Basic Concepts

CT relies on the differential attenuation of x-ray beams passing through tissues to produce an image. The patient lies on the CT table, with his or her long axis aligned along the longitudinal (z) axis of the scanner. The x-ray tube and detector, housed in a gantry, rotate 360 degrees around the patient so the x-ray beam strikes the patient in the transverse (x/y) axis. Conceptually, the slab of tissue imaged can be divided into many small volume elements (voxels), each with x, y, and z dimensions. The degree to which each of these voxels attenuates the x-ray beam is derived by analyzing the data from all the different angular projections, using a reconstruction method known as convolution-backprojection. The computed attenuation value of each voxel is then converted into a gray-scale value of Hounsfield units (HU) and displayed. The attenuation of distilled water at 0° Celsius and 1 bar of pressure is defined as 0 HU. The attenuation of air at the same standard pressure and temperature conditions is defined as −1000 HU.

The spatial resolution of the CT image depends in part on voxel size. Ideally, each voxel of data would be very small to provide high spatial resolution. Each voxel would also ideally be isotropic (having equal dimensions in all three planes) to provide for excellent image reconstructions in any arbitrary plane. It has been relatively easy to achieve high in-plane resolution (along the x/y axes), to the order of 0.5 to 0.7 mm.¹ It has proved difficult to achieve high resolution in the longitudinal or z-plane, because longitudinal resolution is determined by the slice thickness. Use of thin submillimeter slices reduces the length of tissue that can be scanned in a reasonable time or increases the scan time for equal lengths of tissue imaged. Evolution of CT technology over the years can be seen in part as the pursuit of this isotropic resolution.

Conventional CT

In early-generation CT scanners, each CT slice was acquired by one 360-degree rotation of the gantry around the patient. The scan table was then advanced one slice thickness and the process was repeated to obtain the adjacent slice. Because the electrical cables were attached directly to the gantry, the gantry had to stop after each scan to “unwind” the cabling before advancing to obtain the next slice. This type of scanning, known as step-by-step or conventional scanning, is relatively time consuming and prone to respiratory misregistration. It has largely been replaced by spiral or helical CT, which uses slip ring technology to eliminate the cable problem.

Spiral CT

Spiral CT was developed in the early 1990s to improve scan speed and flexibility. In spiral CT, the x-ray tube and detectors rotate continuously about the patient while the scan table advances the patient continuously through the gantry. As a result, the x-ray beam traces a helical path through the patient and provides a “spiral” of image data. Because the patient is intentionally moved through the gantry during scanning, there is significant motion artifact. However, computational methods known as z-interpolation were specifically developed to manage the spiral dataset and to eliminate the motion artifact caused by patient translation. For any image position along the z-axis of the patient, z-interpolation re-forms the spiral data to fit on a single plane. The conventional convolution-backprojection algorithm for data analysis can then be applied.

Spiral CT does not depend on direct, one-to-one correspondence between scan position and image slice, so image slices can be reconstructed anywhere along the z-axis at different slice thicknesses and varying intervals. This flexibility is an important advantage of spiral CT over conventional CT. Overlapping slices can be acquired with no increase in radiation dose to the patient, resulting in high-quality multiplanar reconstructions. Because scan time is fast, spiral CT examinations can be performed in a single breath-hold to reduce respiratory misregistration and motion artifact, and injected contrast agents can be imaged more quickly over greater lengths of tissue to perform CT angiography (CTA).

One technical factor unique to spiral CT is pitch. Pitch is the ratio of table displacement per 360-degree gantry rotation to slice collimation or thickness (table speed × rotation time/slice collimation). A small pitch gives finer spatial resolution along the z-axis of the patient but covers less tissue in a given time and delivers a higher radiation dose to the patient. A large pitch reduces the radiation dose to the patient but also reduces spatial resolution in the z-axis.

Multislice Spiral CT

The next significant milestone in CT evolution was the introduction of scanners with multiple detector rows. In 1998, all major vendors introduced 4-slice CT scanners capable of acquiring up to four slices per gantry rotation. Instead of a single detector row, multiple detector rows were stacked in the gantry along the z-axis of the patient (Fig. 1-1). The time needed for the gantry to complete a 360-degree revolution (gantry rotation time) was also cut in half from 1 second to 0.5 second. For the same slice thickness, pitch, and scan time, a 4-slice CT scanner could image eight times the distance of a single-slice scanner. Alternatively, the 4-slice scanner could acquire four 1.25-mm slices in half the time that single-slice spiral CT acquired one 5-mm slice. Four-slice CT made higher z-axis resolution feasible for a reasonable anatomic length and scan time.

FIGURE 1-1 Single-slice versus multi-slice CT scanners. The four-slice scanner has multiple detector rows stacked along the z-axis of the patient.

Subsequently, 16-, 40-, and 64-slice scanners were introduced widely for clinical use, the latter in 2004. As a result, slices as thin as 0.5 mm can now be acquired very quickly and over long distances to provide submillimeter resolution in the z-axis, truly isotropic voxels, and isotropic resolution. The advantages of multi-slice CT over single-slice imaging can be summarized as better spatial resolution in the z-axis, faster imaging time, and longer anatomic coverage.

