Learning Point

Whenever the NMV is pushed beyond 90 it is said to be partially saturated. When the NMV is pushed to a full 180 it is said to be fully saturated. If partial saturation of the fat and water vectors occurs T1 weighting results. If however saturation of the fat and water vectors does not occur, proton density weighting results. To understand this, the processes of T1 recovery should be reviewed. Before the application of the first RF pulse, the fat and water vectors are aligned with B0. When the...

Magnetic resonance angiography MRA

A more sophisticated means of imaging the vascular system is with the use of a technique known as magnetic resonance angiography (MRA). Vascular contrast is maximised by enhancing the signal from moving spins in flowing blood and or suppressing the signal from stationary spins residing in tissue. When stationary spins are suppressed, the appearance of vasculature is enhanced by the increased signal from fresh spins which flow into the imaging volume and receive RF excitation for the first time...

Ferromagnetism

Ferromagnetic substances differ a great deal from diamagnetic and paramagnetic substances. When a ferromagnetic substance, such as iron, comes in contact with a magnetic field, the results are strong attraction and alignment. Objects made of substances of this type can become dangerous projectiles when inadvertently brought near a strong magnetic field. They retain their magnetisation even when the external magnetic field has been removed. Therefore, ferromagnetic substances remain magnetic,...

Gradient rise time

The time that it takes for gradients to reach their maximum strength or amplitude is known as the gradient rise time (Fig. 12.1). If the rise time is reduced, time is saved within the pulse sequence which is then translated into shorter overall imaging times. High gradient amplitudes allow for Fig. 12.1 Gradient amplitude versus rise time. shorter rise times. As shown in Fig. 12.2, the application of enough power to create high gradient amplitudes shortens rise times but yields a power...

Info

Fig. 4.9 Pixel size versus matrix size. results in large pixels, whereas a small FOV produces small pixels. Increasing the FOV size therefore decreases the spatial resolution. Spatial resolution and pixel dimension Square pixels always provide better spatial resolution than rectangular pixels as the image is equally resolved along both the frequency and phase axis. If the FOV is square, the pixels are also square if an even matrix is selected, e.g. 256 x 256. If the FOV is square and an uneven...

Relaxivity

When contrast agents are used in computed tomography (CT), the enhancement is due to concentrations of the agent. When contrast agents are used in MRI it is not the agent itself but the effects of the agent that is measured. The effect of a substance on relaxation rate is known as its relaxivity. As previously discussed, water tumbles much faster than the Larmor frequency resulting in inefficient relaxation and persistence of phase coherence. T1 and T2 times are directly affected by local...

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Larmor Equation Spin And Spin Down

Therefore at high field strengths fewer nuclei have enough energy to join the high energy population. This means that the magnitude of the NMV is larger at high field strengths than low field strengths, resulting in improved signal. Each hydrogen nucleus that makes up the NMV is spinning on its axis as in Fig. 1.8. The influence of Bo produces an additional spin, or wobble of the NMV around Bq. This secondary spin is called precession and causes the magnetic moments...

Pulse Sequences

Conventional gradient echo The steady state Coherent residual transverse magnetisation Incoherent residual transverse magnetisation Steady state free precession (SSFP) Ultra-fast sequences Echo planar imaging (EPI) Summary Questions Understanding pulse sequences forms an integral part of learning MRI. Pulse sequences enable us to control the way in which the system applies pulses and gradients. In this way, image weighting and quality is determined. There are many different pulse sequences...

Further Reading

The following texts provide alternative and or more in-depth discussions on many of the topics included in MRI in Practice. Bushong, S. (1996) Magnetic Resonance Imaging Physical and Biological Principles. Moseby St Louis, MO. Cordoza, J. & Herfkens, R. (1994) MRI Survival Guide. Lippincott Raven New York, NY. English, P. & Moore, C. (1995) MRI for Radiographers. Springer Verlag London. Hashemi, R.H. & Bradley, W.G. Jr. (1997) MRI The Basics. Williams and Wilkins Baltimore MD. Kaut, C....

