CT ANGIOGRAPHY

TABLE OF CONTENT

INTRODUCTION

Angiography is a medical imaging technique used to visualize the inside, or lumen, of blood vessels and organs of the body, with particular interest in the arteries, veins and the heart chambers.

Angiography is performed using: ◦ x-rays with catheters ◦ computed tomography (CT) ◦ magnetic resonance imaging (MRI)

CT ANGIOGRAPHY

Produces detailed images of both blood vessels and tissues in various parts of the body.

Contrast agent: Iodinated.

The scan is then performed while the contrast flows through the blood vessels to the various organs of the body.

By 1994 CT angiography began to replace conventional angiography in diagnosing and characterizing most cardiovascular abnormalities.

CT angiography is used to examine blood vessels and the organs supplied by them in various body parts, including:

  • brain
  • neck
  • heart
  • chest
  • abdomen (such as the kidneys and liver)
  • pelvis
  • legs and feet
  • arms and hands

PATIENT PREPARATION

Careful history and clinical examination.

Informed consent.

Patient should be well hydrated.

Fasting 4 hours prior to procedure.

Xylocaine sensitivity test.

Following investigations to be done :

  • Hb% and Hematocrit and platelet count
  • ESR
  • PT, PTT, BT and CT
  • HBsAg and HN
  • Pulse chart

General examination and bruits, if any, should be noted

INDICATIONS

  • aneurysms
  • blockages
  • blood clots
  • congenital (birth-related) abnormalities of the cardiovascular system, including the heart
  • disorganized blood vessels, such as vascular malformations
  • injury
  • tumors
  • vessel rupture or tears
  • Percutaneous interventional vascular procedures.

CONTRA INDICATIONS

  • Bleeding tendencies or anticoagulant therapy leading to a prothrombin time above 30% of the control values.
  • Abnormal renal function.
  • Cardio Vascular diseases like recent MI, overt CCF. Contrast injection may exacerbate cardiac failure.
  • Hepatic failure.
  • History of allergy to CM.
  • Pregnancy.
  • Residual barium from previous studies.

PROCEDURE

Cannula is inserted into the vein of the patient usually in the arm or hand.

With the help of pressure injector, connected to the IV,  will inject contrast media at a specified rate.

The patient is positioned on the CT exam table, usually lying flat on back.

Straps and pillows may be used to help you maintain the correct position and remain still during the exam.

In some cases, especially in children and patients with fragile and small veins, the contrast may be hand-injected using a syringe.

The scan of the desired area is performed.

TECHNIQUE

CT angiography is a contrast CT where images are taken with a certain delay after injection of radio-contrast medium.

The contrast medium is radiodense causing it to light up brightly within the blood vessels of interest.

In order for the CT scanner to be able to scan the correct area where the contrast is, the scanner uses either automatic detectors which start scanning when enough contrast is present, or small test boluses.

With the small test bolus, a small amount of contrast is injected in order to detect the speed that the contrast will move through the blood vessels.

After determining this speed, the full bolus is injected and the scan is begun at the timing determined by the test bolus.

After the scan is completed the images are post-processed to better visualize the vessels and can even be created in the 3D images.

Using contrast media can also help to obtain functional information about tissues.

Often, images are taken both with and without radiocontrast. CT images are called precontrast or native-phase images before any radiocontrast has been administrated, and postcontrast after radiocontrast administration.

BOLUS TRACKING

Bolus tracking is a technique to optimize timing of the imaging.

A small 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 (“R.O.I.”) 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, carotid and hepatic arteries.

The bolus-tracking technique is used to trigger the start of image acquisition, with ROI placement either in the LA or in the AA.

The delay time between reaching the threshold and the start of the acquisition is 5 s in CCTA.

In cranio-caudal direction, CCTA extended from the tracheal carina to just below the diaphragm. The so-called step-and-shoot technique was used, a prospective ECG-gated, sequential CT technique.

WASHOUT

“Washout” is where tissue loads radiocontrast during arterial phase, but then returns to a rather hypodense state in venous or later phases.

This is a property of for example hepatocellular carcinoma as compared to the rest of the liver parenchyma.

PHASES

Depending on the purpose of the investigation, there are standardized protocols for time intervals between intravenous radiocontrast administration and image acquisition, in order to visualize the dynamics of contrast enhancements in different organs and tissues.