Imaging

Normal Appearance of Images

Attenuation is represented in Hounsfield units on a gray scale in which distilled water is set at 0 HU for standard temperature and pressure, and air is set at −1000 HU. Tissues such as bone, which attenuate the x-ray beam more than water, have positive HU values (approximately 1000 HU for bone) and appear very white. Tissues such as fat, which attenuate the x-ray beam less than water, have negative HU values and appear darker than water (−30 to −100 HU for fat).

The human eye can typically differentiate only 60 to 80 different levels of gray. In practice, therefore, the Hounsfield scale must be narrowed to illustrate specific structures of interest. This is achieved by selecting a gray-scale window of displayed Hounsfield units and arbitrarily making all structures above the chosen window white and all structures below the window black. The window width describes the range of Hounsfield values displayed as shades of gray. The window level gives the center value of that gray-scale window. A head CT is typically viewed at window width of 80 HU and window level of 40 HU, which means that 0 HU and 80 HU are the lower and upper limits of the window, respectively, with 40 HU in the center. This relatively narrow window width successfully displays the small differences in attenuation values of the brain. Figure 1-2 emphasizes the importance of choosing appropriate windows to properly display structures of interest and to detect clinically important pathologic processes.

FIGURE 1-2 Importance of window settings. A, Subdural hematomas can be easily missed with narrow window settings because hemorrhage may lie outside the window and appear as bright as adjacent bone. B, However, widening the window (width 150, level 80) shows a very small right frontal subdural hematoma (arrow). C, Normal brain window (width 80, level 40) shows very subtle loss of gray-white differentiation in the right motor cortex (arrow). D, The acute stroke is made more conspicuous (arrow) by narrowing the window (width 8, level 32) to emphasize the small attenuation difference between gray and white matter.

Artifacts

Common artifacts encountered in CT include patient motion, beam hardening, partial volume effects, and metallic object streak artifacts. Patient motion during scanning creates extensive blurring and misregistration of images. This can be partly mitigated by reducing scan times as much as possible. Beam hardening occurs because the energy profile of the x-ray beam changes as it passes through dense objects such as bone. The softer (lower energy) x-rays are absorbed and filtered out by the bone, leaving a beam composed of only harder (higher energy) x-rays. On head CT, beam hardening typically occurs in the posterior fossa between the petrous apices, causing dark horizontal lines across the brain stem and limiting the utility of CT for assessing pathologic processes in this area. Partial volume artifacts ensue when an imaging voxel contains different types of tissue. The attenuation value of the voxel is a numerical average of the attenuation of all the tissues contained within that voxel. If a portion of the voxel has a very high (or low) Hounsfield unit value, that portion may influence the net attenuation of the voxel disproportionately and obscure the presence of other tissues. Like beam hardening, partial volume effects are most troublesome in the posterior fossa, where they cause streaks or bands of light and dark. Reducing scan thickness produces smaller voxels and helps to reduce partial volume effects. Metallic objects such as aneurysm clips or dental hardware generate intense streak artifacts because their exceptionally high density causes beam hardening and partial volume artifacts. The streaks can completely obscure adjacent structures and prevent their evaluation. Figure 1-3 illustrates these typical artifacts.

FIGURE 1-3 Common CT artifacts. A, Beam hardening is seen between the petrous apices, limiting evaluation of the pons. B, Aneurysm clip causes extensive metallic streak artifact. C, Partial volume artifact is seen as streaks throughout the posterior fossa on this 5-mm thick slice. D, Reducing slice thickness to 2.5 mm significantly reduces partial volume artifact.

Specific Uses

Brain CT is most useful in acute settings, especially emergency departments, because of its fast acquisition time, ready accessibility, and lower cost compared with MRI. As the first-line examination after trauma, CT is more sensitive than MRI for detecting skull fractures and radiopaque foreign bodies such as metal or glass.² CT readily identifies acute subdural/epidural and parenchymal hematomas and hemorrhagic contusions and is superior to MRI for detecting acute subarachnoid hemorrhage.³ CT is particularly helpful for identifying calcification and assessing pathologic processes of bone, both of which may narrow a differential diagnosis. CT is indispensable for studying patients with cardiac pacemakers, defibrillators, intra-orbital metal, or other implants that contraindicate the use of MRI.

CT angiography (CTA) has become important in the initial evaluation of subarachnoid hemorrhage, achieving 90% to 93% sensitivity for detecting aneurysms according to meta-analyses of older studies.^4,5 The faster scan times available with 16- and 64-slice scanners permit selective capture of the arterial phase of contrast opacification without venous contamination and provide images close to true angiograms. The fast, thinly collimated multi-slice acquisitions now permit CTA to be performed over long distances in short periods of time, so CTA can image the entire region from the base of the heart to the vertex of the skull to evaluate stroke patients for left atrial thrombi and potential occlusions in the cervical and intracranial circulations. Although digital subtraction angiography (DSA) remains the gold standard for angiography at present, the sensitivity and speed of CTA are constantly improving, so CTA will come to rival DSA in the near future.⁶

Analysis

In any acute setting, noncontrast head CT can be used to quickly assess for the three Hs—hemorrhage, herniation, and hydrocephalus—which may necessitate immediate neurosurgical intervention. Figure 1-4 illustrates the utility of CT in the acute setting, as well as its importance in the evaluation of bony lesions.

