MRI of Bone and Soft Tissue Tumors and Tumorlike Lesions

Introduction

Overview of Magnetic Resonance Imaging and its Role in the Evaluation of Musculoskeletal Abnormalities

Magnetic resonance imaging (MRI) is a powerful medical imaging method that has been used extensively in the evaluation of musculoskeletal tumors and other lesions.1-3,7-12,20,23-25,29,31,32,36,38-45,47,48 MRI can provide "in vivo" anatomic images of the human body with high, soft tissue contrast resolution. The magnetic resonance (MR) images can be obtained in multiple planes, that is, sagittal, axial, coronal, or various oblique combinations. The "signal" used to generate an MR image comes from hydrogen nuclei (protons) within a human body. In essence, MRI is a hydrogen scan.

The hydrogen nucleus has a net charge of + 1 and spins at a frequency that is dependent on the ambient magnetic field and its particular physical characteristic known as the gyromagnetic ratio. The spinning charge of each hydrogen nucleus gives off a tiny magnetic field perpendicular to the axis of spin, thus acting like a tiny bar magnet. Outside the bore of a magnet, the net magnetic properties (magnetic moment) of a person will be zero because the spinning hydrogen nuclei will be oriented randomly, resulting in an overall cancellation of the sum total of tiny magnetic fields. Once placed into a high field-strength magnet, spinning hydrogen nuclei within the human body become aligned or magnetized along its magnetic field. This net magnetization of hydrogen nuclei is oriented in a low-energy alignment (ground state) that is parallel to the magnetic field of the magnet. The hydrogen nuclei spin (precess) at a frequency proportional to their specific gyromagnetic ratio and the magnetic field, in a relationship known as the Larmor equation. The precessional frequency of hydrogen nuclei at 1.5 tesla (T) is 64 megahertz (MHz).

To generate an MR signal, energy is transferred to the hydrogen nuclei within the magnet by using a radio frequency (RF) pulse at the Larmor frequency. The Larmor frequency is dependent on the field strength of the magnetic device and the gyromagnetic ratio, which is specific for the element or molecule of interest. For MRI, that element is the hydrogen nucleus. The hydrogen nuclei absorb this energy and move out of their ground-state alignment. When the RF pulse is turned off, the energy absorbed by the hydrogen nuclei is emitted at the same frequency. This emitted energy, or MR signal, can be detected by the receiver coils (which act like antennae) in the magnet, and used to produce an MR image. Soft tissue contrast results from: (1) the densities of protons (hydrogen nuclei) within different tissues, (2) the different rates which the protons in various tissues realign themselves with the magnetic field of the magnet (also referred to as T1 relaxation, longitudinal or spin lattice relaxation), and (3) rates of signal decay or dephasing (also referred to as T2 relaxation, transverse or spin spin relaxation). Using these biophysical properties of different normal and abnormal tissues allows MRI to have greater soft tissue contrast than computed tomography (CT).

The main components of a typical MRI scanner include: (1) a large-bore magnet with high field strength (0.3-3.0T); (2) radio frequency (RF) coils within the magnet which can transmit and receive properly-tuned RF pulses, as well as set spatially-dependent magnetic fields (gradients) that allow localization of specific regions of anatomic interest; and (3) a computer which operates the device as well as processes the RF signal data received from the patient to form an anatomic image. To generate an MR image, a person is placed onto a table which can move into specific locations within the bore of the magnet. Once in the magnet, the operator selects programs which include the RF pulse sequences necessary to generate images with the desired contrast parameters based on the proton densities, T1 and T2 values of the various tissues. The data received from the subject or patient are processed by the computer using computer algorithms (2D or 3D Fourier transformation). The images are displayed on the monitor console and transferred to film or other computers. Many systems store the image data on digital tape or optical discs for easy retrieval.

Not all patients can have MRI examinations. Intracranial aneurysm clips, cardiac pacemakers, and metallic foreign bodies in the eyes are absolute contraindications for MRI. In addition, the presence of surgical clips, metallic rods, wires, and other orthopedic hardware can produce artifacts obscuring visualization of the anatomic structures in the region of interest.

