CT Image Production- Image Processing

Which of these algorithms are preferred for viewing bone?

Edge-enhancing algorithms Edge-enhancing algorithms sharpen the appearance of bone and enhance visibility of fractures or other bone pathologies. All other pathologies listed are forms of smoothing algorithms used for viewing soft tissues, not bone.

What reconstruction algorithm should be used to create detailed images of the boney trabeculae?

Edge-enhancing algorithms Several variables affect the detail, or spatial resolution, of an image. The best detail and best spatial resolution can be achieved by using thin slices and edge enhancing algorithms. This creates more image noise, but it allows for the visualization of smaller parts.

Using a larger reconstruction slice thickness results in which of the following? 1-Decreased image noise 2-Decreased spatial resolution 3-More total image slices

1 and 2 only A larger slice thickness is both good and bad. Thicker slices result in less image noise because there is more information in each slice. However, the spatial resolution that decreases. This is because more data is average together, and some of the smaller details will get lost in the slice. Using thicker slices actually results in fewer total slices in the image series. It would be like cutting a loaf of bread with thicker slices. It's the same loaf, but there are fewer total slices.

What mathematical process prevents star-like artifacts from appearing on the CT images?

Back-projection filter Filtered back projection is the specific aspect of reconstruction used to prevent star artifacts on the CT image. Unfiltered back projection creates the same image, but the image will be filled with star like artifacts.

An ROI indicating an average HU of +39 mostly likely corresponds to which of the following?

Blood Different tissues in the body have different average hounsfield units. Fat is significantly less than zero. Water is approximately equal to zero. Bone is as high as +1,000. This leaves us with blood, which is very close to +39.

Which of these CT protocols generally use thin slices? a. Circle of Willis for arterial-venous malformation b. Abdomen for ulcerative colitis c. Chest for cancer evaluation d. Head for hemorrhage

Circle of Willis for arterial-venous malformation Compared to the other scan protocols, The Circle of Willis can will certainly use the thinnest slices. The Circle of Willis includes several tiny blood vessels, which must be evaluated using thinner slices. Remember, thinner slices are better for evaluating smaller parts.

The mathematical filtration of CT data to enhance the appearance of specific tissue types is called:

Convolution Convolution is part of the reconstruction process. This is what the computer system uses to enhance the appearance of certain tissues. For example, the bone algorithm convolutes the scan data into image data that best demonstrates bone.

Compared to a 1.25 mm image data, 3.75 mm image data will demonstrate:

Decreased spatial resolution Thicker slices will always result in LESS image noise and LESS spatial resolution. Thicker slices are not as good at imaging small parts, which is why the spatial resolution decreases.

By definition, the acquisition thickness is the same as the:

Detector row thickness The acquisition thickness is the same as the detector row thickness. If the detector rows are 2.5 mm thick, the acquisition thickness would be described as 2.5 mm. This can only be changed by combining detector rows.

An ROI placed over an abnormal lesion yields an average CT number of -52. This lesion may be composed of:

Fat Only a few types of tissue will have a hounsfield value less than zero. This includes air and lungs, as well as fat. In this question, the only possible answer is fat. Water, blood, and cerebrospinal fluid will all have hounsfield values equal to 0 or more.

The process of reconstruction in the CT computer system creates:

Image Data

Where is raw CT data stored?

In the CT computer system Raw data is stored in the CT computer system. This is not the same as the image data. This data is never viewed on the computer or sent to PACS, but it is used for retrospective reconstructions.

During a routine CT scan of a pediatric chest, the radiologist requests that the technologist reconstruct the image data with 1.25 mm slices rather than 2.5 mm slices. One negative effect of this change will be:

Increased image noise In this question, the slice thickness is being decreased from 2.5 mm down to 1.25 mm. The only negative effect is increased image noise, because there is less information to go into each slice. The spatial resolution increases, but this is a positive effect!

Estimating an unknown value in a series of known values is a mathematical process called:

Interpolation Interpolation is the process of estimating an unknown value when the surrounding values are known. Interpolation is an important part of helical scanning. This is what allows the spiral-shaped raw data to be converted into axial image data.

Which of these image-processing techniques in CT are only used in helical scanning?

Interpolation Interpolation is the process of estimating an unknown value when the surrounding values are known. Interpolation is an important part of helical scanning. This is what allows the spiral-shaped raw data to be converted into axial image data.

If the reconstruction slice thickness is set to 3.0 mm, which of these statements are also true?

The image data is 3.0 mm thick Reconstruction is a process through which image data is created. The image data is organized into slices, just like a loaf of bread is organized as slices. If the reconstruction slice thickness is set to 3.0 mm, the image data will be created at 3.0 mm. The reconstruction slice thickness is the same as the image slice thickness.

