This method of radiology provides a three dimensional view of the internal structures of the body.

Computed tomography (CT) is the imaging technique that creates two-dimensional cross-sectional images from three-dimensional body structures. To obtain image, CT uses a mathematical technique called reconstruction. Although it is important for any individual studying the CT science to recognize that CT is a mathematical process, the author’s emphasis in this chapter is on physical principles only. In general sense, a CT image is the result of “breaking apart” a three-dimensional structure and mathematically putting it back together again and displaying it as a two-dimensional image on a television screen. The primary task of any CT system is to accurately reproduce the internal structures of the body as two-dimensional cross-sectional images. Moreover, CT is capable to overcome superimposition of structures and demonstrate even slight or vanishingly small differences in tissue contrast—this superior feature is inherent for CT.

5.2.1.1 Roentgen generation

The goal of computed tomography is to differentiate tissues and structures within the human body in a cross-sectional manner. What CT measures is the degree by which material affects roentgen radiation as it passes through. Conventional X-ray images are projection images that display the cumulative roentgen attenuation of an entire object. Computed tomography is a technique that acquires a large number of projection images from different rotational angles and uses the cumulative attenuation profiles to calculate the spatial distribution of the roentgen attenuation throughout the interrogated cross-sectional plane. In cardiac imaging, where temporal resolution is essential, projections acquired during half of a full system rotation are sufficient for image reconstruction.

The core components of a CT system are the X-ray tube and the roentgen detectors, which are positioned at opposing sides of the scanner gantry. The high-power roentgen generator produces X-rays with a variable energy level; the maximum kV level is selectable, generally between 70 and 150 kV. The tube potential (kV) relates to the energy level of the photons, where X-ray with a higher kV are more penetrating than the weaker low kV photons. The X-ray beam is not a monochromatic source at a single kV level, but will include a spectrum of energy levels. The lowest energy levels, which do not contribute to the image generation, are filtered out before they enter the gantry. The number of photons per time unit (flux) is expressed in milli-Ampere. The point where electrons from the cathode collide with the anode to generate roentgen photons is called the focal spot. A small focal spot improves spatial resolution, but creates more heat at a constant flux. The anode rotates to better disperse the heat. Repetitive alternation of the focal spot (or more accurately focal ring) by electromagnetically changing the direction of the electrons allows for double-image sampling. Acquisition of slightly overlapping images using the “flying focal spot” improves the image quality in the longitudinal direction. Collimators are used to shape the X-ray beam to fit the detectors on the opposite side of the gantry.

7 Summary

We have summarized direct and iterative approaches to 2D and 3D tomographic reconstruction for x-ray CT, PET and SPECT. With the exception of the rebinning algorithms, which can be used in place of fully 3D reconstruction methods, the choice of direct reconstruction algorithm is determined primarily by the data collection geometry. On the other hand, the iterative approaches (ART, ML and MAP) can be applied to any collection geometry in PET and SPECT. Furthermore, after appropriate modifications to account for differences in the mapping from image to data, these methods are also applicable to transmission PET and SPECT data. X-ray CT data are not Poisson so that a different likelihood model is required if ML or MAP methods are to be used.

Image processing for computed tomography remains an active area of research. In large part development is driven by construction of new imaging systems which are continuing to improve the resolution of these technologies. Carefully tailored reconstruction algorithms will help to realize the full potential of these new systems. In the realm of x-ray CT, new spiral and cone-beam systems are extending the capabilities of CT systems to allow fast volumetric imaging for medical and other applications. In PET and SPECT, recent developments are also aimed at achieving high resolution volumetric imaging through combinations of new detector and collimator designs with fast, accurate reconstruction algorithms. In addition to advances resulting from new instrumentation developments, current areas of intense research activity include theoretical analysis of algorithm performance, combining accurate modeling with fast implementations of iterative methods, direct methods that account for factors not included in the line integral model, and development of methods for fast dynamic volumetric (4D) imaging.

SUMMARY

Computed Tomography (CT) is an advanced industrial x-ray nondestructive testing method. CT provides quantitative 3-D information that enables the detection and precise location of defects; the accurate measurement of internal dimensions; and the measurement and mapping of density distributions. CT technology is very versatile and is not restricted by the size, shape or composition of an object as long as adequate x-ray transmission can be achieved. The performance capability of state-of-the-art industrial CT systems, including their use in automated inspection operations, are reviewed. CT applications are discussed and are illustrated by CT images.

10.3.1 Computed tomography

Computed tomography (CT) is one of the conventional methods to detect cancer and monitor its progression. It helps in determining the size and shape of the tumor, CT scans are a noninvasive procedure, free of any painful steps and it takes 10–30 min. CT is majorly done using x-rays that provides more detailed image as CT scans are cross-sectional images of the body such as the bones, organs and soft tissues produced by the computer. The CT scan results helps in obtaining clear images to identifying the blood vessels which act as the source of nutrients for the tumor cells. CT-guided biopsy uses CT scans to direct the needles to individual tumor locations to remove a specific pair of tissue. CT scans can also be used to inject potential cancer therapeutic drugs on to the tumor, combined with a process called radiofrequency ablation (RFA), where heat is used to destroy the tumor. Continuous CT scans are done on the patients to monitor the effect of the cancer treatments on the tumor cells. Modern CT procedures use special contrast materials for clearer images; these contrast agents can be injected into the veins or introduced into the rectum via enema (CT Scan for Cancer. American Cancer Society, 2015).

