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How to use this book

Introduction

This book has been written with the intention of providing a step-by-step explanation of the most common examinations currently carried out using magnetic resonance imaging (MRI). It is divided into two parts.

Part 1 contains reviews or summaries of those theoretical and practical concepts that are frequently discussed in Part 2. These are:

• parameters and trade-offs
• pulse sequences
• flow phenomena and artefacts
• gating and respiratory compensation (RC) techniques
• patient care and safety
• contrast agents.

These summaries are not intended to be comprehensive but contain only a brief description of definitions and uses. For a more detailed discussion of these and other concepts, the reader is referred to the several MRI physics books now available. MRI in Practice by C. Westbrook, C. Kaut Roth and John Talbot (Wiley Blackwell, 2011, fourth edition) describes them in more depth.

Part 2 is divided into the following examination areas:

• head and neck
• spine
• chest
• abdomen
• pelvis
• upper limb
• lower limb.

Each anatomical region is subdivided into separate examinations. For example, the section entitled Head and Neck includes explanations on imaging the brain, temporal lobes, pituitary fossa, etc. Under each examination, the following categories are described:

• basic anatomy
• common indications
• equipment
• patient positioning
• suggested protocol
• image optimization
• patient considerations
• contrast usage.

Basic anatomy

Simple anatomical diagrams are provided for most examination areas to assist the reader.

Common indications

These are the most usual reasons for scanning each area, although occasionally some rarer indications are included.

Equipment

This contains a list of the equipment required for each examination and includes coil type, gating leads, bellows and immobilization devices. The correct use of gating and RC is discussed in Part 1 (see Gating and respiratory compensation techniques). The coil types described are the most common currently available. These are as follows.

• Volume coils that both transmit and receive radio-frequency (RF) pulses and are specifically called transceivers. Most of these coils are quadrature coils, which means that they use two pairs of coils to transmit and receive signal, so improving the signal to noise ratio (SNR). They have the advantages of encompassing large areas of anatomy and yielding a uniform signal across the whole field of view (FOV). The body coil is an example of this type of coil.
• Linear phased array coils consist of multiple coils and receivers. The signal from the receiver of each coil is combined to form one image. This image has the advantages of both a small coil (improved SNR) and those of the larger volume coils (increased coverage). Therefore linear phased array coils can be used either to examine large areas, such as the entire length of the spinal cord, or to improve signal uniformity and intensity in small areas such as the breast. Linear phased array coils are commonly used in spinal imaging.
• Volume phased array (parallel imaging) uses the data from multiple coils or channels arranged around the area under examination to either decrease scan time or increase resolution. Additional software and hardware are required. The hardware includes several coils perpendicular to each other or one coil with several channels. The number of coils/channels varies but commonly ranges from 2 to 32. During acquisition, each coil fills its own lines of k-space (e.g. if two coils are used together, one coil fills the even lines of k-space and the other the odd lines. k-space is therefore filled either twice as quickly or with twice the phase resolution in the same scan time). The number of coils/channels used is called the reduction factor and is similar in principle to the turbo factor/echo training length (ETL) in fast spin echo (FSE) (see section on Pulse sequences in Part 1). Every coil produces a separate image that often displays aliasing artefact (see section on Artefacts in Part 1). Software removes aliasing and combines the images from each coil to produce a single image. Most manufacturers offer this technology, which can be used in any examination area and with any sequence.
• Surface/local coils are traditionally used to improve the SNR when imaging structures near to the skin surface. They are often specially designed to fit a certain area and, in general, they only receive signal. RF is usually transmitted by the body coil when using this type of coil. Surface coils increase SNR compared with volume coils. This is because they are placed close to the region under examination, thereby increasing the signal amplitude generated in the coil, and noise is only received in the vicinity of the coil. However, surface coils only receive signal up to the edges of the coil and to a depth equal to the radius of the coil. To visualize structures deep within the patient, either a volume, linear or volume phased array coil or a local coil inserted into an orifice must be utilized (e.g. a rectal coil).

The choice of coil for any examination is one of the most important factors that determine the resultant SNR of the image. When using any type of coil remember to:

• Check that the cables are intact and undamaged.
• Check that the coil is plugged in properly and that the correct connector box is used.
• Ensure that the receiving side of the coil faces the patient. This is usually labelled on the coil itself. Note: Both sides of the coil receive signal, but coils are designed so that one side receives optimum signal. This is especially true of shaped coils that fit a certain anatomical area. If the wrong side of the coil faces the patient, signal is lost and image quality suffers.
• Place the coil as close as possible to the area under examination. The coil should not directly touch the patient's skin as it may become warm during the examination and cause discomfort. A small foam pad or tissue paper placed between the skin surface and the coil is usually sufficient insulation.
• Ensure that the coil does not move when placed on the patient. A moving coil during acquisition means a moving image!
• Always ensure that the receiving surface of the coil is parallel to the Z (long) axis of the magnet. This guarantees that the transverse component of magnetization is perpendicular to the coil and that maximum signal is induced. Placing the coil at an angle to this axis, or parallel to the X or Y axis, results in a loss of signal (Figure 1.1).

Figure 1.1 Correct placement of a flat surface coil in the bore of the magnet. The surface of the coil (shaded) area must be parallel to the Z axis to receive signal. The coil is therefore positioned so that transverse magnetization created in the X and Y axes is perpendicular to the coil.

Patient positioning

This contains a description of the correct patient position, placement of the patient within the coil and proper immobilization techniques. Centring and land-marking are described relative to the laser light system as follows (Figure 1.2):

• The longitudinal alignment light refers to the light running parallel to the bore of the magnet in the Z axis.
• The horizontal alignment light refers to the light that runs from left to right of the bore of the magnet in the X axis.
• The vertical alignment light refers to the light than runs from the top to the bottom of the magnet in the Y axis.

Figure 1.2 Positioning of the alignment lights.

It is assumed in Part 2 that the following areas are examined with the patient placed head first in the magnet:

• head and neck (all areas)
• cervical, thoracic and whole spine
• chest (all areas)
• abdomen (for areas superior to the iliac crests)
• shoulders and upper limb (except where specified).

The remaining anatomical regions are examined with the patient placed feet first in the magnet. These are:

• pelvis
• hips
• lower limbs.

Suggested protocol

This is intended as a guideline only. Almost every centre uses different protocols depending on the type of system and radiological preference. However, this section can be helpful for those practitioners scanning without a radiologist, or where the examination is so rare that perhaps neither the radiologist nor the practitioner knows how to proceed. The protocols given are mainly limited to scan plane, weighting, suggested pulse sequence choices and slice positioning.

It must be stressed that all the protocols listed are only a reflection of the authors' practice and research, and are in no way to be considered the law!

If all your established protocols are satisfactory, this section is included for interest only. If, however, you are unfamiliar with a certain examination, the suggested protocol should be useful.

Occasionally in this section coordinates for slice prescription are given in bold type in millimetres (mm) where...