CT Scan imaging

CT Scans are like x-rays, but take many more images in a precise and controlled fashion that permits the reconstruction of a 3-dimensional image. CT Scans are often taken in 'slices'. The patient may be given oral constrast and IV contrast to enhance the imaging of soft tissues.

Occluding factors aren't as much of an issue with CT scans as they are with x-rays, because the CT takes multiple fan-shaped images at each point as it rotates around the body, which is enough for a computer to reconstruct both the position and density of the objects in the body that absorb the x-rays.

Conventional CT scans are slow and require the patient to be immobilized during the scan. If the chest is being imaged, the patient may be told to hold his or her breath. Alignment problems between slices can sometimes miss a small mass that lies between slices. A conventional CT scan has a slice thickness of 8 mm to 10 mm.

Higher resolution CT scans, known as HRCT, helical CT and spiral CT, are much quicker. Instead of scanning each slice with the patient bed stationary, the bed is moving continuously as the CT scanner scans around the body (i.e., in a spiral pattern). This is fast enough that the patient doesn't need to hold his/her breath. It is also much higher resolution than conventional CT, so in many cases contrast dye will not be needed. A HRCT scan has a slice thickness of 1 mm to 1.3 mm.

A key limitation of HRCT is the higher resolution can lead to false positives. At higher resolutions more anomalies can be seen, but a much greater number of them turn out to be benign.

Another limitation of CT scanning in general is an inability to view very fine details in soft tissues such as muscles or ligaments. An MRI might be more appropriate in such situations.

Patient motion during a CT scan can cause the images to be "blurry". Also, any metal artifacts in the body can cause 'streaks' in the image. Flat tumors may also be harder to image with a CT scan. Scar tissue may also show up on a CT scan.

CT scanning is slower and more expensive than x-rays and ultrasounds. The radiation exposure from a whole-body CT scan is approximately 100 times that of a chest x-ray.

CT scans are not recommended for pregnant women. The intravenous contrast dye may not be recommended for patients who are allergic to iodine or shellfish.

Guidelines for Use of CT in Pregancy

The role of a radiologist is to estimate the fetal risk from known radiation dose from a particular examination and to help formulate a plan that provides minimal fetal radiation exposure but at the same time is able to accurately answer the clinical question. Table 3 presents a comparison of the guidelines proposed by the American Congress of Obstetricians and Gynecologists (ACOG) and those outlined by the ACR (10,14).

Recommendations by the ACOG and the ACR on Use of CT in Pregnancy

There are situations wherein the risk of irradiating the fetus is much less than the risk of not making a critical diagnosis in the mother (9,15), an assertion endorsed by the International Commission on Radiological Protection. For example, to evaluate the seriously injured pregnant patient with blunt abdominal trauma, CT (4) is the most accurate and cost-efficient diagnostic tool available (16,17). We are cautious when performing a CT examination in pregnant patients (11). At our institution, we use a low-dose CT protocol that entails reducing the scan range (if clinically allowable), decreasing the tube current, and increasing the pitch in comparison with those of the standard protocol. In some cases, it may be possible to reduce the kilovoltage without compromising image quality.

The radiation doses resulting in fetal anomalies and risks are far and above those typically seen in medical imaging, as shown in Table 1 (12). When medical imaging is being considered, radiation dose to the fetus is most concerning after multiple consecutive studies have been performed and the accumulation of radiation dose nears the threshold dose.

Overall, the best practice, as emphasized by the 2008 ACR practice guidelines for imaging pregnant or potentially pregnant adolescents and women with ionizing radiation, is as follows (10):

“To maintain a high standard of safety, particularly when imaging potentially pregnant patients, imaging radiation must be applied at levels as low as reasonably achievable (ALARA), while the degree of medical benefit must counterbalance the well-managed levels of risk.”

Radiation dose from a CT scan can be greatly reduced when proper technique is used.

CT Criteria for Adenoma

Several radiologic criteria can be used to diagnose an adenoma. The most well-known technique is to measure the mean attenuation value of the lesion. The rationale for this technique is based on a histologic feature of adenomas—that of abundant intracytoplasmatic lipid in the cortex. In contrast, malignant tumors of the adrenal gland have relatively little intracytoplasmic lipid, which can be detected by CT and MRI. A high correlation between lipid content and density measurements on CT has been shown, and when attenuation levels are between 10 HU and 15 HU or less on thin-section non-enhanced CT images, the diagnostic specificity approaches 100% . Although nonenhanced CT has a very high specificity, sensitivity is low, and higher attenuation values do not exclude the diagnosis of adenoma

Unfortunately, most CT examinations of the abdomen are performed after IV contrast, particularly if the indication is to look for metastatic disease, and mean attenuation measurements of adrenal adenomas may often be greater than 10 to 15 HU on postcontrast images. In this instance, looking at the contrast washout pattern of the adrenal lesion may help to diagnose an adenoma.

