Management

The New Age Two-in-One Imaging: PET / CT

PET/CT is particularly effective in identifying whether cancer is present or not, if it has spread, if it is responding to treatment, and if a person is cancer free after treatment

"CT's ability to measure metabolism has significant implications in localizing the site or origin of epilepsy, because it can vividly illustrate areas where brain actively differs from the normal" - Amita Mitra Rockland Hospital New Delhi

Hailed as the investigation of this century, Positron Emission Tomography - Computed Tomography (better known by its acronym PET-CT) is a medical imaging device which combines in a single gantry system both a Positron Emission Tomography (PET) and an X-Ray Computed Tomography, so that images acquired from both devices can be taken sequentially, in the same session from the patient and combined into a single superposed (co-registered) image. Thus, functional imaging obtained by PET, which depicts the spatial distribution of metabolic or biochemical activity in the body can be more precisely aligned or correlated with anatomic imaging obtained by CT scanning. Two- and three-dimensional image reconstruction may be rendered as a function of a common software and control system.

PET-CT has revolutionised many fields of medical diagnosis, by adding precision of anatomic localisation to functional imaging, which was previously lacking from pure PET imaging. For example, in oncology, surgical planning, radiation therapy and cancer staging have been changing rapidly under the influence of PET-CT availability, to the extent that many diagnostic imaging procedures and centers have been gradually abandoning conventional PET devices and substituting them by PET-CTs. Although the combined device is considerably more expensive, it has the advantage of providing both functions as stand-alone examinations, being, in fact, two devices in one.

The Story of PET/ CT

Doctors, especially cancer surgeons, were often frustrated in trying to match PET images with CT images to determine the precise location of a tumour in relation to an organ or the spinal column. They had little choice other than to 'eyeball' the two separate images and make an educated guess as to the tumour's exact location- until 1992, when engineer Ron Nutt and physicist David Townsend came up with the idea of combining a PET and CT into one machine.

After working on their combined PET and CT concept for three years, Nutt and Townsend installed a prototype machine at the university of Pittsburgh medical centre in 1998.

Time Magazine honored PET /CT as the 'Medical Science Investigation of the year' in 2000, noting that the PET/ CT scanner has 'provided medicine with a powerful new diagnostic tool'.

How Does PET / CT WORK?

Sample image obtained using a combination of PET and CT imaging technology.

The PET/CT systems now in wide clinical use combine a multi-detector PET system with a multi-detector (currently 4-16 slice) Computed Tomography (CT) scanner in a single unit with a patient couch which traverses the bore of both imaging components. Approximately 30 to 60 minutes after intravenous FDG administration the patient is placed on the examination couch. The CT data is acquired first (lasting around 30 seconds) followed by a repeat slower transit of the patient through the bore for PET data acquisition (lasting around 30-45 minutes).

The CT and PET data sets are fused or 'co-registered' electronically by the scanner's computer system and presented to the interpreter on a work station. The data can then be simultaneously and interactively viewed as CT data, PET data, and superimposed CT and PET data in any percentage combination of these data sets desired (e.g. 100 per cent PET data, 100 per cent CT data, 50 per cent CT / 50 per cent PET data).

PET / CT Indications

PET/CT is particularly effective in identifying whether cancer is present or not, if it has spread, if it is responding to treatment, and if a person is cancer free after treatment.

Cancers for which PET/ CT is considered particularly effective include lung, head and neck, colorectal, esophageal, lymphoma, melanoma, and breast, as well as a variety of other tumours.

Early Detection

Series of PET/CT images, showing CT alone (far left), PET alone (center column), and PET/CT (right column).

Because PET/ CT images are biochemical activity, it can accurately characterise some tumours as benign or malignant, thereby avoiding surgical biopsy when the PET/ CT scan is negative. Conversely, because a PET/ CT scan images the entire body, conformation of distant metastases can alter treatment plans, in certain cases, from surgical intervention to chemotherapy. PET/ CT is extremely sensitive in determining the full extent of disease, especially in lymphoma, malignant melanoma, breast, lung and colon cancers.

Conformation of the presence or absence of metastatic disease allows the physician to more effectively decide how to proceed with the patient's management.

