Web Log

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. This method of imaging is used primarily to produce images of arteries, such as the aorta, pulmonary artery, cerebral and carotid arteries. The image shown illustrates this technique on a sagittal MPR (multi planar reformat). The image is demonstrating the blood flow through an abdominal aortic aneurysm or AAA. The bright white on the image is the contrast. You can see the lumen of the aorta in which the contrast is contained, surrounded by a grey 'sack', which is the aneurysm. Images acquired from a bolus track, can be manipulated into a MIP (maximum intensity projection) or a volume rendered image.

B. What is Maximum Intensity Projection? Explain how it works, and provide images as examples of the technique.
A 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. This technique is computationally fast, but the 2D results do not provide a good sense of depth of the original data. To improve the sense of 3D, animations are usually rendered of several MIP frames in which the viewpoint is slightly changed from one to the other, thus creating the illusion of rotation. This helps the viewer's perception to find the relative 3D positions of the object components. However, since the projection is orthographic the viewer cannot distinguish between left or right, front or back and even if the object is rotating clockwise or anti-clockwise.

C. What is Segmentation? Explain how it works, and provide images as examples of the technique.
Segmentation algorithms are often based on the principle of region growing. Placing one or more seed points initiates the segmentation of the target structure. From these seed points, more and more neighboring voxels that fulfill predefined criteria are included in the segmentation. The technique can be defined in two ways: segmentation of the desired tissue or segmentation of the undesired tissue with subsequent removal from the data. The latter method removes only interfering tissue from the CT angiography data and retains soft tissue. The process of segmentation is somewhat automatic, an operator is needed to set additional seeding points.
The Web log is due on April 23rd. Once it is complete, send me an e-mail with the exact web address in the body of the e-mail. The subject of the e-mail should be "Web Log" (exactly that, and nothing else).

Artifacts

1. Misregistration
The severity in misregistration comes more from fluctuations of theheart rate more than the use of Burst or Burst Plus. The higher heartrates are handled very well with the sector mode of scanning especiallywhen that rate is very steady. If the heart rate is making frequent andwide variations then the likelihood of misregistration increases. As tothe effect of pitch on misregistration, the lower the pitch the moreoverlap in the 40 mm acquisition is possible, reducing the artifactpotential.In response to a patient scheduled for a pulmonary vein ablation study,for a patient in active atrial fibrillation the recommendation would beto ignore the wildly fluctuating heart rate and set the system to scanin the segment mode with your lowest pitch capability (0.16 on aVCT--type in 35 for the heart rate to set this low) in order to have asmuch overlap in the data as possible. Second option would be to do thestudy as an ungated study and simply capture the heart in a helicalacquisition using your fastest rotation speed for routine scanning ( 0.4sec ) in order to reduce as much of the cardiac motion as possible. Thecardiac structure itself would be seen very well and 3D post processingto use navigator views of the Ostia of the pulmonary vessels and atrialappendage, as well as vessel analysis to lay out the pulmonary vesselscan still be achieved. In regards to regular CCTA exams, once again, a slower, or lower,pitch will provide more overlap reducing the potential formisregistration. The thing to remember is a lower pitch means more timein the scan field which translates to increased dose possibilities.

FIGURE 4. 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.

2. Scalloping
Scalloping is due to the fact that the slice sensitivity profile is increased in spiral CT so that partial volume artifacts also become stronger. Scalloping is a phenomenon arising, for example, in skull tomographies, particularly in slice positions in which the skull diameter quickly changes its axial direction.

Figure 9. Fibrous dysplasia (16 x 0.625 mm). (A and B) Note the characteristic ground-glass appearance on the axial oblique reformatted images. (C and D) Minimal expansion and endosteal scalloping are well seen on the 3D renderings.

