PACS

Newer Facets in Radiology

PACS allows direct economic savings on film, film packets, film processing chemicals and the redeployment of space previously used for film storage

"Cases that in the past required 30-plus minutes for transmission is now available almost as soon as completed" - Dr Natasha Nanda Radiologist -Piramal Diagnosis Centre Dr LH Hiranandani Hospital Mumbai

Many healthcare facilities today grapple with the issue of replacing a legacy of older picture archiving. The increasing proportion of imaging modalities that generate images in digital form has lead to the development of digital image management systems. Such systems referred to as Picture Archiving and Communication Systems (PACS) are emerging in clinical and radiological environments. A computerised means of replacing the role of conventional radiological film: images are acquired, stored, transmitted, and displayed digitally. PACS dissolves the physical time barriers associated with traditional film-based image retrieval, distribution and display. When such a system is installed throughout the hospital, a filmless clinical environment results. PACS are computers, commonly servers, dedicated to the storage, retrieval, distribution and presentation of images. A PACS consists of four major components: the imaging modalities such as CT and MRI, a secured network for the transmission of patient information, workstations for interpreting and reviewing images and long and short-term archives for the storage and retrieval of images and reports.

Once an image has been acquired onto PACS it cannot be lost, stolen or misfiled (20 per cent of films are missing when required). So no patient appointment is cancelled, no clinical decision deferred, no images are repeated because they are missing and no time is wasted by doctors or staff looking for missing films. All images are available day and night for viewing anywhere within or outside the hospital.

Most PACS handle images from various medical imaging instruments, including Ultrasound (US), Magnetic Resonance (MR), Positron Emission Tomography (PET), Computed Tomography (CT),Digital Radiography (DR), Computed Radiography (CR) and Mammography.

The Major Functions

PACS has four main uses. It replaces hard-copy based means of managing images, such as film archives. With the decreasing price of digital storage, PACS provides a growing cost and space advantage over film archives in addition to the instant access to prior images at the same institution. It expands on the possibilities of conventional systems by providing capabilities of off-site viewing and reporting. It enables practitioners in different physical locations to access the same information simultaneously for tele-radiology.

The numerous PACS terminals throughout the hospital allow simultaneous viewing of the same image. Whereas conventional film can only physically exist in one place at any one time. This means, for example, that a doctor in the Casualty can discuss a patient's images with the radiologist, with both clinicians viewing the images yet neither having left their department. Similarly, by the time a patient has returned to the outpatient department after being sent for an urgent radiological examination, the images will be available on PACS for viewing by the referring doctor.

The PACS database ensures that all images are automatically grouped into the correct examination, are chronologically ordered, correctly orientated and labeled, and can be easily retrieved using a variety of criteria (for example, name, hospital number, date, referring clinician, etc). Working with soft copy images on monitors allows the full gamut of computer tools to be used to manipulate and post-process the Images (crop, rotate, zoom etc). Alteration of the contrast width and level allows soft tissue and bony structures to be well seen on a single exposure. The 'invert grayscale' facility, whereby images may be viewed as black structures on a white background is considered by many to be helpful in precisely identifying the tips of thin lines, such as arterial and venous umbilical catheters.

Value for Money

PACS allows direct economic savings on film, film packets, film processing chemicals and the redeployment of space previously used for film storage.

A full PACS should also interface with existing hospital information systems: HIS and RIS. When a study has been reported by a radiologist the PACS can mark it as read. This avoids needless double-reading. The report can be attached to the images and be viewable via a single interface. The PACS can obtain lists of appointments and admissions in advance, allowing images to be pre-fetched from near line storage. Dictation of reports can be integrated into a single system Images can be compressed either through reversible lossless compression or through lossy compression. Lossless compression algorithms can only achieve relatively low compression rates (ratios of 2:1 up to 4:1). Lossy compression algorithm can provide much higher compression rates. However, through lossy compression, images are slightly degraded.

