Bolus Tracking

Bolus tracking is a technique used in CT angiography to visualize vessels more clearly. This method individualizes the timing of a volume of contrast media delivery to a region of interest. The image is scanned once the region of interest reaches a certain level of contrast. This method of imaging is primarily used to evaluate arteries, such as the aorta, pulmonary artery, cerebral and carotid arteries. Images acquired from a bolus track, can be manipulated into a maximum intensity projection (MIP) or a volume rendered image (VRI).

Maximum Intensity Projection (MIP)

MIP is a three dimensional processing method used in CT and MRI. Blood vessels in CT angiography data sets are best viewed as MIP images. This method of reformation takes the brightest pixels in the image volume and projects them into the final MIP. Since blood usually has a higher pixel intensity than other structures in the background, the blood vessels are displayed brighter than the background.

Segmentation

Segmentation is a process used to simplify and/or change the representation of an image into something that is more meaningful and easier to analyze. A label is assigned to every pixel in an image such that pixels with the same label share certain visual characteristics. Desired tissue is imaged while undesireable tissue can be removed.

Banding Artifacts

In transmission computed tomography, relative x-ray attenuation measurements are made at various angles around a patient's body. These input projection data are reconstructed to yield a cross-sectional view of internal structure. If the body section contains material that severely attenuates the x-ray beam, high-density bands that obliterate internal structure will be produced in the process of image reconstruction. A technique has been developed that removes this imaging artifact. The approach views the affected projection data as misinformation. These data are assigned new values, and image reconstruction is performed without changing existing hardware or software.

Pitch Effect

In spiral CT, errors may arise with an excessively high pitch factor. If pitch is increased while holding kVp, mA, and beam collimation constant, then 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 the pitch.

Scalloping Artifact

Scalloping artifact is due to the fact that the slice sensitivity profile is increased in spiral CT so that the partial volume artifacts become stronger. This type of artifact can arise in skull CT, where the skull diameter quickly changes its axial direction. Reducing the pitch factor makes the artifact less noticeable.

Misregistration Artifacts

Patient motion can cause misregistration artifacts within the image. Small motions can cause blurring and larger physical displacements produce artifacts that appear as double images or ghost images. Proper patient stabilization can prevent voluntary motion however, some involuntary motion may be unavoidable.

Stair stepping artifact

Stair stepping appear around the edges of structures in multiplanar and 3D reconstructions when wide collimation and non-overlapping reconstruction intervals are used. Virtually eliminated in multiplanar and 3D reconstructions of thin slice data from todays multi-slice scanners.

Blood Circulation in the Liver

The liver receives blood from 2 sources. It receives oxygenated blood from the hepatic artery and from the hepatic portal vein it receives deoxygenated blood containing newly absorbed nutrients, drugs and possibly microbes and toxins from the gastrointestinal tract. The blood from the hepatic artery and hepatic portal vein are carried into the liver sinusoids, where oxygen, nutrients and some toxic substances are taken up by hepatocytes. The hepatocytes secrete products they manufacture as well as nutrients important to cell functions back into the blood, which drains into the central vein and eventually passes into the hepatic vein. Blood from the hepatic vein passes into the inferior vena cava, which then drains into the right atrium of the heart.

Branches of the Abdominal Aorta

Visceral Branches

Celiac Artery – This is the first visceral branch from the aorta inferior to the diaphragm. The celiac trunk divides into three branches:
1. Left Gastric artery – smallest of the 3 arteries
Supplies the stomach and esophagus
2. Splenic artery – largest of the 3 arteries
Supplies the spleen
Gives rise to 3 arteries:
* Pancreatic artery – supplies pancreas
* Left gastroepiploic artery – supplies the stomach and greater
omentum
* Short gastric artery – supplies the stomach
3. Common hepatic artery – gives rise to 3 arteries:
* Proper hepatic artery – supplies the liver, gallbladder and stomach
* Right gastric artery – supplies the stomach
* Gastroduodenal artery – supplies the stomach, duodenum of the
small intestine, pancreas, and greater omentum

Superior Mesenteric Artery – Arises from the anterior surface of the abdominal aorta about 1 cm inferior to the celiac artery. Gives rise to 5 branches:
1. Inferior pancreaticoduodenal artery – supplies the pancreas and duodenum
2. Jejunal and ileal arteries – supplies the jejunum and ileum of the small
intestine, respectively
3. Ileocolic artery – supplies the ileum and ascending colon of the large
intestine
4. Right colic artery – supplies the ascending colon
5. Middle colic artery - supplies the transverse colon of the large intestine

Inferior Mesenteric Artery – (not labeled) Arises from the anterior aspect of the abdominal aorta. Gives rise to 3 branches:
1. Left colic artery – supplies transverse colon and descending colon of the
large intestine
2. Sigmoid arteries – supplies the descending colon and sigmoid colon of the
large intestine
3. Superior rectal artery – supplies the rectum of the large intestine

Suprarenal arteries (paired) – Three pairs of suprarenal arteries supply the adrenal glands (superior, middle and inferior). Only the middle pair originates directly from the abdominal aorta. The superior arteries arise from the inferior phrenic artery and the inferior suprarenal arteries originate from the renal arteries.

Renal arteries (paired) – supplies blood to the kidneys, adrenal glands and ureters.

Gonadal arteries (paired) – In males, the gonadal arteries are referred to as the testicular arteries. They supply the testes, epididymis, and ureters. In females, the gonadal arteries are called the ovarian arteries. They are much shorter than the testicular arteries and supply the ovaries, uterine (Fallopian) tubes and ureters.

Parietal Branches (not labeled)

Median sacral artery – supplies the sacrum and coccyx

Inferior phrenic arteries (paired) – Immediately superior to celiac trunk. Distributed to the inferior surface of the diaphragm and adrenal glands.

Lumbar arteries (paired) – supply lumbar vertebrae, spinal cord, meninges and to the muscles and skin of the lumbar region