Bright blood imaging

Echo Image Medical

In addition to making the vessels appear black, vascular structures can also be visualised by making them bright. Several techniques can be used to enhance the signal from flowing blood including gradient echo imaging and or gradient moment rephasing and or contrast enhancement. In gradient echo imaging flowing spins are refocused by the rephasing gradient and hence patent vessels appear bright on the image. As a result this technique can be referred to as bright blood imaging (Fig. 8.3) and...

The remedy

Cross talk can never be eliminated as it is caused by the natural dissipation of energy by the nuclei. Cross excitation can be reduced by ensuring that there is at least a 30 gap between the slices. This is 30 of the slice thickness itself, and reduces the likelihood of RF exciting adjacent slices. For example, if the slice thickness selected is 5 mm use a skip or gap of 2 mm (40 of 5 mm), rather than a 1 mm gap (20 of 5 mm). In addition, most systems excite alternate slices during the...

Parameters and Tradeoffs

Signal to noise ratio (SNR) How to increase SNR Contrast to noise ratio (CNR) Spatial resolution How to increase spatial resolution There are many parameters available to the operator when setting up a sequence. The choice of pulse sequence determines the weighting and the quality of the images as well as their sensitivity to pathology. The timing parameters selected specifically determine the weighting of the images. TR determines the amount of T1 and proton density weighting. Flip angle...

Overcoming the disadvantages of TOFMRA

Therer are a number of ways of overcoming the limitations of TOF-MRA for both 2D and 3D acquisitions. These are listed above and there are several imaging options and protocol modifications that compensate for these pitfalls. To overcome the susceptibility artefacts that are present on any gradient echo sequence, including MRA, short TEs and small voxel volumes should be utilised. In general, longer TEs permit more dephasing and therefore a TE of less than 4 ms minimises this artefact. The...

D fast spin echo

Even with high speed gradients, slice thickness is limited to 1 mm on most imaging systems. 3D fast spin echo acquisitions provide higher resolution and less susceptibility artefact than conventional gradient echo 3D acquisitions. 3D fast spin echo acquistions are acquired by the excitation of a slab (as opposed to a single slice), phase encoded (multiple lines of K space per TR period) and frequency encoded. One of the main applications of 3D fast spin echo is the acquisition of high...

Time of flight MRA

Time of flight MRA (TOF-MRA) produces vascular contrast by manipulating the longitudinal magnetisation of the stationary spins. TOF-MRA uses an incoherent gradient echo pulse sequence in combination with gradient moment rephasing to enhance flow. In TOF-MRA, the TR is kept well below the T1 time of the stationary tissues so that T1 recovery is prevented. This beats down the stationary spins, whilst the in-flow effect from fully magnetised flowing fresh spins produces high vascular signal....

Conventional vascular imaging techniques

These techniques involve using options such as gradient moment rephasing and pre-saturation. As previously discussed in Chapter 6, they are used to reduce motion artefact from flowing nuclei. However, as they give nuclei flowing in blood either signal void or signal enhancement, they also produce contrast between vessels and the surrounding tissue. These techniques can therefore be very useful to demonstrate occlusion of a vessel, if the more recent angiographic procedures are not available....

Flow Phenomena

Phase Mis Mapping Mri

The mechanisms of flow Time of flight phenomenon Entry slice phenomenon in-flow effect Intra-voxel dephasing This chapter specifically explores artefacts produced from nuclei that move during the acquisition of data. Flowing nuclei exhibit different contrast characteristics from their neighbouring stationary nuclei, and originate primarily from nuclei in blood and CSF. The motion of flowing nuclei causes mismapping of signals and results in artefacts known as flow motion artefacts or phase...

B

Fig. 3.3 Steep and shallow gradient slopes. Gradients perform many important tasks during a pulse sequence. As previously described, gradients can be used to either dephase or rephase the magnetic moments of nuclei. Gradients also perform the following three main tasks in encoding. 1 Slice selection - locating a slice within the scan plane selected. 2 Spatially locating encoding signal along the long axis of the anatomy - this is called frequency encoding. 3 Spatially locating encoding signal...