FIGURE 1-4 Uses of CT. A, Extensive right parenchymal and subdural hematomas cause significant right-to-left midline shift, a neurosurgical emergency. B, Acute hydrocephalus with transependymal flow of CSF is seen as low attenuation of periventricular white matter. C, The sphenoid wing hyperostosis associated with this enhancing extra-axial mass is characteristic of meningioma. D, Fibrous dysplasia has a typical CT appearance as an expansile osseous lesion with ground-glass internal matrix.

A sample report is shown in Box 1-1.

FIGURE 1-5 CTA of ruptured right middle cerebral artery (MCA) bifurcation aneurysm. A, The 0.625-mm collimated axial source images obtained on a 64-slice scanner demonstrate the saccular right MCA aneurysm with adjacent intraparenchymal hematoma. Axial (B) and coronal (C) maximum intensity projections (20-mm thickness with interval of 5 mm and 75% overlap) show more of the aneurysm and adjacent vessels with each slice than the thin source images. A small anterior communicating artery aneurysm is seen on the axial image (arrow, B). Coronal (D) and sagittal (E) volume-rendered images are useful to evaluate the relationship of the MCA branches to the aneurysm.

Pitfalls and Limitations

Several important problems do limit the utility of CT. In patients with renal impairment, the use of iodinated intravenous contrast is limited by concerns about contrast-induced nephropathy, generally identified as an increase in serum creatinine concentration after administration of a contrast agent, without an alternative explanation. Although there are no uniform diagnostic criteria (because creatinine levels are not necessarily precise), the two most important risk factors for developing nephropathy are preexisting renal impairment and diabetes.^7,8 Adequate hydration, acetylcysteine, and sodium bicarbonate may help prevent nephropathy in patients with borderline renal function.^9,10

Radiologists are frequently asked what to do with patients who are “allergic” to shellfish or iodine. There is a mistaken assumption that iodine in each of these compounds confers cross-reactivity to iodinated contrast agents. However, there is little to no evidence to indicate that the iodine itself triggers adverse reactions to contrast, seafood, or topical povidone-iodine.¹¹ In patients with a history of significant prior contrast reaction, premedication with histamine blockers and corticosteroids can be performed. Patients describing allergies to seafood should be questioned about the nature of the reaction but only insofar as a history of severe allergy to any food increases the risk of contrast reaction.

Pregnancy and lactation generate additional safety considerations for CT. The radiation dose to the fetus during the mother’s head CT has been estimated at 0 to 1 mGy and is from scattered radiation only. It is generally believed that the risk to the fetus of teratogenesis or childhood cancer is negligible at radiation dosages less than 50 mGy.^12,13 Because the uterus lies outside the field of view and the radiation dose to the fetus is negligible, it is not clear that it is necessary to place lead shielding over the abdomen/pelvis. However, placing shielding may provide reassurance to the patient. Iodinated contrast material should be avoided if possible during pregnancy because of potential concern for fetal hypothyroidism. For lactating women, the traditional recommendation is to discontinue breast feeding for 12 to 24 hours after contrast agent administration and discard the milk.¹⁴

Current Research and Future Direction

CT scanners capable of up to 64-slice acquisitions are in common clinical use and afford submillimeter isotropic resolution, rapid scan times (<5 seconds for head CT), and good coverage (32 to 40 mm z-axis coverage with a single gantry rotation). Because the imaging parameters are now able to meet most clinical demands, it is not clear that increasing the number of slices acquired simultaneously is particularly useful or warranted. Instead, research has focused on meeting specific clinical needs, such as dynamic imaging for perfusion measurements and faster scan times for cardiac imaging.

Increasing the length of coverage along the z-axis may permit an entire organ to be imaged during a single gantry rotation, opening up the potential for dynamic perfusion imaging of individual organs. Ways of increasing the volume of coverage include using flat-panel detectors, manipulating the detector array, and increasing the number of detector rows. Indeed, 256- and 320-slice scanners have been developed and installed in limited capacity, providing 12 to 16 cm of z-axis coverage, although higher data load and cost burden are important considerations.

Dual-energy source CT is another promising area for future development. In this approach, two x-ray tubes and two detectors are housed in the same gantry and are used to deliver two x-ray beams at different voltages (e.g., 80 kV and 140 kV). Advantages of dual-source CT include much faster scan times and higher temporal resolution, which are invaluable for cardiac imaging. Dual-source scanning also has the potential to differentiate between specific tissues such as calcium and blood. This ability can be used to selectively depict a single tissue or selectively delete one tissue from the image. For example, one can accurately subtract bone from CTA images to clearly evaluate vessels at the skull base, an area that has traditionally been difficult to visualize.

Basic Concepts

MR Signal Localization in 2D and 3D Imaging

To localize an echo within the body, one applies small magnetic fields called gradients

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