Studies have shown that adenomas show rapid washout of contrast when compared with nonadenomas and that this characteristic can be detected on delayed images acquired 10 to 15 minutes after the administration of IV contrast (Figure 10). If a relative washout of 40% to 50% is demonstrated, the test has a sensitivity of 83% to 93% and specificity of 93% to 98% for the diagnosis of an adenoma.An absolute washout of 60% is also highly specific and sensitive to make this differentiation. These parameters can be used to differentiate lesions that are nonspecific on nonenhanced CT (lipid-poor adenomas), as well as to characterize lesions identified only after the administration of IV contrast, when the use of a threshold of 10 HU or 15 HU is less likely to be useful for the diagnosis of adenoma.

Recently, a new method to detect the presence of fat within adrenal lesions was described by Bae et al. This study found that an accurate diagnosis of adenoma could be made using a histogram analysis method consisting of selecting a region of interest (ROI) within the adrenal mass and looking at the histogram distribution of pixel attenuation values. In this study, the presence of at least 10% negative pixels (ie, 10% lipid pixels) provided a specificity of virtually 100% for diagnosing adenoma. Although the sensitivity of this method was not high (reported to be 28%), the technique was found to have potential application in cases in which IV contrast was administered but delayed images were not available.

The heart in 3D CT Scan

Technological advances, ever-improving image quality and ever-shorter exposure times enabled CT to develop into one of the most important radiological diagnostic procedures within just a few years. It is used, for example if there is suspicion of bleeding, dilated/expanded blood vessels, a brain tumor or brain edema, or to check on degenerative or age-related changes/lesions, a possible stroke or a suspected fracture of the skull. Radiologists use the whole-body CT to search for tumors and cysts in the chest and abdomen – or to monitor the development of known tumors or changes in inner organs. Similarly, slipped discs, osteoporosis and other degenerative changes or bone fractures can be safely and quickly diagnosed with the help of computed tomography. A cardiac CT can generate a three-dimensional image of the coronary arteries and any changes in them. This method can be used to visualize calcifications and deposits in the coronary vessels, the first signs of atherosclerosis. A cardiac CT can show both the inside of the blood vessels and the walls. Further advantages stem from the three-dimensional nature of the image: the separate parts of the heart are shown in their true proportions. Moreover, the physician can turn the 3D object any way he or she wants. This makes it easier to recognize important details.

Advantages of CT Brain

 

 

• CT is much faster than MRI, making it the study of choice in cases of trauma and other acute neurological emergencies
• CT can be obtained at considerably less cost than MRI, and is sufficient to exclude many neurological disorders
• CT is less sensitive to patient motion during the examination. because the imaging can be performed much more rapidly
• CT may be easier to perform in claustrophobic or very heavy patients
• CT provides detailed evaluation of cortical bone
• CT allows accurate detection of calcification and metal foreign bodies
• CT can be performed at no risk to the patient with implantable medical devices, such as cardiac pacemakers, ferromagnetic vascular clips, and nerve stimulators

What happens during a CT scan?

You will remove any metallic objects which could diminish the quality of the images (this includes jewelry, glasses, dentures, and hair clips). You may also be asked remove your clothing and put on a patient gown. A technologist will help you to lie face up on the scanner table, with your head toward the “doughnut hole” of the CT scanner. The technologist will position you on the table, and a device to hold your head in place may be used. Then he or she leaves the exam room and goes to the control room, where you can still communicate by intercom.

An intravenous dye (contrast dye) may be given, through injection. This can help to highlight any areas of abnormality in the scan.

While CT images are being taken, it is important to lie still on the table, which will be moving very slowly in order to image the brain. It is normal for the CT scanner to make a whirring noise during the exam, so you should not be alarmed. The table will be moving a few millimeters at a time in order to obtain images of small slices of the brain, until the exam is finished. The procedure usually takes between 20 minutes and an hour.

Is a CT scan dangerous?

Far more X-rays are involved in a CT scan than in ordinary X-rays, so doctors do not recommend CT scans without a good medical reason.

However, the amount of radiation a patient is exposed to is small, and therefore are unlikely to cause any long term harm.

The risk is greatest to those who are pregnant, as radiation exposure can cause harm to the fetus, and therefore CT sans are contraindicated in pregnant women, unless the benefits of performing the scan far outweigh the risks.

Risks are also greater in children, when compared to adults, and therefore a CT is only recommended if a child has a serious condition that puts them at greater risk.

Some patients may experience side-effects due to allergic reactions to the liquid dye injected into the veins.

In very rare cases, this dye has been known to damage already weakened kidneys.

It is important to let the X-ray doctors or technicians know if you have any allergies, asthma or kidney trouble, prior to having the X-ray dye injected.