The process of assigning a specific level of edge-enhancement to the CT image data is controlled by mathematical filters called:

Reconstruction algorithms Reconstruction is the mathematical process that converts raw data into image data. This process includes assigning some level of edge enhancement. Reconstruction is accomplished buy a mathematical filters that are sometimes called reconstruction algorithms, reconstruction filters, or even reconstruction kernels. All of these terms refer to the same thing. The bone algorithm and standard algorithm are common examples of these reconstruction algorithms.

Iterative reconstruction is an alternative to traditional reconstruction and helps to:

Reduce patient dose Iterative reconstruction is a complex process. The primary benefits of this reconstruction process is a reduction in patient dose while at the same time increasing image quality.

After performing an abdominal scan for trauma, the radiologist requests reconstructed images of the lumbar spine. The raw data is not saved on the CT scanner. This will require:

Rescanning the patient Retrospective reconstruction can never be done without raw data on the CT scanner. If the raw data is gone there is absolutely no way to reconstruct new images. In this scenario, the patient must be scanned again.

After performing a thoracic spine scan for trauma, the radiologist requests images of the entire boney thorax. The raw data is still saved on the CT scanner. This will require:

Retrospectively reconstructing images with a different display field-of-view When scanning a patient, the raw data usually includes significantly more patient anatomy than the image data. The raw data will include everything in the scan field of view, which fills almost the entire gantry. Thoracic spine images use a display field of view that is much smaller than that. As long as the raw data is still on the computer, the entire bony thorax can be reconstructed retrospectively without rescanning the patient. This is accomplished by reconstructing the data into a different display field of view.

In the reconstructed image data, the distance from the start of one slice to the start of the next slice is defined as the:

Slice interval Slice interval is defined as the distance from one slice to the next slice. The slice interval might be the same as the slice thickness, but not always.

A patient in the emergency department is suspected of having a blowout fracture of right maxillary sinus. Structural damage to the eye will be best demonstrated with which of these filters?

Smoothing algorithms All soft tissues, including the soft tissues of the eye, should be evaluated using images Reconstructed with a smoothing algorithm. This makes soft tissues look smooth and easier to see. The other algorithms listed create more edge enhancement, which is good for viewing bones but not good for viewing soft tissues.

Select an algorithm that would be useful for viewing soft tissue changes around the cranium? Multiple correct answers are available.

Smoothing algorithms Unlike the bone algorithm, the standard algorithm and smoothing algorithms decrease edge enhancement. These types of algorithms are commonly used to view soft tissues, including soft tissues around the cranium. Soft tissues should not be viewed with any kind of edge-enhancing or bone algorithm. This would create more image noise that makes the soft tissues difficult to see. Standard algorithms Unlike the bone algorithm, the standard algorithm and smoothing algorithms decrease edge enhancement. These types of algorithms are commonly used to view soft tissues, including soft tissues around the cranium. Soft tissues should not be viewed with any kind of edge-enhancing or bone algorithm. This would create more image noise that makes the soft tissues difficult to see.

Which of these scenarios will result in gaps in the reconstructed image information?

When the slice interval is greater than the slice thickness Normally, the slice interval should be set equal to the slice thickness. In this scenario, one slice ends and the next begins with no gaps. Gaps are created in the data set when the slice interval is greater than the slice thickness. It would be like adding gaps in between the slices of a loaf of bread.

Thick reconstruction slices (greater than 3 mm) are NOT suitable for which of the following? Multiple correct answers are available. a. Multiplanar reformations b. Angiographic imaging c. Abdominal imaging d. Thoracic imaging

a. Multiplanar reformations Thicker slices are good for viewing images with minimal noise, but there are some times that thick slices should never be used. Thick slices should never be used for multiplanar reformations as this results in blurriness in the images. Thick slices should also never be used four protocols requiring high spatial resolution, such as angiographic imaging. b. Angiographic imaging

Select all parameters that CAN be changed retrospectively. a. Scan start and end locations b. Slice thickness c. Display Field-of-View d. Reconstruction algorithm e. Scan field-of-view f. Acquisition thickness g. Slice interval

a. Scan start and end locations b. Slice thickness c. Display Field-of-View d. Reconstruction algorithm g. Slice interval Some aspects of CT images can be changed retrospectively, but some cannot. In this example, notice that the scan field of view and acquisition thickness cannot be changed. Think of the scan field of view as being the raw data. We cannot create more raw data without rescanning the patient. The acquisition thickness is the thickness of the detector rows used during scanning. Obviously there is no way of changing this without scanning the patient again.