3. Banding
Band artifacts due to bulk motion were investigated in images acquired with fast gradient echo sequences. A simple analytical calculation shows that the width of the artifacts has a square-root dependence on the velocity of the imaged object, the time taken to acquire each line of k-space and the field of view in the phase-encoding direction. The theory furthermore predicts that the artifact width can be reduced using parallel imaging by a factor equal to the square root of the acceleration parameter. The analysis and results are presented for motion in the phase-and frequency-encoding directions and comparisons are made between sequential and centric ordering. The theory is validated in phantom experiments, in which bulk motion is simulated in a controlled and reproducible manner by rocking the scan table back and forth along the bore axis. Preliminary cardiac studies in healthy human volunteers show that dark bands may be observed in the endocardium in images acquired with nonsegmented fast gradient echo sequences. The fact that the position of the bands changes with the phase-encoding direction suggests that they may be artifacts due to motion of the heart walls during the image acquisition period. One of problems with the current cardiac CT imaging is the banding artifacts, i.e., horizontal shifts in multiplanar reformatted (MPR) or three-dimensional (3-D) images.

Figure 6a. CT images of the posterior fossa show the dark banding that occurs between dense objects when only calibration correction is applied (a) and the reduction in artifacts when iterative beam hardening correction is also applied (b). (Reprinted, with permission, from reference 1.)

4. Stair-Stepping
The stair-step artifact associated with surfaces or object borders inclined relative to the table translation direction (8,9). Stair-step artifacts characteristically deteriorate the appearance of two-dimensional reformation and 3D-rendered objects and may affect the accuracy of volume or diameter measurements of structures within the scanned.
PURPOSE: Stair-step artifacts in helical computed tomography (CT) are associated with inclined surfaces in longitudinal sections. The authors investigated the origin and the characteristics of the artifacts. MATERIALS AND METHODS: A cone phantom and a skull were dry-scanned with a helical CT scanner, and images were reconstructed by using the half- scan interpolation algorithm with combinations of detector collimation (1 and 5 mm), table feed (1, 2, 5, and 10 mm), and reconstruction interval (1, 2, 5, and 10 mm). RESULTS: Stair-step artifacts were perceived in most instances. Stair-step artifacts arose from two sources: large reconstruction intervals and asymmetric helix interpolation, forming isoclosed curves and spirallike patterns in three-dimensional axial views, respectively. CONCLUSION: To eliminate the stair-step artifacts, both the collimation and the table feed should be less than the longitudinal dimension of the important feature on inclined surfaces, and the reconstruction interval should be less than the table feed. Adaptive interpolation may correct the artifacts. Figure 1. Schematic depicts measurement of stair-step artifacts in a 45° inclined acrylic rod phantom (attenuation, 130 HU) immersed in vegetable oil (attenuation, -100 HU). A profile of 360 radial distance (r) measurements were obtained between the computed centerline path ( ) and the surface of the rod, with use of an attenuation threshold of 0 HU. SDr in the coronal plane served as the measure of artifactual surface distortion of the rod.

5. Pitch Effect
Pitch refers to the ratio between the rate at which the table the patient lies on moves through the scanner and the rate at which the scanner spins. Pitch can be increased by about half to reduce radiation dose by about a third, without any loss of image quality. With single-slice helical CT, an increased pitch can decrease the radiation dose to the patient if all other parameters are constant.
(COULD NOT FIND AN IMAGE FOR THIS)

Cross-sectional CT images of the abdomen

Key:#1 = L5#2 = Ascending Colon#3 = Descending Colon#4 = Ureters (left & right)#5 = Iliac Crest#6 = Gluteus Medius#7 = Iliacus Muscle#8 = Psoas Muscle#9 = Rectus Abdominis#10 = Common Iliac Arteries#11 = Common Iliac Veins

Key:#1 = Liver#2 = Kidney#3 = Duodenum#4 = Jejunum (not visible on CT)#5 = Ascending Colon#6 = Descending Colon#7 = Ureter (not visible on CT)#8 = Psoas Muscle#9 = Rectus Abdominis#10 = Quadratus Lumborum#11 = Aorta#12 = Inferior Vena Cava

