Helical CT Imaging Characteristics

CT was developed in the early 1960s with the first clinical system installed in 1971 [26]. Today, there are five generations of CT scanners characterized by different scanning conditions and properties. Helical or spiral CT is the latest generation of scanners that combines a continuous rotation of the X-ray source and ring of detectors with a continuous movement of the examination table. Hence, data are acquired continuously while the patient is moved through the gantry [27, 28]. Figure 4.3 shows a drawing of the helical CT scanning principle and associated orientations [7]. Figure 4.4 shows a typical two-dimensional (2-D) CT slice through the abdomen. The major organs and structures are labeled including a tumor on the head of the pancreas (Arrow B).

Basic CT principles and operation description can be found in a variety of journal articles and dedicated books [27-29]; Van Hoe and Baert [7] provide a comprehensive summary of the advantages of helical CT relative to standard CT scans for pancreatic tumors, including a description of the various imaging parameters. Kopecky and colleagues from Indiana University Medical Center provide an excellent illustrative presentation of the physical characteristics of helical CT at http://www.indyrad.iupui.edu/public/lectures/multislice/sld001.htm. Herein we focus on the helical CT imaging parameters that may be of interest to the medical imaging scientist and engineer and pertinent to computer applications and CAD development. These parameters include the following:

Figure 4.4: Contrast enhanced, helical CT scan through the abdomen and the head of the pancreas obtained with a reconstruction width of 8 mm (equal to slice thickness). A = liver; B = head of pancreas with tumor; C = bowel; D = spleen; E = right adrenal; F = aorta.

(a) Slice thickness (mm): This is equal to the collimation of the X-ray beam. Maximum slice thickness depends on the size of the detectors and is typically about 8-10 mm. Thick slices are used for general abdominal scans and they usually have better contrast resolution than do thin slices, which in turn have better spatial resolution and require higher radiation dose. Thin slices are used for small organs or to evaluate and review a region of interest in more detail. Pancreatic scans may be performed in either thick (8 mm) scans or thin (less than 4 mm) scans or combinations depending on the case and/or the protocol requirements. Different slice thicknesses result in different image properties and noise characteristics so they have to be carefully considered in computer applications including registration, reconstruction, and segmentation.

(b) Incrementation: This is related to the longitudinal or 2-axis resolution and is also referred to as step or index. It defines the distance between consecutive slices. It is usually equal to the slice thickness leaving no gaps between consecutive slices. In special cases, e.g., small organs or three-dimensional (3-D) reconstructions, it may be smaller than slice thickness yielding an overlap between consecutive slices. Incrementation may be changed retrospectively in helical CT.

(c) Resolution: The field of view is related to the spatial resolution of the CT slices or the in-plane resolution. Based on the field of view and the size of the 2-D matrix (X and Y dimensions in pixels (see Fig. 4.3)) the spatial resolution or pixel size can be determined. The dynamic resolution or pixel depth is determined by the characteristics of the detector.

(d) Exposure: kVp and mAs are the two parameters that influence exposure with kVp defining the beam quality or the average intensity and mAs defining the quantity of the beam. For larger patients, the mAs may be increased.

(e) Pitch: This is the ratio of the distance traveled by the table during one full rotation of the detector (gantry) to the beam collimation. For example, a pitch of 1.0 corresponds to a scan with a beam collimation of 10 mm, 1-sec duration of a 360 degree rotation, and a rate of table movement of 10 mm/sec. By doubling the rate of table translation, the pitch is increased proportionally. In pancreatic screening scans the table speed is on the order of 15 mm/rotation and the pitch is 6. In diagnostic high-resolution scans the table speed is on the order of 6 with a pitch of 6. The higher the pitch the shorter the scan time. For multislice helical CT, pitch is also defined as the ratio of the table travel per gantry rotation to the nominal slice thickness. This is a more ambiguous definition, not applicable to single-slice helical CT, but often used by the manufacturers of multislice CT scanners [30].

(f) Contrast type and amount: For pancreatic scans, a contrast material (e.g., Omnipaque 320 or 350) is administered intravenously prior to imaging at a volume of 100-120 cc. Water, gastograffin, or barium is usually given as oral contrast. There is 50-60 sec scan delay to allow for optimum imaging of the pancreas after the administration of the contrast material.

(g) Data reconstruction interval: This is the thickness of the reconstructed slices and is usually equal to the scanned slice thickness.

Three-dimensional reconstructed CT volumes may be used to generate 2-D views of the organ in the coronal (XZ plane) and sagittal ( YZ plane) modes in additional to the traditional transaxial view (XYplane) (Fig. 4.3).

A standard CT scan of the abdomen and pelvis consists of about 40 slices. An abdominal high-resolution scan of the pancreas may consist of 5-40 slices depending on the selected slice thickness and the orientation of the gland [31]. Slice thickness, slice interval, and slice starting point may be selected retrospectively in helical CT scans because of the continuous nature of the process. This is an advantage of the helical over the conventional CT imaging where these parameters need to be determined at the beginning of the scan at the risk of improper imaging of a lesion [32].

