Six-and-a half years after Pembroke, Ont., dentist Christy Natsis drove across the centre line of a highway and killed Bryan Casey, a 50-year-old father of three, the Ontario Court of Appeal will hear arguments today as to why she should either be acquitted or face a new trial.

After a 55-day trial in Ottawa that stretched over more than three years, Natsis was found guilty of impaired and dangerous driving of a motor vehicle causing death by a judge on May 29, 2015.

Six months after her conviction, Justice Neil Kozloff sentenced her to five years in prison.

But Tuesday, Natsis' counsel will argue in a Toronto courtroom that Kozloff erred in allowing a constable's expert witness testimony.

'You killed a human being. How inhumane.'-Lee Ellen Carroll, widow of Bryan Casey 

At Natsis' emotional sentencing, Casey's widow LeeEllen Carroll placed photos of her husband on the witness box as she read her victim impact statement to the packed courtroom. 

LeeEllen Carroll wife Bryan Casey May 29 2015 verdict Natsis

Bryan Casey's widow, LeeEllen Carroll, outside an Ottawa court in May 2015 said her family is "relieved that justice is being done." (CBC News)

"You killed a human being. How inhumane. His name is Bryan Casey," she said, looking at Natsis, who at times hung her head.

"He was someone's son ... [a] father of three young children. They are too pained to be here. He was a man, my loving husband."

"You ask me if it hurts? Only when I breathe," she said at the end of her statement.

Natsis, a mother of two teenagers, didn't testify in her own defence, but did address the court at the time of her sentencing.

'I stand here, feeling great shame, regret, remorse.'- Christy Natsis

The Pembroke, Ont., dentist said she was haunted by the statement Carroll made to reporters about the looks of pain on the children's faces when Carroll told them their father was dead.

"I stand here, feeling great shame, regret, remorse," she said, adding that she was not looking for sympathy and that she knows her pain pales in comparison to the Casey family's suffering.

She told the court that since the crash, she entered a period of "deep reflection" and was committed to the positive changes she'd made in her life. 

After her sentencing in November 2015, Natsis was granted bail by the Court of Appeal of Ontario in Toronto, according to her lawyer Matthew Gourlay.

She was in custody serving a 40-day sentence for breaching her release conditions by purchasing two bottles of vodka in the months following the crash.

Natsis hired Henein's law firm

Both Gourlay and Marie Henein are listed as counsel on the Natsis appeal. Henein successfully defended former CBC broadcaster Jian Ghomeshi on four counts of sexual assault and one count of choking in 2016. 

marie henein interview

Marie Henein is listed as counsel on the Christy Natsis appeal. Henein successfully defended former CBC broadcaster Jian Ghomeshi on four counts of sexual assault and one count of choking in 2016.

The Crown's case against Natsis included eyewitnesses at both the Kanata bar and the scene after the crash who testified that she was impaired and that they could smell alcohol on her. 

The court also heard from a driver who was behind Natsis on the road and described how her SUV swerved onto the shoulder of the highway.

Data retrieved from the on-board computer modules from both vehicles involved in the crash indicated Natis's SUV made no attempt to brake before the head-on collision, while Casey hit the brakes hard.

Defence argues judge shouldn't have allowed testimony

Natsis' defence counsel, headed by Ottawa lawyer Michael Edelson, was successful in convincing Kozloff that breath samples taken from his client were inadmissible because her charter right to legal counsel was violated. 

Edelson also tried to have the entire testimony of OPP Const. Shawn Kelly, the lead collision investigator, thrown out for being biased against Natsis. The lawyer argued that the officer had made mistakes in his technical report of the collision, had buried witness statements helpful to Natsis and inappropriately viewed himself as an investigator rather than a collision expert witness. 

In the end, the judge did toss significant portions of Kelly's testimony, but ruled his technical analysis of the crash was admissible.

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Christy Natsis was found guilty of impaired and dangerous driving of a motor vehicle causing death after driving across the centre line of Highway 17 in 2011.

Constable's shortcomings tainted the pool of evidence: defence 

The fact the judge allowed some of Kelly's testimony, and that of two other OPP collision experts, is the basis for the defence's appeal. 

In a written factum, Natsis's lawyers argue the trial judge made errors regarding the reliability and the admissibility of the officers who investigated the collision. 

The document goes further: "Kelly's shortcomings tainted this entire pool of evidence, irreparably."

In their own written factum, Crown prosecutors state: "The trial judge's decision to admit some of Kelly's evidence was reasonably open to him. As such, there is no basis for appellate intervention." 

The appeal hearing is scheduled to only last one day.

With files from Kristy Nease