpg

Figure 4: The images of brain shows different image contrast. a) T1-weighted image – CSF (black), grey matter (grey), white matter (light grey) b) T2-weighted image – CSF (white), grey matter (grey), white matter (dark grey) c) PD weighted image – CSF (dark), grey matter (light grey) white matter (dark grey) (Weegenaar, n.d.).

5. Effect of Pathological and Contrast Media (CM) to Relaxation Time

Pathological processes in some tissue will alter the relaxation time hence produces changes in the signal intensity and contrast of the tissues in MR images (Berquist, 2012). Pathologic tissues usually have longer T1 and T2 compared to normal tissue, hence, the tissue will manifest high contrast compared to its surrounding (Yoshioka et al., 2009).

C:UsersAbd RahimPicturesfid.png

Table 1: The table shows different pathologies that affect image contrast in MRI (Hesselink, n.d.).

Both T1 and T2 relaxation time can be shortened by using contrast media to detect pathological tissue (Saeed and Wilson, 2012). Pathological tissue in TI image will appear bright as CM increase the signal intensity while in T2 image, CM will produces broader lines with increased intensity (Yoshioka et al., 2009).

T1 CM3.png

Figure 5: T1 post-contrast demonstrating edema throughout the right fronto-parietal region with an enhancing, necrotic lesion within the anterior right frontal lobe (Chambless et al., 2010).

6. Cell Characteristics Factor That Affect Relaxation Times

Different characteristics of cell will affect both T1 and T2 relaxation time (Shen, 2012). The molecules of tissue which are closely packed together will induce faster energy transfer between the particles. This characteristic is related to T2 relaxation time (Westbrook et al., 2011).

Besides, molecules that have low inherent energy also have short T1 relaxation time. It is related to the ability of molecular lattice to easily absorb energy from hydrogen nuclei. It is related in T1 relaxation time (Westbrook et al., 2011).

7. Partial Saturation Pulse Sequences

The pulse sequence parameters should be adjusted so that a high signal can be achieved (Pinus and Mohamed, 2010). Partial saturation is different from saturation recovery pulse sequence based on the time interval given between pulses. (Holma and Sidarosa, 2006). In partial saturation pulse sequence, TR short is applied so that the protons have not fully relaxed (Jordan et al., 2013). T1WI will be produced in this pulse sequence. However, the sequences is said to be saturated if the second pulse is applied immediately after the first pulse, hence produce PDWI (Hashemi et al., 2010).

C:UsersAbd RahimPicturesx1y0.png

Figure 6: The sequences of RF pulse in a partial saturation recovery (Hashemi et al., 2010).

CONCLUSION

The principles of MRI should be understood clearly to obtain images that can use for diagnosis. The importance of knowing the principles also need to be applied not just in theory but also during performing MRI examination. The functions of each parameters used in MRI also should be recognized as they play vital role in producing different type of images.

(1091 words)

REFERENCES

Berquist, T. H. 2012. MRI of the Musculoskeletal System. Sixth ed. Philadelphia, USA: Lippincott Williams & Wilkins.

Borisjuk, L., Rolletschek, H. & Neuberger, T. 2013. Nuclear magnetic resonance imaging of lipid in living plants.

Chambless, L. B., Angel, F. B., Abel, Y. W., Xia, F. & Weaver, K. D. 2010. Delayed cerebral radiation necrosis following treatment for a plasmacytoma of the skull.

Chavhan, G. B., Babyn, P. S., Thomas, B., Shroff, M. M. & Haacke, E. M. 2009. Principles, Techniques, and Applications of T2*- based MR Imaging and Its Special Applications.

Chen, Q., Marble, A. E., Colpitts, B. G. & Balcom, B. J. 2005. The internal magnetic field distribution, and single exponential magnetic resonance free induction decay, in rocks. Journal of Magnetic Resonance, 

Fonseca, M. G. 2013. Spin Echo Magnetic Resonance Imaging.

Gultekin, D. H. & Gore, J. C. 2005. Temperature dependence of nuclear magnetization and relaxation.

Hancock, J. 2014. MRI Study Guide, Australian Instistute of Radiography.

Hashemi, R. H., Bradley, W. G. & Lisanti, C. J. 2010. MRI: The Basics. Third edition ed. Philadelphia, USA: Lippincott, Williams & Wilkins.

Holma, D. A. & Sidarosa, K. 2006. Slice profile optimization in arterial spin labeling using presaturation and optimized RF pulses. Magnetic Resonance Imaging.

Hsu, J.-J. & Lowe, I. J. 2006. Encoding to the longitudinal magnetization for MR imaging and flow velocity mapping.

Jordan, C. D., Saranathan, M., Bangerter, N. K., Hargreaves, B. A. & Gold, G. E. 2013. Musculoskeletal MRI at 3.0 T and 7.0 T: A comparison of relaxation times and image contrast. European Journal of Radiology, 82, 734-739.

Key, A. W. 2007. Magnetic Resonance Imaging.

Kobzar, K., Ehni, S., Skinner, T. E., Glaser, S. J. & Luy, B. 2012. Exploring the limits of broadband 90 and 180 universal rotation pulses.

Lakrimi, M., Thomas, A. M., Hutton, G., Kruip, M., Slade, R., Davis, P., Johnstone, A. J., Longfield, M. J., Blakes, H., Calvert, S., Smith, M. & Marshall, C. A. 2011. The principles and evolution of magnetic resonance imaging.

Macdonald-Jankowski, D. S. 2006. Magnetic Resonance Imaging. Part 1: The Basic Principles.

Pang, Y. & Shen, G. X. 2007. Improving excitation and inversion accuracy by optimized RF pulse using genetic. 30 January 2007 ed.

Piechnik, S. K., Ferreira, V. M., Lewandowsk, A. J., Ntusi, N. A., Banerjee, R., Holloway, C., Hofman, M. B., Sado, D. M., Maestrini, V., White, S. K., Lazdam, M., Karamitsos, T., Moon, J. C., Neubauer, S., Leeson, P. & Robson, M. D. 2013. Normal variation of magnetic resonance T1 relaxation times in the human population at 1.5 T using ShMOLLI. Journal of Cardiovascular Magnetic Resonance.

Pinus, A. B. & Mohamed, F. B. 2010. fMRI Scanning Methodologies.

Saeed, M. & Wilson, M. 2012. Value of MR contrast media in image-guided body interventions. World Journal of Radiology.

Shen, J. 2012. Preserving the excitation profile of small flip angle RF pulses in the presence of rapid transverse relaxation. Journal of Magnetic Resonance.

Westbrook, C., Roth, C. K. & Talbot, J. 2011. MRI in practice, West Sussex, USA, Blackwell Publishing Ltd.

Yoshioka, H., Schlechtweg, P. M. & Kose, K. 2009. Magnetic Resonance Imaging.

1

Cite This Work

To export a reference to this article please select a referencing stye below:

Reference Copied to Clipboard.
Reference Copied to Clipboard.
Reference Copied to Clipboard.
Reference Copied to Clipboard.
Reference Copied to Clipboard.
Reference Copied to Clipboard.
Reference Copied to Clipboard.

Related Services

View all

DMCA / Removal Request

If you are the original writer of this essay and no longer wish to have your work published on the UKDiss.com website then please: