| Dr Priya Chudgar |
Medical imaging has become an essential component in many fields of clinical practice; and oncology is not an exception. Varied imaging modalities with newer techniques and software not only help in early diagnosis, but also in treatment planning and follow ups. Newer advances with MDCT applications and PET has brought a paradigm shift in oncology. Accurate detection and preoperative evaluation helps in surgical planning. Regular monitoring helps to follow up during treatment and post-treatment. Thus, imaging plays an important and integral role in oncology. Here we discuss newer imaging techniques and advances vis-a-vis their role in oncology.
Ultrasound
Recent improvements in ultrasound such as tissue harmonic imaging, nonlinear signal processing and 2D matrix array transducers have introduced newer possibilities and paved the way for useful 3D imaging, while fast computing has allowed the production of real-time 3D scans (so-called 4D US imaging)
Volumes can be displayed as series of multiplanar reformats or rendered 3D images, which improve appreciation of the relative position of structures, including flowing blood. Currently, the main clinical applications are in obstetrics but the approach shows promise in breast and prostate cancer and reveals the complexity of tumour vascularity in a thorough manner. In interventional procedures, 4D ultrasounds are promising for needle biopsy guidance while 3D ultrasounds are used to guide radioactive seed implants in the prostate and for the breasts.
MDCT and its advanced applications
| 3D sonogram of breast accurately depicts size and morpholgy of lesion |
The advent of multi-detector CT scanners has brought a new era in field of radiology. This produces faster and better scans with shorter breath hold and less amount of iodinated contrast. It proves a blessing for morbid patients with poor renal function who require repeated scans. Newer machines also produce lesser radiation hazards.
MDCT with excellent angiography views serve as a guideline for oncosurgeons, while hepatic volumetric helps to predict tumour volume. Lymph node detection undoubtedly helps in tumour staging, while CT with stereotactic guidance helps in radiotherapy planning.
Image-processing softwares help to localise the tumour regions; image measurements help to quantify the tumour properties; image visualisations provide intuitive ways to present the tumour; image registrations help to fuse two images so that different tumour properties can be combined in one view; finally, CAD could be used in the clinical diagnosis/ detection of tumours.
Medical image processing has evolved into an established discipline. It is a very active and fast-growing field. Image processing techniques have already shown great potential in detecting and analysing tumours in clinical images and this trend will undoubtedly continue into the future.
MRI with spectroscopy and other applications
| Newer generation CT scan machine has revolutionised onco-imaging |
High performance MRI systems with newer sequences using diffusion, perfusion and dynamic contrast has furthered onco imaging. MRIs, with increasingly sophisticated imaging capabilities serve as problem-solving tools in most of the cases.
Neuroradiology and imaging of brain tumours is not complete without MRI and spectroscopy. Brain tissue is complex and is composed of many metabolites, some of which have unique magnetic resonance frequencies. However, most conventional MRI scans depend only on water and fat peaks to generate sufficient signal to generate an image. By selectively measuring the peaks of other metabolites relative to water, a spectrum that contains important clinical information can be generated. Two metabolites of particular importance in the brain are N-acetyl acetate (NAA) and choline (Ch) (31). NAA is a structural component of intact neural tissue. Choline is a membrane component of cells. In tumours, NAA would be expected to decrease in concentration whereas choline would increase in concentration. Thus, the ratio of NAA/ Ch decreases in tumours compared to normal brain tissue, and this ratio appears to have prognostic information. Tumours with low NAA/ Ch ratio have poorer prognosis.
PET technology with combined PET/ CT systems
These systems are especially helpful for detailed morphological and functional evaluation of disease. PET-CT has revolutionised onco imaging by adding precision of anatomic localisation to functional imaging. Surgical planning, radiation therapy and cancer staging have been changing rapidly under the influence of PET-CT. A PET/ CT system significantly decreases the number of equivocal findings.
Mammography and related newer techniques
| MR spectroscopy helps to predict tumour metabolites |
Discussion about onco-imaging cannot be complete without mammography. Mammography screening programmes helps for early detection of breast cancer, thus reducing death from breast cancer. Computer aided detection helps to pick up cancer, missed on mammography.
Elastography uses principle of the tissue’s distortion (strain) under an applied stress (e.g., compression via the transducer), known as elasticity imaging or elastography. The images produced have very high contrast and may significantly improve lesion detection within the breast, prostate and liver.
HIFU
HIFU or high-intensity focused ultrasound surgery, as a therapeutic technique is not a new concept but recent advances in probe design and alternate ultrasonic imaging methods make it likely to become a realistic clinical tool in the near future. HIFU uses a highly focused ultrasound beam to coagulate a well-defined volume of tissue by heating it to above 50 degree celcius. Maintenance of this temperature for one to seconds results in cell death, and a single ultrasound exposure destroys a cigar-shaped volume of tissue of 0.5 ml. The surrounding tissue is not damaged and there is a very sharp line of demarcation between coagulated and viable tissue. This completely non-invasive technique has been used to treat malignant tumours of the liver, prostate and kidney and benign breast via a percutaneous or transrectal approach without the need for general anaesthesia. Currently, HIFU tissue ablation damage is best observed using MRI, however it often renders the treatment cumbersome and expensive. Since B-mode ultrasound cannot distinguish between coagulated and normal tissue, alternate ultrasonic imaging methods such as elastography, reflex transmission imaging and thermal imaging are likely candidates to depict the tissue damage. HIFU could also be deployed intraoperatively, e.g., in the treatment of liver metastases.
Ultrasound drug and gene delivery
| PET/ CT picks up disease activity and reduces false positive findings |
Exposure to ultrasound causes a transient increase in cell membrane permeability, an effect known as sonoporation. Using this technique, tissues can be targetted to stimulate cellular uptake of a drug (e.g., a chemotherapeutic agent) or a gene. Sonoporation requires high acoustic powers (higher than that used in diagnosis and equivalent to those used in physiotherapy) but the power needed is markedly reduced when micro bubbles are also present. A drug or gene can be incorporated in or on the surface of the micro bubbles and tracked in the circulation with an imaging beam; when they are exposed to high power US, the micro bubbles rupture, releasing the agent near the target tissue. In the case of oncological drugs, this has the advantage of decreasing the dose of the drug needed, so reducing systemic side effects. Encouraging initial in vitro studies have demonstrated sonoporation without inducing cell death.
Conclusion
This topic of newer advances in onco imaging is unending. Though the list may look elaborate, it is only like tip of iceberg. Still newer and better applications are emerging. Be it contrast enhanced ultrasound or MR lymphangiography, ongoing research will throw light into clinical and advanced applications of many such techniques. No cancer patient can do without digital radiograph, mammogram, periodic ultrasound or cross-sectional imaging. Evolving role of PET CT will help for accurate staging and hence further management. Future developments with advent of molecular imaging and many more advances will eventually help to increase overall life expectancy.