Abstract
Ultrasound is the investigation of choice for the initial assessment of fibroids. However, the density, number, size or location of the fibroids can degrade the image quality due to the attenuation of the sound waves. Magnetic resonance imaging (MRI) does not suffer the same limitations and is not operator-dependent; it provides detailed and consistent assessment, making it the ideal tool to evaluate fibroid characteristics over time, such as change in size or amount of necrosis following treatment.
4.1 Introduction and Overview
Ultrasound is the investigation of choice for the initial assessment of fibroids. However, the density, number, size or location of the fibroids can degrade the image quality due to the attenuation of the sound waves. Magnetic resonance imaging (MRI) does not suffer the same limitations and is not operator-dependent; it provides detailed and consistent assessment, making it the ideal tool to evaluate fibroid characteristics over time, such as change in size or amount of necrosis following treatment.
In this chapter, the basic MRI technique and features of the various types of fibroids will be discussed with the aid of illustrations.
4.2 MRI Technique
The physics of MRI is complex and beyond the scope of this book. This section covers the basic principles of how an image is produced [1].
MRI takes advantage of the properties of hydrogen atoms in the body. As a spinning charged particle, each hydrogen proton produces a small magnetic field known as the magnetic moment that aligns with the stronger magnetic field of the MRI scanner. The majority of the protons’ magnetic moments form north–south pairs, creating a net magnetism of zero. However, a small excess of protons are unpaired, making their magnetic moments detectable.
A radiofrequency (RF) pulse excites the protons to a higher energy state. On termination of the pulse, the protons return to their original state and the excess energy is released as a radiofrequency signal. The signal from the unpaired protons is detected, amplified, and transformed into an image.
Various parameters are used to manipulate the initial RF pulse and the returning signal; this creates the variety of tissue contrast in the different MRI sequences. The commonest MRI sequences are T1 and T2. Tissues with inherently high signal intensity (bright) on T1 include fat, blood and protein. Fluid and fat have inherently high signal intensity on T2. Gadolinium is used as a contrast agent for MRI and returns high signal intensity on T1 sequences.
The magnetic field produced by a typical MRI scanner (1.5 tesla) is 30,000 times stronger than the Earth’s magnetic field strength. Any ferromagnetic object is a potential projectile that could cause significant harm to the patient or scanner. The strong magnetic field can also have an adverse effect on medical equipment, notably pacemakers; therefore, safety is paramount.
4.3 Features of Fibroids
The typical features of a fibroid on MRI are isodense signal intensity (similar to the skeletal muscle) on T1, low signal intensity (dark) on T2, and a well-defined capsule (usually with a smooth border). The position of a fibroid is assessed by examining the scans in the sagittal, axial and coronal planes (Figures 4.1 and 4.2).
Figure 4.1 Multiple fibroids. (a) Sagittal T2. (b) Axial T2. (c) Sagittal T1 fat-saturated post-contrast. (d) Axial T1.
Subserosal (arrow), intramural (solid arrowheads), submucosal (arrowheads). Typical fibroids are of low signal intensity on T2 and inconspicuous on T1. Fibroids have variable degrees of enhancement following gadolinium administration.
Figure 4.2 Large pedunculated subserosal fibroid (sagittal T2).
The fibroid (arrow) is of low T2 signal intensity and indents the urinary bladder (UB). It has a narrow pedicle (solid arrowheads). There is also a posterior intramural fibroid (arrowhead) distorting the endometrial cavity.
The pattern of enhancement of a fibroid following the MRI contrast medium varies greatly depending on the vascular supply. This particular feature can be used to assess the therapeutic effect of an embolization procedure (Figure 4.3).
Figure 4.3 Posterior intramural fibroid before and after embolization (sagittal T1 post-contrast). (a) Pre-embolization. (b) 3 months post-embolization. (c) 2 years post-embolization.
Note the diminishing enhancement and size of the fibroid after embolization.
The type of degeneration in a fibroid determines its appearance on the MRI sequences [2]. Necrosis of a fibroid may produce disconcerting features that can be misleading. An intact smooth capsule with no other features of invasion or spread, such as lymphadenopathy, is reassuring and points to a benign necrotic process (Figure 4.4).
Figure 4.4 Benign necrotic fibroid. (a) Axial T2. (b) Axial T1 without contrast. (c) Axial T1 fat-saturated post-contrast.
10 cm fibroid with an irregular central area of necrosis (arrow) that is of high signal intensity on T2, low signal intensity on T1 and does not enhance. The fibroid has a smooth intact capsule (solid arrowheads), best seen on T2 and post-contrast sequences.
The main MRI feature of a fibroid undergoing haemorrhagic degeneration (also known as red degeneration) is the presence of blood products of various ages within a well-encapsulated fibroid. The form of haemoglobin within the haemorrhage determines how it appears on T1 and T2 sequences, and varies according to the age of the blood products. For example, a predominance of the methaemoglobin form in the intracellular space leads to high signal intensity on T1 and intermediate to low signal intensity on T2 scans. Haemosiderin is of low signal intensity on both T1 and T2 sequences and surrounds the haemorrhagic area (Figure 4.5).
Figure 4.5 Benign haemorrhagic degeneration. (a) Axial T1. (b) Axial T2.
The central necrotic area contains blood products of different ages. The areas of high signal intensity on T1 and low signal intensity on T2 represent the products with the highest concentration of methaemaglobin (arrow). The haemosiderin rim is of low signal intensity on T1 and T2 (solid arrowhead).
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