Bolus tracking is a technique used in computed tomography imaging, to visualise vessels more clearly. A bolus of radio-opaque contrast media is injected into a patient via a peripheral intravenous cannula. Depending on the vessel being imaged, the volume of contrast is tracked using a region of interest at a certain level and then followed by the CT scanner once it reaches this level. Images are acquired at a rate as fast as the contrast moving through the blood vessels. For example, 20 ml. of Visipaque 320 followed by 40 ml. of a saline flush is injected at 4cc/sec. Scanning is begun 10 seconds after the start of injection, scanning 10 scans 3 seconds apart. The scan with the brightest contrast in the ascending aorta is then selected.

Maximum Intensity Projection (MIP) is a computer visualization method for 3D data that projects in the visualization plane the voxels with maximum intensity that fall in the way of parallel rays traced from the viewpoint to the plane of projection. This implies that two MIP renderings from opposite viewpoints are symmetrical images.

MIP is used for the detection of lung nodules in lung cancer screening programs which utilise computed tomography scans. MIP enhances the 3D nature of these nodules, making them stand out from pulmonary bronchi and vasculature.



Segmentation refers to the process of partitioning a digital image into multiple segments (sets of pixels). The goal of segmentation is to simplify and/or change the representation of an image into something that is more meaningful and easier to analyze. Image segmentation is typically used to locate objects and boundaries (lines, curves, etc.) in images. More precisely, image segmentation is the process of assigning a label to every pixel in an image such that pixels with the same label share certain visual characteristics.

The result of image segmentation is a set of segments that collectively cover the entire image, or a set of contours extracted from the image (see edge detection). Each of the pixels in a region are similar with respect to some characteristic or computed property, such as color, intensity, or texture. Adjacent regions are significantly different with respect to the same characteristic(s).

Here is a picture of segmentation of the Aorta lumen and aneuryism.

Artifacts in Spiral CT

MISREGISTRATION ARTIFACTS

Patient motion can cause misregistration artifacts, which usually appear as shading or streaking in the reconstructed image. The use of positioning aids is sufficient to
prevent voluntary movement in most patients. However, in some cases it may be necessary to immobilize the patient by means of sedation. Using as short a scan time as possible helps minimize artifacts when scanning regions prone to movement. Respiratory motion can be minimized if patients are able to hold their breath. Manufacturers minimize motion artifacts by using overscan and underscan modes, software correction, and cardiac gating.In this image a misregistration artifact seen on attenuation-corrected image (left) but not on non–attenuation-corrected image (right). Artifact is result of patient motion between CT and PET acquisitions



SCALLOPING

Scalloping Artifact are caused because the slice sensitivity profile is increased in spiral CT so that partial volume artifacts also become stronger. Scalloping can occur in skull CTs, in slices in which the skull diameter quickly changes its axial direction. This can be corrected by reducing the pitch.

BANDING

Band artifacts are sometimes seen in the endocardium as dark bands or horizontal shifts in multiplanar reformatted (MPR) or three-dimensional (3-D) images. This is caused by motion.




STAIR-STEPPING

Stair step artifacts appear around the edges of structures in multiplanar and
three-dimensional reformatted images when wide collimations and nonoverlapping reconstruction intervals are used. They are less severe with helical scanning, which permits reconstruction of overlapping sections without the extra dose to the patient
that would occur if overlapping axial scans were obtained. Stair step artifacts are virtually eliminated in multiplanar and three-dimensional reformatted images from thin-section data obtained with today’s multisection scanners.



PITCH EFFECT

In single CT spiral scanner, the pitch is the table movement per tube rotation/slice collimation. In multi-sliceCT spiral scanners, the definition is table movement per
rotation/single slice collimation. If pitch is increased, than the table speed increases, mAs decreases, patient dose decreases, and either the effective slice
width increases or the image noise increases. So for reducing the artifacts due to spiral rotation, we should decrease pitch. (No Image)

Abdominal Slices

This is an abdominal CT scan showing anatomy and an aneurism.

The Hepatic Portal System

The circulatory system of the liver is unlike that seen in any other organ. Of great importance is the fact that a majority of the liver's blood supply is venous blood. Seventy five percent of the blood supply to the liver is from the portal vein. Venous blood returning from the small intestine and stomach drains into the superior and inferior mesenteric veins; these two vessels are then joined by the splenic vein just posterior to the neck of the pancreas to form the portal vein. This then splits to form the right and left branches, each supplying about half of the liver. The remaining twenty five pecent of the blood supply to the liver is arterial blood from the right and left hepatic arteries.

Terminal branches of the hepatic portal vein and hepatic arteries empty together and mix as they enter the sinusoids in the liver. Sinusoids are distensible vascular channels lined with highly "holey" endothelial cells and bounded circumferentially by hepatocytes. Here, the blood is screened by specialised macrophages (Kupffer cells) to remove any pathogens that manage to get past the GI defences. The plasma is filtered through the endothelial lining of the sinusoids and bathes the hepatocytes; these cells contain vast numbers of enzymes capable of braking down and metabolising most of what has been absorbed. Some of the blood is stored in the liver for later use, or emptied into the central vein of each lobe. Central veins join into hepatic veins, which leave the liver and empty into the inferior vena cava.

Branches of the Abdominal Aorta

The abdominal portion of the aorta begins at the level of the diaphragm, crossing it via the aortic hiatus at the vertebral level of T12. The aortic branches supply blood to the abdominal cavity. From superior to inferior, they are listed below, with the level and the main area that they supply:

Inferior phrenic: diaphragm (T12)

Suprarenal: adrenal glands

Celiac Trunk: (Anterior) Divides into:

• Lt. Gastric: stomach
• Common Hepatic: liver
• Splenic: Spleen

Superior Mesenteric: intestines (L1)

Renal: Kidneys (L2)

Gonadal: Ovarian or Testicular

Lumbar: Abdominal wall & Spinal Cord (L1-L4)

Inferior Mesenteric: Distal Colon (L3)

CT Physics Class

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