Checking for Recurrences

PET/ CT is currently considered to be the most accurate diagnostic procedure to differentiate tumour recurrences from radiation necrosis or post-surgical changes in many types of cancer. Such an approach allows for the development of a more rational treatment plan for the patient.

Assessing the Effectiveness of Chemotherapy

The level of tumour metabolism is compared on PET/CT scans taken before and after a chemotherapy cycle. PET/CT can provide important about the effectiveness of a chemotherapy treatment plan. Treatment regime can be altered or modified according to the PET /CT scan evidence of treatment response.

Neurology

The CT image (above left) shows a mass in the right lung. The combined PET/CT image (above right) reveals the metabolic activity of that mass, as well as its precise location in the lung. The fused image can help physicians with diagnosing and staging the disease, as well as tailoring the treatment plan

PET/ CT's ability to measure metabolism has significant implications in localizing the site or origin of epilepsy, because it can vividly illustrate areas where brain actively differs from the normal. PET/ CT can also be used to differentiate Alzheimer's disease, Pick's disease from other causes of dementia in cases where the clinical picture is atypical. The device can also evaluate extent of stroke and recovery following therapy as well as image malignant brain tumours.

Cardiology

By measuring both perfusion and metabolic activity within the heart, PET / CT scans can pinpoint areas of decreased blood flow such as that caused by artery blockages and can differentiate infracted non viable myocardium from viable one. This information is particularly important in patients who have had previous myocardial infarction and who are being considered for a revascularisation procedure.

Radiotherapy

For uses in stereotactic radiation therapy of cancer, specific fiducial marks are placed in the patient's body before acquiring the PET-CT images. The slices thus acquired may be transferred digitally to a linear accelerator which is used to perform precise bombardment of the target areas using high energy photons (radiosurgery). PET/CT is most beneficial in evaluating patients with known or suspected cancer. The new PET/CT systems have been shown to be highly accurate in the diagnosis, staging and restaging of cancer, and in monitoring the effects of therapy. PET/CT has become the standard of care for oncology imaging.

Procedure for PET / CT Imaging

An example of how PET-CT works in the work-up of FDG (flurodeoxyglucose) metabolic mapping follows:

• Before the exam, the patient undergoes a minimum of eight-hour fasting and rest;
• In the day of the exam, the patient rests lying for a minimum of 15 minutes, in order to quiet down muscular activity, which might be interpreted as abnormal metabolism.
• An intravenous bolus injection of a dose of recently produced 2-FDG or 3-FDG is made, usually by a vein in one of the arms. Dosage ranges from 0.1 to 0.2 mCi per kg of body weight.
• After one or two hours, the patient is placed into the PET-CT device, usually lying in a supine position with his/ her arms resting at the sides, or brought together above the head, depending on the main Region Of Interest (ROI).
• An automatic bed moves head first into the gantry, first obtaining a topogram, also called a scout view, which is a kind of whole body flat sagital section, obtained with the X-ray tube fixed into the upper position.
• The operator uses the PET-CT computer console to identify the patient and examination, delimit the caudal and rostral limits of the body scan onto the scout view, selects the scanning parameters and starts the image acquisition period, which follows without human intervention;
• The patient is automatically moved head first into the CT gantry, and the x-ray tomogram is acquired.
• Now the patient is automatically moved through the PET gantry, which is mounted in parallel with the CT gantry, and the PET slices are acquired.
• The patient may now leave the device, and the PET-CT software starts reconstructing and aligning the PET and CT images.

A whole body scan, which usually is made from mid-thighs to the top of the head, takes from five minutes to 40 minutes depending on the acquisition protocol and technology of the equipment used. FDG imaging protocols acquires slices with a thickness of two to three mm. Hypermetabolic lesions are shown as false color-coded pixels or voxels onto the gray-value coded CT images. Standardised uptake values are calculated by the software for each hypermetabolic region detected in the image. It provides a quantification of size of the lesion, since functional imaging does not provide a precise anatomical estimate of its extent. The CT can be used for that, when the lesion is also visualised in its images (this is not always the case when hypermetabolic lesions are not accompanied by anatomical changes).

Image Reconstruction and Interpretation

Taking the molecular imaging concept of PET one step further is the combined imaging modality positron emission tomography/ computed tomography (PET/CT). PET/ CT fuses functional information in the form of PET data and anatomic information in the form of CT data acquired almost simultaneously so that these information sets can be viewed and interpreted together. In PET/CT, both the multi-detector CT apparatus and the PET detectors are mounted in the same gantry, one immediately behind the other. Both PET and CT scanning are performed with the patient lying in the same position on the imaging table resulting in optimal correlation of anatomic and metabolic information. For interpretation, the PET data is actually superimposed upon the CT data (co-registration) resulting in improved anatomic localisation of normal and abnormal FDG activity. This fusion process has proven beneficial in more exactly localising tissues involved by tumour. Better co-registration is especially significant in regions of complex anatomy, such as in the abdomen and in the head and neck. More exact localisation of the involved tissues results in more accurate staging and more appropriate treatment planning including surgical therapy, radiotherapy, and medical therapy.

3D techniques have better sensitivity (because more coincidences are detected and used) and therefore less noise, but are more sensitive to the effects of scatter and random coincidences, as well as requiring correspondingly greater computer resources. The advent of sub-nanosecond timing resolution detectors affords better random coincidence rejection, thus favoring 3D image reconstruction

Safety

PET scanning is non-invasive, but it does involve exposure to ionising radiation. The total dose of radiation is not insignificant, usually around 11 mSv. This can also be compared to 2.2 mSv average annual background radiation, 0.02 mSv for a chest x-ray and 6.5 - 8 mSv for a CT scan of the chest, according to the Chest Journal and ICRP.

Benefits

The benefits of a combined PET/CT scanner include:

• Greater detail with a higher level of accuracy; because both scans are performed at one time without the patient having to change positions, there is less room for error.
• Greater convenience for the patient who undergoes two exams (CT and PET) at one sitting, rather than at two different times.

Risks

• Because the doses of radiotracer administered are small, diagnostic nuclear medicine procedures result in low radiation exposure, acceptable for diagnostic exams. Thus, the radiation risk is very low compared with the potential benefits.
• Nuclear medicine diagnostic procedures have been used for more than five decades, and there are no known long-term adverse effects from such low-dose exposure.
• Allergic reactions to radiopharmaceuticals may occur but are extremely rare and are usually mild. Nevertheless, you should inform the nuclear medicine personnel of any allergies you may have or other problems that may have occurred during a previous nuclear medicine exam.
• Injection of the radiotracer may cause slight pain and redness which should rapidly resolve.
• Women should always inform their physician or radiology technologist if there is any possibility that they are pregnant or if they are breastfeeding their baby.

Limitations

Limitations to the widespread use of PET arise from high cost of cyclotrons needed to produce the short-lived radionucleides for PET scanning and the need for specially adapted on-site chemical synthesis apparatus to produce radiopharmaceuticals. Because half life of F-18 is about two hours, the prepared dose of a radiopharmaceutical bearing this nuclide will undergo multiple half lives of decay during the working day. This necessitates frequent recalliberation of the remaining dose and careful planning with respect to patient scheduling.

3D techniques have better sensitivity (because more coincidences are detected and used) and therefore less noise, but are more sensitive to the effects of scatter and random coincidences, as well as requiring correspondingly greater computer resources. The advent of sub-nanosecond timing resolution detectors affords better random coincidence rejection, thus favouring 3D image reconstruction hours to days for the radiotracer to accumulate in the part of the body under study and imaging may take up to several hours to perform, though in some cases, newer equipment is available that can substantially shorten the procedure time.

The resolution of structures of the body with nuclear medicine may not be as clear as with other imaging techniques, such as CT or MRI. However, nuclear medicine scans are more sensitive than other techniques for a variety of indications, and the functional information gained from nuclear medicine exams is often unobtainable by any other imaging techniques.

PET scanning can give false results if chemical balances within the body are not normal. Specifically, test results of diabetic patients or patients who have eaten within a few hours prior to the examination can be adversely affected because of altered blood sugar or blood insulin levels.

A person who is very obese may not fit into the opening of a conventional PET/CT unit.

dramitamitra@yahoo.com