Despite its many advantages, e.g., high-spatial resolution and ability to identify vascular involvement, helical CT fails to detect small (< 2cm) tumors, hepatic metastasis, and peritoneal implants, and also has low specificity to other pancreatic pathology [33]. Most patients with pancreatic cancer are evaluated by combinations of imaging tests rather than a single test. In addition to the limitations of the imaging technique, pancreatic tumor measurements and evaluation during treatment monitoring are done visually using subjective criteria by a single expert or, at best, a panel of experts. Hence, there is significant interobserver, and possibly intraobserver, variability in the evaluation of response to treatment and management of the patients with pancreatic cancer.

Important to the development of image processing techniques for pancreatic cancer applications is the knowledge of the clinical imaging characteristics of the normal and abnormal pancreas. The normal pancreas is relatively easy to delineate on CT slices. Understanding how the image of the normal pancreas may be distorted by disease and particularly pancreatic masses (benign or malignant) is the basis for selecting robust features for the development of automated segmentation, classification, registration, and reconstruction methodologies. The most important features used by the radiologists and oncologists in the evaluation of pancreatic adenocarcinoma on radiologic images are summarized in Table 4.1. These features are merely general observation that may not always hold.

There is variability in the imaging characteristics of pancreatic tumors on CT images that increases detection and diagnostic difficulty relative to other organ abnormalities. Table 4.1 summarizes the characteristics of adenocarcinoma

Table 4.1: Helical CT imaging characteristics of the normal pancreas and changes induced by pancreatic adenocarcinoma [2, 31, 34-36]

Normal pancreas

• Uniform density (image intensity) throughout, slightly lower than that of the liver, no calcifications. Contrast material increases pancreatic density uniformly. Approximately equal density to the spleen, kidneys, and skeletal muscle.

• The contour is smooth with a faint lobulation is some cases.

• A fat plane usually surrounds the normal pancreas with the exception of very thin patients. The fat plane appears as an area of lower intensity than the gland area on the CT scan.

• The anterior-posterior diameter of the normal pancreas averages 3 cm in the head, 2.5 cm in the body, and 2 cm in the tail.

• Organ tends to taper uniformly from head to the tail.

• Reports suggest that the ratio between the transverse diameter of the accompanying vertebral body and the pancreas can be used as a guide for normalcy.

• There is usually a fatty appearance due to the gland's nature.

Abnormal pancreas (Adenocarcinoma)

• Variable imaging characteristics; tumors generally appear isodense to normal pancreatic tissue in enhanced studies. Some adenocarcinomas may show central necrosis or appear as hyperdense areas relative to the rest of the pancreas.

• Abrupt transition to the smooth contour may occur due to the presence of a mass.

• The fat plane is usually disrupted or disappears due the presence of a mass or other disease.

• Changes in the size of the pancreas may occur due to the presence of large masses.

• Duct dilation is one of the most significant consequence of pancreatic adenocarcinomas.

• Alterations occur in organs and structures adjacent to the pancreas due to the presence of masses.

• Gland areas are enhanced with contrast material that could allow separation from normal tissues. Usually appear as hypodense areas due to poor arterial blood supply.

because it is the most common type of pancreatic cancer and, hence, the one better understood. Similar imaging and evaluation procedures are initially followed for all pancreatic tumor types.

All pancreatic tumors are better visualized when intravenous contrast material is used. Only necrotic tumors and very large tumors can be identified without contrast enhancement. Endocrine tumors often have associated calcifications and are less likely to have central necrosis than do adenocarcinomas.

They also enhance more than normal tissue during the initial phases of contrast administration [36]. Cystic neoplasms have a variety of appearances. They can appear solid secondary to the multiple tiny nonvisible cysts or they can appear as multiple small cysts or as "multilocular-appearing mass" with thin septations [37]. Alterations in the bowel, blood vessels, or ducts within or adjacent to the pancreas may be caused by all types of pancreatic tumors and are important features in the identification of pancreatic abnormalities [38].

Once diagnosed, pancreatic tumors are surgically removed or treated. The resection of pancreatic tumors is based on the identified tumor size and the presence or absence of additional abnormal signs on the abdominal CT scans. Resection is determined by three imaging criteria:

• Tumor size (less than 4 cmusually); tumors greater than 5 cm are resectable in less than 10% of the cases.

• Vascular invasion, in particular invasion of superior mesenteric artery/vein or portal vein.

• Presence of malignant ascites, nodal disease outside of the area of resection, liver metastases, or peritoneal carcinomatosis.

Presence of metastatic disease, involvement of the mesenteric, and invasion of the portal or superior mesenteric vein are all indicators of nonresectable disease [3].

In addition to the imaging characteristics of pancreatic cancer, clinical findings contribute to the diagnosis and management of the disease. Clinical and demographic characteristics that may be useful in CAD development include [3]

• age; one of the most significant risk factors for pancreatic cancer.

• presence of jaundice that is usually associated with adenocarcinoma of the pancreatic head; resectability rate of pancreatic tumors is noted to be higher in these patients than in patients not presenting with jaundice.

• abdominal pain that may be used as a survival predictor; shorter survival intervals are associated with greater pain reported prior to surgery.

• weight loss and anorexia symptoms.

Clinical and demographic characteristics play a role in feature selection for clustering and classification. In the past, few CAD application incorporated image and nonimage characteristics in algorithm design. New directions in medical image analysis and processing clearly demonstrate the need to consider the patient as a whole and integrate information from a variety of sources to achieve high performances.

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Responses

  • edilio
    How are slice incrementation changed retrospectively with helical ct?
    5 years ago

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