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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...

Info

The decisions made when setting up a pulse sequence depend on the area to be examined, the condition and co-operation of the patient and the clinical throughput required. There are really no rules in MRI - this can be very frustrating when trying to learn, but also makes the subject interesting and challenging. Every facility has protocols established with the cooperation of the manufacturer and the radiologist. However, here are a few tips for optimising image quality. Table 4.2 Parameters and...

Learning Point

This is analogous to the hour and minute hand of a clock. Both hands travel at different speeds around the clock the hour hand moves through 360 in 12 h, whereas the minute hand moves the same distance in 1 h. However at certain times of the day, the hands are superimposed or in phase, i.e. at 12 noon, 1.05 am, 2.10 am, 3.15 am etc. (Fig. 7.23). When fat and water are in phase their signals add constructively, and when they are out of phase their signals cancel each other out (Fig. 7.24). This...

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

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...