a Depends on degree of fat deposition.
b Depends on the hemoglobin concentration and hematocrit.
c Depends on age and fat deposition.
d Very limited evaluation secondary to partial volume effect.
CSF = cerebrospinal fluid.
Although the density of the salivary glands is variable, the parotid glands tend to be slightly lower in density relative to muscle, secondary to a higher fat content and become progressively more fat replaced over time. The CT density of parotid glands varies from −10 to +30 H. The submandibular glands are denser than parotid glands and are equivalent in density to muscle. The submandibular glands vary in density from +30 to +60 H.
Figure 2.7. CT angiogram (CTA) of the neck at the level of the parotid gland demonstrating the retromandibular vein and adjacent external carotid artery (large white arrow). Note the right cervical lymphangioma (thin white arrow) associated with the tail of the right parotid gland.
CT angiography (CTA) is a powerful method, which allows visualization of arterial vasculature, demonstrating the vascular anatomy of arteries and veins. CTA can be critical in preoperative evaluation to determine the degree of vascularity of lesions and plan an appropriate surgical approach to minimize blood loss or perform preoperative embolization. CTA is obtained with fast image acquisition over a defined region of interest while administering a rapid IV contrast bolus timed to arrive in the region of interest during image acquisition. CTA images may be rendered in 3D data sets and rotated in any plane (Figure 2.7). CTA is not only useful for preoperative planning, but it can also be quite useful in diagnosis of salivary gland vascular pathology such as aneurysms, or arteriovenous fistulas (AVFs) (Wong et al. 2004).
CT scanning, as with all imaging modalities, is prone to artifacts. Artifacts can be caused by motion, very dense or metallic implants (dental amalgam), and volume averaging. Motion artifact is common and may result from breathing, swallowing, coughing, or sneezing during the image acquisition or from an unaware or uncooperative patient. Metallic implants cause complete attenuation of X‐rays in the beam and result in focal loss of data and bright and dark steaks in the image. Because the image is created from a three‐dimensional section of tissue averaged to form a two‐dimensional image, the partial volume or volume averaging artifact results from partial inclusion of structures in adjacent images. Finally, the beam hardening artifact is produced by attenuation of low‐energy X‐rays, by dense objects, from the energy spectrum of the X‐ray beam, resulting in a residual average high‐energy beam (or hard X‐rays), which results in loss of data and dark lines on the image. This phenomenon is often seen in the posterior fossa of head CT scans caused by the very dense petrous bones. Multi‐detector row CT scanner can help reduce metallic artifacts using advanced algorithms, and reduce motion artifacts secondary to faster scanning speeds.
Advanced computed tomography
Newer CT techniques including CT perfusion and dynamic contrast‐enhanced multi‐slice CT have been studied. Dynamic multi‐slice contrast‐enhanced CT is obtained while scanning over a region of interest and simultaneously administering IV contrast. The characteristics of tissues can then be studied as the contrast bolus arrives at the lesion and “washes in” to the tumor, reaches a peak presence within the mass, and then decreases over time, i.e. “washes out.” This technique has demonstrated differences in various histologic types of tumors, for example, with early enhancement in Warthin's tumor with a time to peak at 30 seconds and subsequent fast washout. The malignant tumors show a time to peak at 90 seconds. The pleomorphic adenomas demonstrate a continued rise in enhancement in all four phases (Yerli et al. 2007).
CT perfusion attempts to study physiologic parameters of blood volume, blood flow, mean transit time, and capillary permeability surface product. Statistically significant differences between malignant and benign tumors have been demonstrated with the mean transit time measurement. A rapid mean transit time of less than 3.5 seconds is seen with most malignant tumors, but with benign tumors or normal tissue the mean transit time is significantly longer (Rumboldt et al. 2005).
MAGNETIC RESONANCE IMAGING (MRI)
Magnetic resonance imaging (MRI) represents imaging technology with great promise in characterizing salivary gland pathology. The higher tissue contrast of MRI, when compared to CT, enables subtle differences in soft tissues to be demonstrated. Gadolinium contrast‐enhanced MRI further accentuates the soft‐tissue contrast. Subtle pathologic states such as perineural spread of disease are better delineated when compared with CT. This along with excellent resolution and exquisite details make MRI a very powerful technique in head and neck imaging, particularly at the skull base. This notwithstanding, its susceptibility to motion artifacts and long imaging time as well as contraindication due to claustrophobia, pacemakers, aneurysm clips, deep brain and vagal nerve stimulators limit its usefulness in the general population as a routine initial diagnostic and follow‐up imaging modality. Many of the safety considerations are well defined and detailed on the popular website, www.mrisafety.com.
MRI Technique
Although the physics and instrumentation of MRI are beyond the scope of this text, a fundamental understanding of the variety of different imaging sequences and techniques should be understood by clinicians to facilitate reciprocal communication of the clinical problem, and understanding of imaging reports.
In contrast to CT, which is based on the use of ionizing radiation, MRI utilizes a high magnetic field and pulsed radiofrequency waves to create an image or obtain spectroscopic data. MRI is based on the proton (hydrogen ion) distribution throughout the body. The basic concept is that protons are normally oriented in a random state. However, once placed in the imaging magnet, a high magnetic field, a large proportion of protons align with the magnetic field. The protons remain aligned and precess (spin) in the magnetic field until an external force acts upon them and forces them out of alignment. This force is an applied radiofrequency pulse, applied for a specified time and specified frequency by an antenna called a transmit coil. As the protons return to the aligned state, they give off energy in the form of their own radiofrequency pulse, determined by their local chemical state and tissue structure. The radiofrequency pulse given off is captured by an antenna, called a receive coil. The energy of the pulse and location are recorded and the process repeated multiple times and averaged, as the signal is weak. The recorded signal is used to form the image. Several different types of applied pulse sequences of radio waves result in different types of images.
The impact of MRI is in the soft‐tissue contrast that can be obtained, noninvasively. The relaxation times of tissues can be manipulated to bring out soft‐tissue detail. The routine sequences used in clinical scanning are spin‐echo (SE), gradient echo (GRE), and echo‐planar (EPI). Typical pulse sequences for head and neck and brain imaging include spin‐echo T1, spin‐echo T2, proton density (PD), fluid attenuation inversion recovery (FLAIR), diffusion weighted images (DWI), post‐contrast T1 and STIR. A variant of the spin‐echo, the fast spin‐echo sequence (FSE) allows for a more rapid acquisition of spin‐echo images. Any one of these can be obtained in the three standard orientations of axial, coronal, and sagittal planes. Oblique planes may be obtained in special circumstances.
Spin‐echo T1
On T1 weighted images, a short repetition time (tr) and short