Key:#1 = Abdominal Aorta#2 = Duodenum#3 = Gallbladder#4 = Head of Pancreas#5 = Inferior Vena Cava#6 = Left Kidney#7 = Left Renal Vein#8 = Liver#9 = Psoas Muscle#10 = Right Kidney#11 = Stomach#12 = Superior Mesenteric Artery#13 = Tail of Pancreas#14 = Transverse Colon

Key:#1 = Abdominal Aorta#2 = Inferior Vena Cava#3 = Left Crus of Diaphragm#4 = Liver#5 = Right Crus of Diaphragm#6 = Spleen#7 = Stomach

Blood Circulation in the Liver

The liver's place in the circulation

Blood flow

The liver receives blood from two sources. Oxygenated blood is supplied in the hepatic artery, a branch of the celiac trunk from the abdominal aorta. Venous blood from the entire gastrointestinal tract (containing nutrients from the intestines) is brought to the liver by the hepatic portal vein. On reaching the liver the portal vein divides into thousands of which pass in between the lobules and terminate in the sinusoids. The blood leaves the liver via a central vein in each lobule, which drains in the hepatic vein.

Functions
The circulation of blood in the liver is so arranged that very large volumes of blood come into close contact with the cells of the lobules.The cells are thus in a favorable position both to absorb materials from the blood and also secret materials into it. This they do all the time, for the real task of the liver is to maintain in the blood the correct concentrations of many of its constituents. Hepatocytes carry out most of the tasks attributed to the liver, but the phagocytic Kupffer cells that line the sinusoids are responsible for cleansing the blood.

Formation of bile
When the red cells of the blood become worn out they are destroyed by the cells of the reticuloendothelial system. In this process bilirubin is formed, and this is carried by the blood to the liver. Together with several other substances it is secreted by the liver as bile.

The plasma proteins
In the blood plasma there are three soluble proteins which are essential for our well-being. These proteins are albumin, globulin, and fibrinogen, and all of them are largely formed in the liver. Albumin and globulin are important, for they control the amount of water that the blood draws out of the tissues as it flows through the capillaries. Fibrinogen is the precursor of the substance fibrin which is responsible for the formation of the blood clots which form on the top of wounds.

Various branches of the abdominal Aorta

KEY
A - Abdominal Aorta
G - Adrenolumbar Artery
B - Celiac Trunk
H - Renal Artery
C - Hepatic Artery
I - Gonadal Arteries
D - Left Gastric Artery
J - Inferior Mesenteric Artery
E - Splenic Artery
K - Iliolumbar Artery
F - Superior Mesenteric Artery
L - Femoral Artery

The abdominal aorta is a large-lumened, unpaired arterial vessel that is part of the main trunk of the systemic arterial system. As such, the abdominal aorta supplies oxygenated blood, pumped by the left ventricle of the heart, to the abdominal and pelvic organs and structures via visceral and parietal arterial branches.
The abdominal aorta and its major arterial branches are highly elastic. During systole (heart muscle contraction), the aortic and arterial walls expand to accommodate the increased blood flow. Correspondingly, the vessels contract during diastole and elastin fibers assure that this contraction also serves to drive blood through the arterial vessels.

As the thoracic aorta passes through the aortic hiatus (an opening in the diaphragm) it becomes the abdominal aorta. The abdominal aorta ultimately branches into left and right common iliac arteries. The common iliac arteries then branch into internal and external iliac arteries to supply oxygenated blood to the organs and tissues of the lower abdomen, pelvis, and legs.

Major branches of the abdominal aorta include, ventrally, the celiac branches, and superior and inferior mesenteric arteries. On the dorsal side of the aorta are the lumbar and median sacral branch arteries. Lateral to the aorta are the inferior phrenics, middle supernal, renal, and ovarian or testicular arteries. Because the branches from the abdominal aorta are large, the aorta rapidly decreases in size as it courses downward (inferiorly) through the abdomen.

The celiac trunk divides into three major branches: the left gastric artery to the stomach, the hepatic artery to the lobes of the liver, and splenic artery--surrounded by a plexus of nerves--that ultimately terminates in branches entering the hilus of the spleen.

CT Physics

This is CT Physics Spring 2009