3.2 Computed Tomography (CT)

CT images are known for their very excellent contrast sensitivity (on the order of 0.2% or better), low noise and dramatic cross-sectional images. The first unit of industrial CT was manufactured in 1983 by America ARA Corp. and its newest one is ARA COR ICT 1500 System, whose source can be 420 kV or 9MeV, and its spatial resolution is 25μ m. The largest object—weight 10t, length 2.8m, diameter Φ 1.6m can be tested by such system. The Second Intenational Industrial CT Conference was taken place in 1991 in U.S.A. Up to date, America has produced more than 100 units of industrial CT and the yearly gross output value in 1992 by the equipments of industrial CT will be more than 200 million U.S. dollars. China began study industrial CT since 1985 and reached 1mm for spatial resolution in some simple equipments of industrial CT.

9.4.2 CT-guided thermal ablation techniques

CT is not as operator dependent as ultrasound. It covers the critical organs and structures that need to be avoided. CT scanning is much less sensitive to body habitus than ultrasound and images are not affected by bowel gas. The target lesion should ideally be visible on a noncontrast CT examination. Ablation can be performed using a conventional CT scanner or a CT scanner with real-time fluoroscopic capability (Ortiz-Alvarado and Anderson, 2010).

Ablation is most often performed under ultrasound or noncontrast CT guidance. These modalities are sufficient for lesion detection and probe placement in most cases, but neither can depict the exact ablation margins during treatment. The real-time CT–ultrasound fusion imaging matches a preprocedural volumetric CT to real-time ultrasound images (Mcwilliams et al., 2010). An electromagnetic tracking system mounted on the ultrasound probe provides the position and orientation of the probe and permits representation of the corresponding multiplanar reformatted CT image in the same plane and position. A feasibility study showed high and consistent levels of matching accuracy between preprocedural CT and real-time ultrasound imaging in an in vitro liver model, with a mean registration error of only 3 mm (Crocetti et al., 2008).

The fusion imaging technology uses computer programs to analyze preprocedural CT, segment the tumor volume to be treated, design the desired needle approaches for the necessary number of overlapping ablation spheres, and provide real-time guidance during needle placement. CT-integrated robots that partially or fully automate the needle placement process based on a user-defined treatment plan have been designed in tumor ablation (Wood et al., 2007).

CT scanning

CT imaging uses X-rays in conjunction with computing algorithms to image the body. In CT, an X-ray generating tube opposite an X-ray detector (or detectors) in a ring-shaped apparatus rotate around a patient producing a computer-generated cross-sectional image (tomogram). CT is acquired in the axial plane, while coronal and sagittal images can be tendered by computer reconstruction. Radiocontrast agents are often used with CT for enhanced delineation of anatomy. Although radiographs provide higher spatial resolution, CT can detect more subtle variations in attenuation of X-rays. CT exposes the patient to more ionizing radiation than a radiograph. Spiral multi-detector CT utilizes 8, 16, 64, or more detectors during continuous motion of the patient through the radiation beam to obtain much finer detail images in a shorter exam time. With rapid administration of IV contrast during the CT scan, these fine detail images can be reconstructed into 3D images of carotid, cerebral and coronary arteries, CTA (CT angiography). CT scanning has become the test of choice in diagnosing some urgent and emergent conditions such as cerebral hemorrhage, pulmonary embolism (clots in the arteries of the lungs) aortic dissection (tearing of the aortic wall), appendicitis, diverticulitis, and obstructing kidney stones. Continuing improvements in CT technology including faster scanning times and improved resolution have dramatically increased the accuracy and usefulness of CT scanning and consequently increased utilization in medical diagnosis.

The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield at EMI Central Research Labs, Great Britain in 1972. EMI owned the distribution rights to The Beatles music and it was their profits which funded the research. Sir Hounsfield and Alan McLeod McCormick shared the Nobel Prize for Medicine in 1979 for the invention of CT scanning. The first CT scanner in North America was installed at the Mayo Clinic in Rochester, MN in 1972.

2.4.10 Benefits and limitations of CT-based diagnosis

CT test for COVID-19 can be considered as an opening into the disease pathology that could provide important insights into diagnosis and progression [77]. CT-scan may facilitate rapid diagnosis of COVID-19 as there is a specific infection pattern characterizing the typical CT features observed in frontier radiological reports. CT-scan can early detect COVID-19 in suspected patients bearing negative RT-PCR tests, even in asymptotic patients or before the onset of the symptoms [24].

On the contrary, performing CT routinely for a large population is not feasible and carries further risks, mainly due to limited personal protective equipment (PPE) resources, increased risk of viral transmission due to the proximity of patients and radiology technicians, and exposure to additional ionizing radiation [67]. Similar to plain radiography, CT utilizes X-ray radiation to produce images; however, the radiation doses from CT are higher due to multiple exposures [31].

5.3.7 Computed Tomography

CT is helpful in the diagnosis of late CP and its associated complications. CT is limited in the detection of early CP. The usual CT findings in CP are dilation of the pancreatic duct, pancreatic calcifications, and parenchymal atrophy. Additionally, CT findings of CP can be made on the size and shape of the gland. Parenchymal atrophy is seen in half of patients with chronic pancreatitis [80]. However, parenchymal atrophy is neither sensitive nor specific and can be part of the normal aging process. Many patients with severe exocrine insufficiency can have a normal appearing pancreas on CT. Unfortunately there is a poor correlation between pancreatic morphology and exocrine insufficiency from chronic pancreatitis [81]. Intraductal pancreatic calcifications are the most specific and reliable CT signs of chronic pancreatitis; however, these appear late in the disease process or in patients with severe disease [82]. Therefore, it is generally accepted that CT can detect CP with severe or advanced disease. However, MRCP, MRI, and EUS are more sensitive in diagnosis early or mild pancreatitis.