The main phases thereof are as follows:

PHASE TIME FOR INJECTION TIME FOR BOLUS TRACKING TARGETED STRUCTURE AND FINDING
NECT-calcifications
-fat in tumors such as adrenocortical adenomas
– fat stranding as seen in inflammation such as appendicitis diverticulitis and omental infarction.
Pulmonary arterial phase 6-13 sec– pulmonary embolism (can use bolus tracking in pulmonary trunk +6 seconds)
Pulmonary venous phase 17-24 sec
Early systematic arterial phase 15-20 secimmediately – arteries without enhancement of organs and other soft tissues.
arterial phase /early venous portal phase 35-40 sec15-20 sec– all structures that get their blood supply from the arteries have optimal enhancement
– some enhancement of the portal vein
Pancreatic phase 30-50 sec20-30 sec– pancreatic cancers become hypodense compared to the parenchyma.
Hepatic or late portal phase 70-80 sec50-60 sec– liver parenchyma enhances through portal vein supply, normally with some enhancement of hepatic veins.
Nephrogenic phase 100 sec80 sec– all of the renal parenchyma enhances, including the medulla, allowing detection of small renal cell carcinomas
systematic venous phase 180 sec160 sec– detect venous thrombosis
Delayed phase/ wash out phase 6-15 min6-15 mindisappearance of contrast in all abdominal structures except for fibrosis, which appears more radiodense

AMOUNT OF CONTRAST DOSE

The dose should be adjusted in those not having normal body weight, and in such cases the adjustment should be proportional to the lean body mass of the person.

In obese patients, the Boer formula is the method of choice (at least in those with body mass index (BMI) between 35 and 40):

For men: Lean body mass = (0.407 × W) + (0.267 × H) − 19.2

For women: Lean body mass = (0.252 × W) + (0.473 × H) − 48.3

INFLUENCE OF SPATIAL RESOLUTION

In-plane spatial resolution is predominantly determined by detector geometry and the convolution kernel

The major advantage of more detector rows is higher through-plane resolution by reducing the width of a single detector row from 1–1.25 mm (four–detector row CT) to 0.5–0.6 mm (64–detector row CT) .

Typical in-plane resolution with application of a CT angiography protocol (64 × 0.6-mm detector configuration, 120 kV, 140 mAs [effective], field of view of 120 mm, medium sharp convolution kernel) is 0.6–0.7 mm and through-plane resolution is 0.5–0.7 mm, thus providing isotropic data.

Isotropic data allow image reconstruction in arbitrarily chosen planes without loss of spatial resolution and minimization of partial volume effects.

CONTRAST MATERIAL INJECTION

Short scan times require short contrast material injection.

The injection protocols need to be simple and standardized to guarantee excellent and reproducible results.

To deliver an appropriate amount of iodine, injection rates of 4–5 mL/sec and highly concentrated contrast medium (iodine, 350–370 mmol/mL) are preferable.

The utility of the contrast media bolus can be increased if a saline bolus is appended.

Flushing of the veins reduces streak artifacts due to beam hardening, especially at the thoracic inlet. 

IMAGE RECONSTRUCTION

—To reduce image noise, images may be reconstructed slightly thicker than the detector collimation, for example with a 0.75-mm section thickness from a data set acquired with 64 × 0.6-mm detector collimation.

—The reconstruction algorithm (convolution kernel) influences the spatial resolution in plane.

—The ideal kernel would combine low image noise and sharp edge definition, maintaining good low-contrast resolution.

—Soft kernels reduce image noise and allow smooth surfaces with rendering techniques, improving the visualization of aneurysms and vascular malformations.

—Sharper kernels improve edge definition and reduce blooming effects from calcifications, necessary for stenosis measurements, at the expense of higher image noise.

—The field of view also affects image quality, especially the quality of 3D reformations, which benefit from a small and isotropic voxel size.

POST PROCESSING TECHNIQUE

Image processing involves traditional operations such as multiplanar reformation (MPR) and maximum intensity projection (MIP), as well as surface and volume rendering.

Because bone and calcifications are seen as a particular problem in CT angiography, a variety of different approaches have been advocated to cope with this problem.

Editing, volume cropping, manipulation with transfer functions, and segmentation are common but time-consuming techniques, not convenient in the emergency setting.

Visualization with interactive MPR, sliding thin-slab MIP, or standardized volume rendering presets in combination with clip planes is more appropriate.

Sophisticated operations like volume rendering with 2D transfer functions or bone subtraction are emerging techniques that enhance the visualization of vascular disease with minimal user interaction.