Major advantages of MRI for musculoskeletal imaging include excellent soft-tissue contrast resolution, multiplanar imaging capabilities, dynamic rapid data acquisition, and various available contrast agents. MRI has proven to be a powerful imaging modality for abnormalities involving fat, muscles, nerves, bone and bone marrow, and has been used in the evaluation of:

neoplasms of the muscles, fat, nerves, bone, and meninges

response of neoplasms to neoadjuvant (preoperative chemotherapy) and postoperative chemotherapy and/or radiation treatment

residual and/or recurrent tumor after surgery

disorders of histogenesis

congenital and developmental musculoskeletal anomalies

traumatic lesions

hemorrhage

ischemia and infarction of muscles, fat and bone marrow

infectious and noninfectious inflammatory diseases

metabolic disorders.

MR data can also be used to generate images of arteries and veins (MR angiography) in displays similar to conventional angiography. The appearance of blood vessels on MR images depends on various factors such as the type of MRI pulse sequence, pulsatility and range of velocities in the vessels of interest, and size, shape, and orientation of the vessels relative to the image plane. Useful anatomic information of blood vessels can be gained by using spin echo pulse sequences which can display patent vessels as zones of signal void (black-blood images), or gradient recall echo (GRE) pulse sequences which display the moving hydrogen atomic-nuclei (protons) in blood as zones of high signal (bright-blood images). Other options with clinical MRI scanners include: magnetic resonance spectroscopy (acquisition of spectral data to characterize the biochemical properties of selected regions of interest in the soft tissues); diffusion-weighted imaging (evaluation of different rates of proton diffusion between normal and abnormal tissue); and perfusion imaging (evaluation of the differences in rates of contrast enhancement between normal and abnormal tissue).

The appearance of muscle, fascia, tendons, ligaments, and bone cortex and marrow depends on the MRI pulse sequence used as well as the age of the patient imaged. In addition to the standard spin echo or fast spin echo sequences that are commonly used for evaluation of the musculoskeletal system, other MRI pulse sequences or imaging options are sometimes used such as: inversion recovery techniques (STIR-short Tan inversion recovery used for fat signal suppression, FLAIR-fluid attenuated inversion recovery used for fluid signal suppression); GRE imaging with or without MR angiography; magnetic transfer; diffusion/perfusion MR imaging; and frequency selective chemical saturation. Detailed discussions of these sequences and options can be found elsewhere in the literature.

Bone formation occurs by either enchondral or membranous ossification. Longitudinal growth occurs by enchondral bone formation in which a calcified cartilaginous matrix at the growth (physeal) plates is remodeled into bone.41 The physeal plate contains four parallel zones oriented perpendicular to the long axis of bone. The four zones from peripheral (nearest the epiphysis) to proximal (nearest the metaphysis) are (1) resting zone, (2) proliferating zone, (3) hypertrophic zone, and (4) calcifying zone. Active cartilage cell division and maturation occurs in the proliferating and hypertrophic zones. Osteoid matrix formation and mineralization occurs in the calcifying zone, also referred to as the zone of provisional calcification. At the adjacent metaphyseal region (primary spongiosa), remodeling of bone occurs with osteoclastic activity. The resting, proliferating, and hypertrophic zones are radiolucent, and on MRI have high signal on T2-weighted imaging (WI) and frequency selective (FS) T2WI, whereas the zone of provisional calcification has attenuation similar to mature mineralized bone on radiographs and CT, and has low signal on T2WI and FS T2WI.41 With membranous bone formation, bone cells form directly from the periosteum (long bones, facial bones, clavicle) for axial growth, or dura (calvarium) without intervening growth plates. The periosteum has low signal on T1WI and T2WI and is attached to the outer surface of the bone cortex in the meta-diaphyseal regions by collagen fibers (fibers of Sharpey), but is absent from the articular ends of the bones; it is composed of an outer fibrous layer and an inner cellular layer referred to as the cambrium.48 Osteoblastic activity occurs in the cambrium, and is responsible for increasing the diameter of bone during growth in childhood. The periosteum is loosely attached to the cortex in children, whereas it is firmly attached in adults. Reactivation of the perisoteum in adults can occur as a result of trauma, infection, or neoplasms. Perisoteal membranous bone formation occurs with induction of fibroblasts (in the fibrous layer or adjacent...