There is a centralised storage system with a long term archive of one month on optical disks and a short term storage system of 10 days on a random array of inexpensive disks called RAID or Redundant Array of Inexpensive Discs. It is a powerful and inexpensive storage solution which provides much faster disk access while completely protecting against the failure of one (or even two) discs in the physical RAID array. In addition, the RAID management software has an error correcting algorithm which is able to rewrite any lost data with a high degree of accuracy, so that data loss is very unlikely. Data stored on a RAID array is typically backed up to tape or copied, in real time, to a slower, inexpensive disc in another machine at another location. Images are retrieved in two seconds from short term storage and in approximately two minutes from long term storage. Textual demographic data are carried by slower ethernet protocol.

Main Features

There are two features of PACS software which facilitate and automate interface - prefetching and Default Display Protocols (DDPs).

Prefetching: It is the process whereby previous images on a patient are automatically retrieved from the long term archive onto the short term server, prior to the acquisition and viewing of the current imaging examination on that patient. This means that only a configurable number of examinations from the same modality and of the same body part as that currently being imaged are prefetched from the long-term archive. Prefetching is triggered by the HIS or the RIS, since it is these computer systems which act as the repository for data such as hospital admissions, outpatient clinic appointments, and the scheduling of further examinations.

Default Display Protocols: This software feature automatically arranges and displays the current image, with relevant past images on the PACS monitors in the correct chronological order, in a manner which allows immediate comparative viewing without the need for time consuming dragging of images into a preferred configuration.

When PACS has been installed it is vital to guard against a total system failure as a hospital cannot function without an imaging service. PACS image backup is a critical, but sometimes overlooked part of the PACS architecture. There are several methods of backing up the images, but they typically involve automatically sending copies of the images to a separate computer for storage, preferably off-site.

One of the most significant developments in PACS over the past couple of years has been the exploitation of conventional web browser technology to access images from a PACS server and displays them on ordinary desktop personal computers. This has provided a cheap and easy means of reviewing images without the need for expensive specialised PACS review workstations. It has also facilitated the development of teleradiology. By designing an appropriate tool for displaying and manipulating images and linking them to existing 'www' browsers, it is possible to use the existing web for communicating and remote consultations of images. A programme that displays images called 'Osiris' is the one that is widely used in some Hospitals for viewing images. Also, because the Osiris programme is written for different hardware platforms (Macintosh, PC Windows, and UNIX) it is ideally suited to be used as a document viewer in the conventional web architecture.

RoI

The installation of a hospital PACS is expensive. A reasonable conclusion is that PACS will pay for itself in about five years after installation from direct and estimated indirect cost savings. Cost varies, depending on the performance level of a chosen system. Low-end equipment can range from Rs 20 lakh. High performance systems exceed five crore.

Improvement in networking has moved connections from less than 1,000bps (bits/second) to speeds approaching 1,000Mbps (megabits/sec). Cases that in the past required 30-plus minutes for transmission is now available almost as soon as completed.

An Efficient Tool

Stability of the PACS is related not only to delivery of images but also to archival storage. It is critical that PACS performance and stability address both. The newer generation of PACS vendors also have divisions for data storage and migration. PACS can serve as an internal tool for the radiologists and an external one for the referring physician.

Functionally for the radiologist it is crucial for effective and efficient reading of films with capability to perform advanced reconstructions on any plane-MIP and MPR. These are fundamental tools for reading any high-quality MR or CT exam. Reconstruction is a natural evolution of advanced imaging, a process that should be seamless and simple to execute on a PACS workstation. Images are readily available for consultation between the patient and their primary care or referring physician, without incurring any additional costs, often as the patient leaves the centre and heads back to that doctor. Even collaborate with other radiologists on difficult cases.

The Pioneer of PACS

Dr Harold Glass, a medical physicist in London in the early 1990s transformed Hammersmith Hospital in London as the first filmless hospital in the United Kingdom. Dr Glass died a few months after the project came live but is credited with being one of the pioneers of PACS.

natasha.nanda@pds.piramal.com