BONE SUBTRACTION CT ANGIOGRAPHY

For bone subtraction CT angiography, non-enhanced and contrast-enhanced spiral CT data sets are required. The non-enhanced scan may be a diagnostic scan performed to rule out hemorrhage or ischemia or a low-dose scan performed for subtraction purposes only.

After loading both data sets, processing is performed automatically without any user interaction.

The algorithm selectively eliminates bone from the CT angiography data set, retaining both soft tissue and contrast-enhanced vessels.

Pixels in the non-enhanced data set with a CT value above a certain threshold are defined as bone and used to iteratively register the non-enhanced data set to the CT angiography data set. Registration is rigid (translation and rotation) and based on mutual information.

PRECAUTIONS

To prevent artifact:

Patient may be asked to hold their breath during the scanning.

◦ Occasionally, sedation may be needed for children to keep them still during scanning.

Reducing exposure:

◦ Children are more sensitive to radiation than adults, the scan is usually performed with an appropriate amount of radiation delivered for the size of the child.

◦ For children and adults of reproductive age, radiation protective shields are used for protection to reproductive parts.

TECHNICAL CONSIDERATION

A good CT angiography study requires more consideration and careful planning than a convention-al sequential CT scan.

Patient preparation, adequate scan protocol and knowledge of post-processing techniques with their advantages and pitfalls are important issues to be dealt with in order to obtain a satisfactory final result.

MDCT is the technology of choice for acquiring data for 3D post-processing.

It delivers faster scan times for a given region, together with other key features like narrow collimation, increased spatial resolution and less partial volume averaging.

RISKS

Harms of overuse of CT angiography include radiation exposure and the possibility of finding then seeking treatment for a clinically insignificant pulmonary embolism which ought not be treated.

Adverse reactions

A reaction may occur whenever iodine contrast is injected.

These reactions range in severity and it is difficult to predict if they will occur.

With the current practice of using low-osmolar contrast these adverse reactions only occur in ~0.1% of cases. 

The severity of the reaction can be :

÷Mild – no treatment required: nausea, vomiting, and/or hives.

÷Moderate – requires treatment: severe hives, lightheadedness or brief loss of consciousness, mild bronchospasm, and/or increased heart rate.

÷Severe – requires immediate treatment: severe bronchospasm, throat swelling, seizure, severe low blood pressure, and/or cardiac arrest.

 patient with a history of allergy to contrast may be advised to take medications such as corticosteroids or histamine (H1) blockers before CTA to lessen the risk of allergic reaction or to undergo a different exam that does not call for contrast material injection.

Patients should also be well hydrated in order to minimize possible adverse effects of contrast

KIDNEYS

—Historically it has been thought that contrast material can lead to contrast-induced nephropathy (also called CIN) in any patient.

However, recent studies have shown that the risk of kidney injury caused by contrast agent in patients with no history of kidney problems occurs extremely infrequently.

The use of CTA in people with kidney failure, kidney disease or long-standing severe diabetes should be weighed carefully as the use of IV iodine contrast material may further harm kidney function.

The decision not to use contrast agents must be weighed against the possibility of misdiagnoses if contrast is not used.

RADIATION

Compared with other imaging modalities, CTA is associated with a significant dose of ionizing radiation. Varying significantly with patient age, sex, and exam protocol, radiation risk models predict coronary CTA to increase lifetime cancer risk. The radiation risk presented by CTA is much lower than invasive angiography procedures.

CT angiography should not be performed in patients who are pregnant as the contrast and radiation may lead to harm to the fetus. The extent of harm to the fetus has not been fully determined

APPLICATIONS

  • Circle of Willis.
  • Subclavian arteries
  • Thoracic & abdominal aorta
  • Renal vasculature
  • Abdominal viscera vasculature
  • Coronary CT angiography
  • Aorta and great arteries
  • Pulmonary arteries
  • Carotid, vertebral and intracranial vessels
  • peripheral arteries

ADVANTAGES

CT angiography may give more precise anatomical detail than MRI, particularly in small blood vessels.

Many patients can undergo CT angiography instead of a conventional catheter angiography (catheterization) to diagnose blood vessel problems.

Lower cost examination compared to catheter angiography.

DISADVANTAGES

There is always a slight chance of cancer from excessive exposure to radiation. However, the benefit of an accurate diagnosis far outweighs the risk.

If a large amount of x-ray contrast material leaks out from the vein being injected and spreads under the skin where the IV is placed, it may damage the skin, blood vessels and nerves.

There might be a risk of serious allergic reaction to contrast materials that contain iodine

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